<- BCP Index (101..200)
BCP 195
(also RFC 8996, RFC 9325)
[Note that this file is a concatenation of more than one RFC.]
Internet Engineering Task Force (IETF) Y. Sheffer
Request for Comments: 7525 Intuit
BCP: 195 R. Holz
Category: Best Current Practice NICTA
ISSN: 2070-1721 P. Saint-Andre
&yet
May 2015
Recommendations for Secure Use of Transport Layer Security (TLS)
and Datagram Transport Layer Security (DTLS)
Abstract
Transport Layer Security (TLS) and Datagram Transport Layer Security
(DTLS) are widely used to protect data exchanged over application
protocols such as HTTP, SMTP, IMAP, POP, SIP, and XMPP. Over the
last few years, several serious attacks on TLS have emerged,
including attacks on its most commonly used cipher suites and their
modes of operation. This document provides recommendations for
improving the security of deployed services that use TLS and DTLS.
The recommendations are applicable to the majority of use cases.
Status of This Memo
This memo documents an Internet Best Current Practice.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
BCPs is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7525.
Sheffer, et al. Best Current Practice [Page 1]
RFC 7525 TLS Recommendations May 2015
Copyright Notice
Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Sheffer, et al. Best Current Practice [Page 2]
RFC 7525 TLS Recommendations May 2015
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. General Recommendations . . . . . . . . . . . . . . . . . . . 5
3.1. Protocol Versions . . . . . . . . . . . . . . . . . . . . 5
3.1.1. SSL/TLS Protocol Versions . . . . . . . . . . . . . . 5
3.1.2. DTLS Protocol Versions . . . . . . . . . . . . . . . 6
3.1.3. Fallback to Lower Versions . . . . . . . . . . . . . 7
3.2. Strict TLS . . . . . . . . . . . . . . . . . . . . . . . 7
3.3. Compression . . . . . . . . . . . . . . . . . . . . . . . 8
3.4. TLS Session Resumption . . . . . . . . . . . . . . . . . 8
3.5. TLS Renegotiation . . . . . . . . . . . . . . . . . . . . 9
3.6. Server Name Indication . . . . . . . . . . . . . . . . . 9
4. Recommendations: Cipher Suites . . . . . . . . . . . . . . . 9
4.1. General Guidelines . . . . . . . . . . . . . . . . . . . 9
4.2. Recommended Cipher Suites . . . . . . . . . . . . . . . . 11
4.2.1. Implementation Details . . . . . . . . . . . . . . . 12
4.3. Public Key Length . . . . . . . . . . . . . . . . . . . . 12
4.4. Modular Exponential vs. Elliptic Curve DH Cipher Suites . 13
4.5. Truncated HMAC . . . . . . . . . . . . . . . . . . . . . 14
5. Applicability Statement . . . . . . . . . . . . . . . . . . . 15
5.1. Security Services . . . . . . . . . . . . . . . . . . . . 15
5.2. Opportunistic Security . . . . . . . . . . . . . . . . . 16
6. Security Considerations . . . . . . . . . . . . . . . . . . . 17
6.1. Host Name Validation . . . . . . . . . . . . . . . . . . 17
6.2. AES-GCM . . . . . . . . . . . . . . . . . . . . . . . . . 18
6.3. Forward Secrecy . . . . . . . . . . . . . . . . . . . . . 18
6.4. Diffie-Hellman Exponent Reuse . . . . . . . . . . . . . . 19
6.5. Certificate Revocation . . . . . . . . . . . . . . . . . 19
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 21
7.1. Normative References . . . . . . . . . . . . . . . . . . 21
7.2. Informative References . . . . . . . . . . . . . . . . . 22
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 26
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 27
Sheffer, et al. Best Current Practice [Page 3]
RFC 7525 TLS Recommendations May 2015
1. Introduction
Transport Layer Security (TLS) [RFC5246] and Datagram Transport
Security Layer (DTLS) [RFC6347] are widely used to protect data
exchanged over application protocols such as HTTP, SMTP, IMAP, POP,
SIP, and XMPP. Over the last few years, several serious attacks on
TLS have emerged, including attacks on its most commonly used cipher
suites and their modes of operation. For instance, both the AES-CBC
[RFC3602] and RC4 [RFC7465] encryption algorithms, which together
have been the most widely deployed ciphers, have been attacked in the
context of TLS. A companion document [RFC7457] provides detailed
information about these attacks and will help the reader understand
the rationale behind the recommendations provided here.
Because of these attacks, those who implement and deploy TLS and DTLS
need updated guidance on how TLS can be used securely. This document
provides guidance for deployed services as well as for software
implementations, assuming the implementer expects his or her code to
be deployed in environments defined in Section 5. In fact, this
document calls for the deployment of algorithms that are widely
implemented but not yet widely deployed. Concerning deployment, this
document targets a wide audience -- namely, all deployers who wish to
add authentication (be it one-way only or mutual), confidentiality,
and data integrity protection to their communications.
The recommendations herein take into consideration the security of
various mechanisms, their technical maturity and interoperability,
and their prevalence in implementations at the time of writing.
Unless it is explicitly called out that a recommendation applies to
TLS alone or to DTLS alone, each recommendation applies to both TLS
and DTLS.
It is expected that the TLS 1.3 specification will resolve many of
the vulnerabilities listed in this document. A system that deploys
TLS 1.3 should have fewer vulnerabilities than TLS 1.2 or below.
This document is likely to be updated after TLS 1.3 gets noticeable
deployment.
These are minimum recommendations for the use of TLS in the vast
majority of implementation and deployment scenarios, with the
exception of unauthenticated TLS (see Section 5). Other
specifications that reference this document can have stricter
requirements related to one or more aspects of the protocol, based on
their particular circumstances (e.g., for use with a particular
application protocol); when that is the case, implementers are
advised to adhere to those stricter requirements. Furthermore, this
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RFC 7525 TLS Recommendations May 2015
document provides a floor, not a ceiling, so stronger options are
always allowed (e.g., depending on differing evaluations of the
importance of cryptographic strength vs. computational load).
Community knowledge about the strength of various algorithms and
feasible attacks can change quickly, and experience shows that a Best
Current Practice (BCP) document about security is a point-in-time
statement. Readers are advised to seek out any errata or updates
that apply to this document.
2. Terminology
A number of security-related terms in this document are used in the
sense defined in [RFC4949].
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
3. General Recommendations
This section provides general recommendations on the secure use of
TLS. Recommendations related to cipher suites are discussed in the
following section.
3.1. Protocol Versions
3.1.1. SSL/TLS Protocol Versions
It is important both to stop using old, less secure versions of SSL/
TLS and to start using modern, more secure versions; therefore, the
following are the recommendations concerning TLS/SSL protocol
versions:
o Implementations MUST NOT negotiate SSL version 2.
Rationale: Today, SSLv2 is considered insecure [RFC6176].
o Implementations MUST NOT negotiate SSL version 3.
Rationale: SSLv3 [RFC6101] was an improvement over SSLv2 and
plugged some significant security holes but did not support strong
cipher suites. SSLv3 does not support TLS extensions, some of
which (e.g., renegotiation_info [RFC5746]) are security-critical.
In addition, with the emergence of the POODLE attack [POODLE],
SSLv3 is now widely recognized as fundamentally insecure. See
[DEP-SSLv3] for further details.
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RFC 7525 TLS Recommendations May 2015
o Implementations SHOULD NOT negotiate TLS version 1.0 [RFC2246];
the only exception is when no higher version is available in the
negotiation.
Rationale: TLS 1.0 (published in 1999) does not support many
modern, strong cipher suites. In addition, TLS 1.0 lacks a per-
record Initialization Vector (IV) for CBC-based cipher suites and
does not warn against common padding errors.
o Implementations SHOULD NOT negotiate TLS version 1.1 [RFC4346];
the only exception is when no higher version is available in the
negotiation.
Rationale: TLS 1.1 (published in 2006) is a security improvement
over TLS 1.0 but still does not support certain stronger cipher
suites.
o Implementations MUST support TLS 1.2 [RFC5246] and MUST prefer to
negotiate TLS version 1.2 over earlier versions of TLS.
Rationale: Several stronger cipher suites are available only with
TLS 1.2 (published in 2008). In fact, the cipher suites
recommended by this document (Section 4.2 below) are only
available in TLS 1.2.
This BCP applies to TLS 1.2 and also to earlier versions. It is not
safe for readers to assume that the recommendations in this BCP apply
to any future version of TLS.
3.1.2. DTLS Protocol Versions
DTLS, an adaptation of TLS for UDP datagrams, was introduced when TLS
1.1 was published. The following are the recommendations with
respect to DTLS:
o Implementations SHOULD NOT negotiate DTLS version 1.0 [RFC4347].
Version 1.0 of DTLS correlates to version 1.1 of TLS (see above).
o Implementations MUST support and MUST prefer to negotiate DTLS
version 1.2 [RFC6347].
Version 1.2 of DTLS correlates to version 1.2 of TLS (see above).
(There is no version 1.1 of DTLS.)
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3.1.3. Fallback to Lower Versions
Clients that "fall back" to lower versions of the protocol after the
server rejects higher versions of the protocol MUST NOT fall back to
SSLv3 or earlier.
Rationale: Some client implementations revert to lower versions of
TLS or even to SSLv3 if the server rejected higher versions of the
protocol. This fallback can be forced by a man-in-the-middle (MITM)
attacker. TLS 1.0 and SSLv3 are significantly less secure than TLS
1.2, the version recommended by this document. While TLS 1.0-only
servers are still quite common, IP scans show that SSLv3-only servers
amount to only about 3% of the current Web server population. (At
the time of this writing, an explicit method for preventing downgrade
attacks has been defined recently in [RFC7507].)
3.2. Strict TLS
The following recommendations are provided to help prevent SSL
Stripping (an attack that is summarized in Section 2.1 of [RFC7457]):
o In cases where an application protocol allows implementations or
deployments a choice between strict TLS configuration and dynamic
upgrade from unencrypted to TLS-protected traffic (such as
STARTTLS), clients and servers SHOULD prefer strict TLS
configuration.
o Application protocols typically provide a way for the server to
offer TLS during an initial protocol exchange, and sometimes also
provide a way for the server to advertise support for TLS (e.g.,
through a flag indicating that TLS is required); unfortunately,
these indications are sent before the communication channel is
encrypted. A client SHOULD attempt to negotiate TLS even if these
indications are not communicated by the server.
o HTTP client and server implementations MUST support the HTTP
Strict Transport Security (HSTS) header [RFC6797], in order to
allow Web servers to advertise that they are willing to accept
TLS-only clients.
o Web servers SHOULD use HSTS to indicate that they are willing to
accept TLS-only clients, unless they are deployed in such a way
that using HSTS would in fact weaken overall security (e.g., it
can be problematic to use HSTS with self-signed certificates, as
described in Section 11.3 of [RFC6797]).
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RFC 7525 TLS Recommendations May 2015
Rationale: Combining unprotected and TLS-protected communication
opens the way to SSL Stripping and similar attacks, since an initial
part of the communication is not integrity protected and therefore
can be manipulated by an attacker whose goal is to keep the
communication in the clear.
3.3. Compression
In order to help prevent compression-related attacks (summarized in
Section 2.6 of [RFC7457]), implementations and deployments SHOULD
disable TLS-level compression (Section 6.2.2 of [RFC5246]), unless
the application protocol in question has been shown not to be open to
such attacks.
Rationale: TLS compression has been subject to security attacks, such
as the CRIME attack.
Implementers should note that compression at higher protocol levels
can allow an active attacker to extract cleartext information from
the connection. The BREACH attack is one such case. These issues
can only be mitigated outside of TLS and are thus outside the scope
of this document. See Section 2.6 of [RFC7457] for further details.
3.4. TLS Session Resumption
If TLS session resumption is used, care ought to be taken to do so
safely. In particular, when using session tickets [RFC5077], the
resumption information MUST be authenticated and encrypted to prevent
modification or eavesdropping by an attacker. Further
recommendations apply to session tickets:
o A strong cipher suite MUST be used when encrypting the ticket (as
least as strong as the main TLS cipher suite).
o Ticket keys MUST be changed regularly, e.g., once every week, so
as not to negate the benefits of forward secrecy (see Section 6.3
for details on forward secrecy).
o For similar reasons, session ticket validity SHOULD be limited to
a reasonable duration (e.g., half as long as ticket key validity).
Rationale: session resumption is another kind of TLS handshake, and
therefore must be as secure as the initial handshake. This document
(Section 4) recommends the use of cipher suites that provide forward
secrecy, i.e. that prevent an attacker who gains momentary access to
the TLS endpoint (either client or server) and its secrets from
reading either past or future communication. The tickets must be
managed so as not to negate this security property.
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3.5. TLS Renegotiation
Where handshake renegotiation is implemented, both clients and
servers MUST implement the renegotiation_info extension, as defined
in [RFC5746].
The most secure option for countering the Triple Handshake attack is
to refuse any change of certificates during renegotiation. In
addition, TLS clients SHOULD apply the same validation policy for all
certificates received over a connection. The [triple-handshake]
document suggests several other possible countermeasures, such as
binding the master secret to the full handshake (see [SESSION-HASH])
and binding the abbreviated session resumption handshake to the
original full handshake. Although the latter two techniques are
still under development and thus do not qualify as current practices,
those who implement and deploy TLS are advised to watch for further
development of appropriate countermeasures.
3.6. Server Name Indication
TLS implementations MUST support the Server Name Indication (SNI)
extension defined in Section 3 of [RFC6066] for those higher-level
protocols that would benefit from it, including HTTPS. However, the
actual use of SNI in particular circumstances is a matter of local
policy.
Rationale: SNI supports deployment of multiple TLS-protected virtual
servers on a single address, and therefore enables fine-grained
security for these virtual servers, by allowing each one to have its
own certificate.
4. Recommendations: Cipher Suites
TLS and its implementations provide considerable flexibility in the
selection of cipher suites. Unfortunately, some available cipher
suites are insecure, some do not provide the targeted security
services, and some no longer provide enough security. Incorrectly
configuring a server leads to no or reduced security. This section
includes recommendations on the selection and negotiation of cipher
suites.
4.1. General Guidelines
Cryptographic algorithms weaken over time as cryptanalysis improves:
algorithms that were once considered strong become weak. Such
algorithms need to be phased out over time and replaced with more
secure cipher suites. This helps to ensure that the desired security
properties still hold. SSL/TLS has been in existence for almost 20
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RFC 7525 TLS Recommendations May 2015
years and many of the cipher suites that have been recommended in
various versions of SSL/TLS are now considered weak or at least not
as strong as desired. Therefore, this section modernizes the
recommendations concerning cipher suite selection.
o Implementations MUST NOT negotiate the cipher suites with NULL
encryption.
Rationale: The NULL cipher suites do not encrypt traffic and so
provide no confidentiality services. Any entity in the network
with access to the connection can view the plaintext of contents
being exchanged by the client and server. (Nevertheless, this
document does not discourage software from implementing NULL
cipher suites, since they can be useful for testing and
debugging.)
o Implementations MUST NOT negotiate RC4 cipher suites.
Rationale: The RC4 stream cipher has a variety of cryptographic
weaknesses, as documented in [RFC7465]. Note that DTLS
specifically forbids the use of RC4 already.
o Implementations MUST NOT negotiate cipher suites offering less
than 112 bits of security, including so-called "export-level"
encryption (which provide 40 or 56 bits of security).
Rationale: Based on [RFC3766], at least 112 bits of security is
needed. 40-bit and 56-bit security are considered insecure today.
TLS 1.1 and 1.2 never negotiate 40-bit or 56-bit export ciphers.
o Implementations SHOULD NOT negotiate cipher suites that use
algorithms offering less than 128 bits of security.
Rationale: Cipher suites that offer between 112-bits and 128-bits
of security are not considered weak at this time; however, it is
expected that their useful lifespan is short enough to justify
supporting stronger cipher suites at this time. 128-bit ciphers
are expected to remain secure for at least several years, and
256-bit ciphers until the next fundamental technology
breakthrough. Note that, because of so-called "meet-in-the-
middle" attacks [Multiple-Encryption], some legacy cipher suites
(e.g., 168-bit 3DES) have an effective key length that is smaller
than their nominal key length (112 bits in the case of 3DES).
Such cipher suites should be evaluated according to their
effective key length.
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RFC 7525 TLS Recommendations May 2015
o Implementations SHOULD NOT negotiate cipher suites based on RSA
key transport, a.k.a. "static RSA".
Rationale: These cipher suites, which have assigned values
starting with the string "TLS_RSA_WITH_*", have several drawbacks,
especially the fact that they do not support forward secrecy.
o Implementations MUST support and prefer to negotiate cipher suites
offering forward secrecy, such as those in the Ephemeral Diffie-
Hellman and Elliptic Curve Ephemeral Diffie-Hellman ("DHE" and
"ECDHE") families.
Rationale: Forward secrecy (sometimes called "perfect forward
secrecy") prevents the recovery of information that was encrypted
with older session keys, thus limiting the amount of time during
which attacks can be successful. See Section 6.3 for a detailed
discussion.
4.2. Recommended Cipher Suites
Given the foregoing considerations, implementation and deployment of
the following cipher suites is RECOMMENDED:
o TLS_DHE_RSA_WITH_AES_128_GCM_SHA256
o TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256
o TLS_DHE_RSA_WITH_AES_256_GCM_SHA384
o TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384
These cipher suites are supported only in TLS 1.2 because they are
authenticated encryption (AEAD) algorithms [RFC5116].
Typically, in order to prefer these suites, the order of suites needs
to be explicitly configured in server software. (See [BETTERCRYPTO]
for helpful deployment guidelines, but note that its recommendations
differ from the current document in some details.) It would be ideal
if server software implementations were to prefer these suites by
default.
Some devices have hardware support for AES-CCM but not AES-GCM, so
they are unable to follow the foregoing recommendations regarding
cipher suites. There are even devices that do not support public key
cryptography at all, but they are out of scope entirely.
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RFC 7525 TLS Recommendations May 2015
4.2.1. Implementation Details
Clients SHOULD include TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256 as the
first proposal to any server, unless they have prior knowledge that
the server cannot respond to a TLS 1.2 client_hello message.
Servers MUST prefer this cipher suite over weaker cipher suites
whenever it is proposed, even if it is not the first proposal.
Clients are of course free to offer stronger cipher suites, e.g.,
using AES-256; when they do, the server SHOULD prefer the stronger
cipher suite unless there are compelling reasons (e.g., seriously
degraded performance) to choose otherwise.
This document does not change the mandatory-to-implement TLS cipher
suite(s) prescribed by TLS. To maximize interoperability, RFC 5246
mandates implementation of the TLS_RSA_WITH_AES_128_CBC_SHA cipher
suite, which is significantly weaker than the cipher suites
recommended here. (The GCM mode does not suffer from the same
weakness, caused by the order of MAC-then-Encrypt in TLS
[Krawczyk2001], since it uses an AEAD mode of operation.)
Implementers should consider the interoperability gain against the
loss in security when deploying the TLS_RSA_WITH_AES_128_CBC_SHA
cipher suite. Other application protocols specify other cipher
suites as mandatory to implement (MTI).
Note that some profiles of TLS 1.2 use different cipher suites. For
example, [RFC6460] defines a profile that uses the
TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256 and
TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384 cipher suites.
[RFC4492] allows clients and servers to negotiate ECDH parameters
(curves). Both clients and servers SHOULD include the "Supported
Elliptic Curves" extension [RFC4492]. For interoperability, clients
and servers SHOULD support the NIST P-256 (secp256r1) curve
[RFC4492]. In addition, clients SHOULD send an ec_point_formats
extension with a single element, "uncompressed".
4.3. Public Key Length
When using the cipher suites recommended in this document, two public
keys are normally used in the TLS handshake: one for the Diffie-
Hellman key agreement and one for server authentication. Where a
client certificate is used, a third public key is added.
With a key exchange based on modular exponential (MODP) Diffie-
Hellman groups ("DHE" cipher suites), DH key lengths of at least 2048
bits are RECOMMENDED.
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RFC 7525 TLS Recommendations May 2015
Rationale: For various reasons, in practice, DH keys are typically
generated in lengths that are powers of two (e.g., 2^10 = 1024 bits,
2^11 = 2048 bits, 2^12 = 4096 bits). Because a DH key of 1228 bits
would be roughly equivalent to only an 80-bit symmetric key
[RFC3766], it is better to use keys longer than that for the "DHE"
family of cipher suites. A DH key of 1926 bits would be roughly
equivalent to a 100-bit symmetric key [RFC3766] and a DH key of 2048
bits might be sufficient for at least the next 10 years
[NIST.SP.800-56A]. See Section 4.4 for additional information on the
use of MODP Diffie-Hellman in TLS.
As noted in [RFC3766], correcting for the emergence of a TWIRL
machine would imply that 1024-bit DH keys yield about 65 bits of
equivalent strength and that a 2048-bit DH key would yield about 92
bits of equivalent strength.
With regard to ECDH keys, the IANA "EC Named Curve Registry" (within
the "Transport Layer Security (TLS) Parameters" registry [IANA-TLS])
contains 160-bit elliptic curves that are considered to be roughly
equivalent to only an 80-bit symmetric key [ECRYPT-II]. Curves of
less than 192 bits SHOULD NOT be used.
When using RSA, servers SHOULD authenticate using certificates with
at least a 2048-bit modulus for the public key. In addition, the use
of the SHA-256 hash algorithm is RECOMMENDED (see [CAB-Baseline] for
more details). Clients SHOULD indicate to servers that they request
SHA-256, by using the "Signature Algorithms" extension defined in
TLS 1.2.
4.4. Modular Exponential vs. Elliptic Curve DH Cipher Suites
Not all TLS implementations support both modular exponential (MODP)
and elliptic curve (EC) Diffie-Hellman groups, as required by
Section 4.2. Some implementations are severely limited in the length
of DH values. When such implementations need to be accommodated, the
following are RECOMMENDED (in priority order):
1. Elliptic Curve DHE with appropriately negotiated parameters
(e.g., the curve to be used) and a Message Authentication Code
(MAC) algorithm stronger than HMAC-SHA1 [RFC5289]
2. TLS_DHE_RSA_WITH_AES_128_GCM_SHA256 [RFC5288], with 2048-bit
Diffie-Hellman parameters
3. TLS_DHE_RSA_WITH_AES_128_GCM_SHA256, with 1024-bit parameters
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RFC 7525 TLS Recommendations May 2015
Rationale: Although Elliptic Curve Cryptography is widely deployed,
there are some communities where its adoption has been limited for
several reasons, including its complexity compared to modular
arithmetic and longstanding perceptions of IPR concerns (which, for
the most part, have now been resolved [RFC6090]). Note that ECDHE
cipher suites exist for both RSA and ECDSA certificates, so moving to
ECDHE cipher suites does not require moving away from RSA-based
certificates. On the other hand, there are two related issues
hindering effective use of MODP Diffie-Hellman cipher suites in TLS:
o There are no standardized, widely implemented protocol mechanisms
to negotiate the DH groups or parameter lengths supported by
client and server.
o Many servers choose DH parameters of 1024 bits or fewer.
o There are widely deployed client implementations that reject
received DH parameters if they are longer than 1024 bits. In
addition, several implementations do not perform appropriate
validation of group parameters and are vulnerable to attacks
referenced in Section 2.9 of [RFC7457].
Note that with DHE and ECDHE cipher suites, the TLS master key only
depends on the Diffie-Hellman parameters and not on the strength of
the RSA certificate; moreover, 1024 bit MODP DH parameters are
generally considered insufficient at this time.
With MODP ephemeral DH, deployers ought to carefully evaluate
interoperability vs. security considerations when configuring their
TLS endpoints.
4.5. Truncated HMAC
Implementations MUST NOT use the Truncated HMAC extension, defined in
Section 7 of [RFC6066].
Rationale: the extension does not apply to the AEAD cipher suites
recommended above. However it does apply to most other TLS cipher
suites. Its use has been shown to be insecure in [PatersonRS11].
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RFC 7525 TLS Recommendations May 2015
5. Applicability Statement
The recommendations of this document primarily apply to the
implementation and deployment of application protocols that are most
commonly used with TLS and DTLS on the Internet today. Examples
include, but are not limited to:
o Web software and services that wish to protect HTTP traffic with
TLS.
o Email software and services that wish to protect IMAP, POP3, or
SMTP traffic with TLS.
o Instant-messaging software and services that wish to protect
Extensible Messaging and Presence Protocol (XMPP) or Internet
Relay Chat (IRC) traffic with TLS.
o Realtime media software and services that wish to protect Secure
Realtime Transport Protocol (SRTP) traffic with DTLS.
This document does not modify the implementation and deployment
recommendations (e.g., mandatory-to-implement cipher suites)
prescribed by existing application protocols that employ TLS or DTLS.
If the community that uses such an application protocol wishes to
modernize its usage of TLS or DTLS to be consistent with the best
practices recommended here, it needs to explicitly update the
existing application protocol definition (one example is [TLS-XMPP],
which updates [RFC6120]).
Designers of new application protocols developed through the Internet
Standards Process [RFC2026] are expected at minimum to conform to the
best practices recommended here, unless they provide documentation of
compelling reasons that would prevent such conformance (e.g.,
widespread deployment on constrained devices that lack support for
the necessary algorithms).
5.1. Security Services
This document provides recommendations for an audience that wishes to
secure their communication with TLS to achieve the following:
o Confidentiality: all application-layer communication is encrypted
with the goal that no party should be able to decrypt it except
the intended receiver.
o Data integrity: any changes made to the communication in transit
are detectable by the receiver.
Sheffer, et al. Best Current Practice [Page 15]
RFC 7525 TLS Recommendations May 2015
o Authentication: an endpoint of the TLS communication is
authenticated as the intended entity to communicate with.
With regard to authentication, TLS enables authentication of one or
both endpoints in the communication. In the context of opportunistic
security [RFC7435], TLS is sometimes used without authentication. As
discussed in Section 5.2, considerations for opportunistic security
are not in scope for this document.
If deployers deviate from the recommendations given in this document,
they need to be aware that they might lose access to one of the
foregoing security services.
This document applies only to environments where confidentiality is
required. It recommends algorithms and configuration options that
enforce secrecy of the data in transit.
This document also assumes that data integrity protection is always
one of the goals of a deployment. In cases where integrity is not
required, it does not make sense to employ TLS in the first place.
There are attacks against confidentiality-only protection that
utilize the lack of integrity to also break confidentiality (see, for
instance, [DegabrieleP07] in the context of IPsec).
This document addresses itself to application protocols that are most
commonly used on the Internet with TLS and DTLS. Typically, all
communication between TLS clients and TLS servers requires all three
of the above security services. This is particularly true where TLS
clients are user agents like Web browsers or email software.
This document does not address the rarer deployment scenarios where
one of the above three properties is not desired, such as the use
case described in Section 5.2 below. As another scenario where
confidentiality is not needed, consider a monitored network where the
authorities in charge of the respective traffic domain require full
access to unencrypted (plaintext) traffic, and where users
collaborate and send their traffic in the clear.
5.2. Opportunistic Security
There are several important scenarios in which the use of TLS is
optional, i.e., the client decides dynamically ("opportunistically")
whether to use TLS with a particular server or to connect in the
clear. This practice, often called "opportunistic security", is
described at length in [RFC7435] and is often motivated by a desire
for backward compatibility with legacy deployments.
Sheffer, et al. Best Current Practice [Page 16]
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In these scenarios, some of the recommendations in this document
might be too strict, since adhering to them could cause fallback to
cleartext, a worse outcome than using TLS with an outdated protocol
version or cipher suite.
This document specifies best practices for TLS in general. A
separate document containing recommendations for the use of TLS with
opportunistic security is to be completed in the future.
6. Security Considerations
This entire document discusses the security practices directly
affecting applications using the TLS protocol. This section contains
broader security considerations related to technologies used in
conjunction with or by TLS.
6.1. Host Name Validation
Application authors should take note that some TLS implementations do
not validate host names. If the TLS implementation they are using
does not validate host names, authors might need to write their own
validation code or consider using a different TLS implementation.
It is noted that the requirements regarding host name validation
(and, in general, binding between the TLS layer and the protocol that
runs above it) vary between different protocols. For HTTPS, these
requirements are defined by Section 3 of [RFC2818].
Readers are referred to [RFC6125] for further details regarding
generic host name validation in the TLS context. In addition, that
RFC contains a long list of example protocols, some of which
implement a policy very different from HTTPS.
If the host name is discovered indirectly and in an insecure manner
(e.g., by an insecure DNS query for an MX or SRV record), it SHOULD
NOT be used as a reference identifier [RFC6125] even when it matches
the presented certificate. This proviso does not apply if the host
name is discovered securely (for further discussion, see [DANE-SRV]
and [DANE-SMTP]).
Host name validation typically applies only to the leaf "end entity"
certificate. Naturally, in order to ensure proper authentication in
the context of the PKI, application clients need to verify the entire
certification path in accordance with [RFC5280] (see also [RFC6125]).
Sheffer, et al. Best Current Practice [Page 17]
RFC 7525 TLS Recommendations May 2015
6.2. AES-GCM
Section 4.2 above recommends the use of the AES-GCM authenticated
encryption algorithm. Please refer to Section 11 of [RFC5246] for
general security considerations when using TLS 1.2, and to Section 6
of [RFC5288] for security considerations that apply specifically to
AES-GCM when used with TLS.
6.3. Forward Secrecy
Forward secrecy (also called "perfect forward secrecy" or "PFS" and
defined in [RFC4949]) is a defense against an attacker who records
encrypted conversations where the session keys are only encrypted
with the communicating parties' long-term keys. Should the attacker
be able to obtain these long-term keys at some point later in time,
the session keys and thus the entire conversation could be decrypted.
In the context of TLS and DTLS, such compromise of long-term keys is
not entirely implausible. It can happen, for example, due to:
o A client or server being attacked by some other attack vector, and
the private key retrieved.
o A long-term key retrieved from a device that has been sold or
otherwise decommissioned without prior wiping.
o A long-term key used on a device as a default key [Heninger2012].
o A key generated by a trusted third party like a CA, and later
retrieved from it either by extortion or compromise
[Soghoian2011].
o A cryptographic break-through, or the use of asymmetric keys with
insufficient length [Kleinjung2010].
o Social engineering attacks against system administrators.
o Collection of private keys from inadequately protected backups.
Forward secrecy ensures in such cases that it is not feasible for an
attacker to determine the session keys even if the attacker has
obtained the long-term keys some time after the conversation. It
also protects against an attacker who is in possession of the long-
term keys but remains passive during the conversation.
Forward secrecy is generally achieved by using the Diffie-Hellman
scheme to derive session keys. The Diffie-Hellman scheme has both
parties maintain private secrets and send parameters over the network
as modular powers over certain cyclic groups. The properties of the
Sheffer, et al. Best Current Practice [Page 18]
RFC 7525 TLS Recommendations May 2015
so-called Discrete Logarithm Problem (DLP) allow the parties to
derive the session keys without an eavesdropper being able to do so.
There is currently no known attack against DLP if sufficiently large
parameters are chosen. A variant of the Diffie-Hellman scheme uses
Elliptic Curves instead of the originally proposed modular
arithmetics.
Unfortunately, many TLS/DTLS cipher suites were defined that do not
feature forward secrecy, e.g., TLS_RSA_WITH_AES_256_CBC_SHA256. This
document therefore advocates strict use of forward-secrecy-only
ciphers.
6.4. Diffie-Hellman Exponent Reuse
For performance reasons, many TLS implementations reuse Diffie-
Hellman and Elliptic Curve Diffie-Hellman exponents across multiple
connections. Such reuse can result in major security issues:
o If exponents are reused for too long (e.g., even more than a few
hours), an attacker who gains access to the host can decrypt
previous connections. In other words, exponent reuse negates the
effects of forward secrecy.
o TLS implementations that reuse exponents should test the DH public
key they receive for group membership, in order to avoid some
known attacks. These tests are not standardized in TLS at the
time of writing. See [RFC6989] for recipient tests required of
IKEv2 implementations that reuse DH exponents.
6.5. Certificate Revocation
The following considerations and recommendations represent the
current state of the art regarding certificate revocation, even
though no complete and efficient solution exists for the problem of
checking the revocation status of common public key certificates
[RFC5280]:
o Although Certificate Revocation Lists (CRLs) are the most widely
supported mechanism for distributing revocation information, they
have known scaling challenges that limit their usefulness (despite
workarounds such as partitioned CRLs and delta CRLs).
o Proprietary mechanisms that embed revocation lists in the Web
browser's configuration database cannot scale beyond a small
number of the most heavily used Web servers.
Sheffer, et al. Best Current Practice [Page 19]
RFC 7525 TLS Recommendations May 2015
o The On-Line Certification Status Protocol (OCSP) [RFC6960]
presents both scaling and privacy issues. In addition, clients
typically "soft-fail", meaning that they do not abort the TLS
connection if the OCSP server does not respond. (However, this
might be a workaround to avoid denial-of-service attacks if an
OCSP responder is taken offline.)
o The TLS Certificate Status Request extension (Section 8 of
[RFC6066]), commonly called "OCSP stapling", resolves the
operational issues with OCSP. However, it is still ineffective in
the presence of a MITM attacker because the attacker can simply
ignore the client's request for a stapled OCSP response.
o OCSP stapling as defined in [RFC6066] does not extend to
intermediate certificates used in a certificate chain. Although
the Multiple Certificate Status extension [RFC6961] addresses this
shortcoming, it is a recent addition without much deployment.
o Both CRLs and OCSP depend on relatively reliable connectivity to
the Internet, which might not be available to certain kinds of
nodes (such as newly provisioned devices that need to establish a
secure connection in order to boot up for the first time).
With regard to common public key certificates, servers SHOULD support
the following as a best practice given the current state of the art
and as a foundation for a possible future solution:
1. OCSP [RFC6960]
2. Both the status_request extension defined in [RFC6066] and the
status_request_v2 extension defined in [RFC6961] (This might
enable interoperability with the widest range of clients.)
3. The OCSP stapling extension defined in [RFC6961]
The considerations in this section do not apply to scenarios where
the DANE-TLSA resource record [RFC6698] is used to signal to a client
which certificate a server considers valid and good to use for TLS
connections.
Sheffer, et al. Best Current Practice [Page 20]
RFC 7525 TLS Recommendations May 2015
7. References
7.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000,
<http://www.rfc-editor.org/info/rfc2818>.
[RFC3766] Orman, H. and P. Hoffman, "Determining Strengths For
Public Keys Used For Exchanging Symmetric Keys", BCP 86,
RFC 3766, April 2004,
<http://www.rfc-editor.org/info/rfc3766>.
[RFC4492] Blake-Wilson, S., Bolyard, N., Gupta, V., Hawk, C., and B.
Moeller, "Elliptic Curve Cryptography (ECC) Cipher Suites
for Transport Layer Security (TLS)", RFC 4492, May 2006,
<http://www.rfc-editor.org/info/rfc4492>.
[RFC4949] Shirey, R., "Internet Security Glossary, Version 2", FYI
36, RFC 4949, August 2007,
<http://www.rfc-editor.org/info/rfc4949>.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008,
<http://www.rfc-editor.org/info/rfc5246>.
[RFC5288] Salowey, J., Choudhury, A., and D. McGrew, "AES Galois
Counter Mode (GCM) Cipher Suites for TLS", RFC 5288,
August 2008, <http://www.rfc-editor.org/info/rfc5288>.
[RFC5289] Rescorla, E., "TLS Elliptic Curve Cipher Suites with SHA-
256/384 and AES Galois Counter Mode (GCM)", RFC 5289,
August 2008, <http://www.rfc-editor.org/info/rfc5289>.
[RFC5746] Rescorla, E., Ray, M., Dispensa, S., and N. Oskov,
"Transport Layer Security (TLS) Renegotiation Indication
Extension", RFC 5746, February 2010,
<http://www.rfc-editor.org/info/rfc5746>.
[RFC6066] Eastlake 3rd, D., "Transport Layer Security (TLS)
Extensions: Extension Definitions", RFC 6066, January
2011, <http://www.rfc-editor.org/info/rfc6066>.
Sheffer, et al. Best Current Practice [Page 21]
RFC 7525 TLS Recommendations May 2015
[RFC6125] Saint-Andre, P. and J. Hodges, "Representation and
Verification of Domain-Based Application Service Identity
within Internet Public Key Infrastructure Using X.509
(PKIX) Certificates in the Context of Transport Layer
Security (TLS)", RFC 6125, March 2011,
<http://www.rfc-editor.org/info/rfc6125>.
[RFC6176] Turner, S. and T. Polk, "Prohibiting Secure Sockets Layer
(SSL) Version 2.0", RFC 6176, March 2011,
<http://www.rfc-editor.org/info/rfc6176>.
[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security Version 1.2", RFC 6347, January 2012,
<http://www.rfc-editor.org/info/rfc6347>.
[RFC7465] Popov, A., "Prohibiting RC4 Cipher Suites", RFC 7465,
February 2015, <http://www.rfc-editor.org/info/rfc7465>.
7.2. Informative References
[BETTERCRYPTO]
bettercrypto.org, "Applied Crypto Hardening", April 2015,
<https://bettercrypto.org/static/
applied-crypto-hardening.pdf>.
[CAB-Baseline]
CA/Browser Forum, "Baseline Requirements for the Issuance
and Management of Publicly-Trusted Certificates Version
1.1.6", 2013, <https://www.cabforum.org/documents.html>.
[DANE-SMTP]
Dukhovni, V. and W. Hardaker, "SMTP security via
opportunistic DANE TLS", Work in Progress, draft-ietf-
dane-smtp-with-dane-16, April 2015.
[DANE-SRV] Finch, T., Miller, M., and P. Saint-Andre, "Using DNS-
Based Authentication of Named Entities (DANE) TLSA Records
with SRV Records", Work in Progress,
draft-ietf-dane-srv-14, April 2015.
[DEP-SSLv3]
Barnes, R., Thomson, M., Pironti, A., and A. Langley,
"Deprecating Secure Sockets Layer Version 3.0", Work in
Progress, draft-ietf-tls-sslv3-diediedie-03, April 2015.
Sheffer, et al. Best Current Practice [Page 22]
RFC 7525 TLS Recommendations May 2015
[DegabrieleP07]
Degabriele, J. and K. Paterson, "Attacking the IPsec
Standards in Encryption-only Configurations", IEEE
Symposium on Security and Privacy (SP '07), 2007,
<http://dx.doi.org/10.1109/SP.2007.8>.
[ECRYPT-II]
Smart, N., "ECRYPT II Yearly Report on Algorithms and
Keysizes (2011-2012)", 2012,
<http://www.ecrypt.eu.org/ecrypt2/>.
[Heninger2012]
Heninger, N., Durumeric, Z., Wustrow, E., and J.
Halderman, "Mining Your Ps and Qs: Detection of Widespread
Weak Keys in Network Devices", Usenix Security Symposium
2012, 2012.
[IANA-TLS] IANA, "Transport Layer Security (TLS) Parameters",
<http://www.iana.org/assignments/tls-parameters>.
[Kleinjung2010]
Kleinjung, T., "Factorization of a 768-Bit RSA modulus",
CRYPTO 10, 2010, <https://eprint.iacr.org/2010/006.pdf>.
[Krawczyk2001]
Krawczyk, H., "The Order of Encryption and Authentication
for Protecting Communications (Or: How Secure is SSL?)",
CRYPTO 01, 2001,
<https://www.iacr.org/archive/crypto2001/21390309.pdf>.
[Multiple-Encryption]
Merkle, R. and M. Hellman, "On the security of multiple
encryption", Communications of the ACM, Vol. 24, 1981,
<http://dl.acm.org/citation.cfm?id=358718>.
[NIST.SP.800-56A]
Barker, E., Chen, L., Roginsky, A., and M. Smid,
"Recommendation for Pair-Wise Key Establishment Schemes
Using Discrete Logarithm Cryptography", NIST Special
Publication 800-56A, 2013,
<http://nvlpubs.nist.gov/nistpubs/SpecialPublications/
NIST.SP.800-56Ar2.pdf>.
[POODLE] US-CERT, "SSL 3.0 Protocol Vulnerability and POODLE
Attack", Alert TA14-290A, October 2014,
<https://www.us-cert.gov/ncas/alerts/TA14-290A>.
Sheffer, et al. Best Current Practice [Page 23]
RFC 7525 TLS Recommendations May 2015
[PatersonRS11]
Paterson, K., Ristenpart, T., and T. Shrimpton, "Tag size
does matter: attacks and proofs for the TLS record
protocol", 2011,
<http://dx.doi.org/10.1007/978-3-642-25385-0_20>.
[RFC2026] Bradner, S., "The Internet Standards Process -- Revision
3", BCP 9, RFC 2026, October 1996,
<http://www.rfc-editor.org/info/rfc2026>.
[RFC2246] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
RFC 2246, January 1999,
<http://www.rfc-editor.org/info/rfc2246>.
[RFC3602] Frankel, S., Glenn, R., and S. Kelly, "The AES-CBC Cipher
Algorithm and Its Use with IPsec", RFC 3602, September
2003, <http://www.rfc-editor.org/info/rfc3602>.
[RFC4346] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.1", RFC 4346, April 2006,
<http://www.rfc-editor.org/info/rfc4346>.
[RFC4347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security", RFC 4347, April 2006,
<http://www.rfc-editor.org/info/rfc4347>.
[RFC5077] Salowey, J., Zhou, H., Eronen, P., and H. Tschofenig,
"Transport Layer Security (TLS) Session Resumption without
Server-Side State", RFC 5077, January 2008,
<http://www.rfc-editor.org/info/rfc5077>.
[RFC5116] McGrew, D., "An Interface and Algorithms for Authenticated
Encryption", RFC 5116, January 2008,
<http://www.rfc-editor.org/info/rfc5116>.
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, May 2008,
<http://www.rfc-editor.org/info/rfc5280>.
[RFC6090] McGrew, D., Igoe, K., and M. Salter, "Fundamental Elliptic
Curve Cryptography Algorithms", RFC 6090, February 2011,
<http://www.rfc-editor.org/info/rfc6090>.
[RFC6101] Freier, A., Karlton, P., and P. Kocher, "The Secure
Sockets Layer (SSL) Protocol Version 3.0", RFC 6101,
August 2011, <http://www.rfc-editor.org/info/rfc6101>.
Sheffer, et al. Best Current Practice [Page 24]
RFC 7525 TLS Recommendations May 2015
[RFC6120] Saint-Andre, P., "Extensible Messaging and Presence
Protocol (XMPP): Core", RFC 6120, March 2011,
<http://www.rfc-editor.org/info/rfc6120>.
[RFC6460] Salter, M. and R. Housley, "Suite B Profile for Transport
Layer Security (TLS)", RFC 6460, January 2012,
<http://www.rfc-editor.org/info/rfc6460>.
[RFC6698] Hoffman, P. and J. Schlyter, "The DNS-Based Authentication
of Named Entities (DANE) Transport Layer Security (TLS)
Protocol: TLSA", RFC 6698, August 2012,
<http://www.rfc-editor.org/info/rfc6698>.
[RFC6797] Hodges, J., Jackson, C., and A. Barth, "HTTP Strict
Transport Security (HSTS)", RFC 6797, November 2012,
<http://www.rfc-editor.org/info/rfc6797>.
[RFC6960] Santesson, S., Myers, M., Ankney, R., Malpani, A.,
Galperin, S., and C. Adams, "X.509 Internet Public Key
Infrastructure Online Certificate Status Protocol - OCSP",
RFC 6960, June 2013,
<http://www.rfc-editor.org/info/rfc6960>.
[RFC6961] Pettersen, Y., "The Transport Layer Security (TLS)
Multiple Certificate Status Request Extension", RFC 6961,
June 2013, <http://www.rfc-editor.org/info/rfc6961>.
[RFC6989] Sheffer, Y. and S. Fluhrer, "Additional Diffie-Hellman
Tests for the Internet Key Exchange Protocol Version 2
(IKEv2)", RFC 6989, July 2013,
<http://www.rfc-editor.org/info/rfc6989>.
[RFC7435] Dukhovni, V., "Opportunistic Security: Some Protection
Most of the Time", RFC 7435, December 2014,
<http://www.rfc-editor.org/info/rfc7435>.
[RFC7457] Sheffer, Y., Holz, R., and P. Saint-Andre, "Summarizing
Known Attacks on Transport Layer Security (TLS) and
Datagram TLS (DTLS)", RFC 7457, February 2015,
<http://www.rfc-editor.org/info/rfc7457>.
[RFC7507] Moeller, B. and A. Langley, "TLS Fallback Signaling Cipher
Suite Value (SCSV) for Preventing Protocol Downgrade
Attacks", RFC 7507, April 2015.
Sheffer, et al. Best Current Practice [Page 25]
RFC 7525 TLS Recommendations May 2015
[SESSION-HASH]
Bhargavan, K., Ed., Delignat-Lavaud, A., Pironti, A.,
Langley, A., and M. Ray, "Transport Layer Security (TLS)
Session Hash and Extended Master Secret Extension", Work
in Progress, draft-ietf-tls-session-hash-05, April 2015.
[Smith2013]
Smith, B., "Proposal to Change the Default TLS
Ciphersuites Offered by Browsers.", 2013,
<https://briansmith.org/browser-ciphersuites-01.html>.
[Soghoian2011]
Soghoian, C. and S. Stamm, "Certified lies: Detecting and
defeating government interception attacks against SSL",
Proc. 15th Int. Conf. Financial Cryptography and Data
Security, 2011.
[TLS-XMPP] Saint-Andre, P. and a. alkemade, "Use of Transport Layer
Security (TLS) in the Extensible Messaging and Presence
Protocol (XMPP)", Work in Progress,
draft-ietf-uta-xmpp-07, April 2015.
[triple-handshake]
Delignat-Lavaud, A., Bhargavan, K., and A. Pironti,
"Triple Handshakes Considered Harmful: Breaking and Fixing
Authentication over TLS", 2014,
<https://secure-resumption.com/>.
Acknowledgments
Thanks to RJ Atkinson, Uri Blumenthal, Viktor Dukhovni, Stephen
Farrell, Daniel Kahn Gillmor, Paul Hoffman, Simon Josefsson, Watson
Ladd, Orit Levin, Ilari Liusvaara, Johannes Merkle, Bodo Moeller,
Yoav Nir, Massimiliano Pala, Kenny Paterson, Patrick Pelletier, Tom
Ritter, Joe St. Sauver, Joe Salowey, Rich Salz, Brian Smith, Sean
Turner, and Aaron Zauner for their feedback and suggested
improvements. Thanks also to Brian Smith, who has provided a great
resource in his "Proposal to Change the Default TLS Ciphersuites
Offered by Browsers" [Smith2013]. Finally, thanks to all others who
commented on the TLS, UTA, and other discussion lists but who are not
mentioned here by name.
Robert Sparks and Dave Waltermire provided helpful reviews on behalf
of the General Area Review Team and the Security Directorate,
respectively.
Sheffer, et al. Best Current Practice [Page 26]
RFC 7525 TLS Recommendations May 2015
During IESG review, Richard Barnes, Alissa Cooper, Spencer Dawkins,
Stephen Farrell, Barry Leiba, Kathleen Moriarty, and Pete Resnick
provided comments that led to further improvements.
Ralph Holz gratefully acknowledges the support by Technische
Universitaet Muenchen. The authors gratefully acknowledge the
assistance of Leif Johansson and Orit Levin as the working group
chairs and Pete Resnick as the sponsoring Area Director.
Authors' Addresses
Yaron Sheffer
Intuit
4 HaHarash St.
Hod HaSharon 4524075
Israel
EMail: yaronf.ietf@gmail.com
Ralph Holz
NICTA
13 Garden St.
Eveleigh 2015 NSW
Australia
EMail: ralph.ietf@gmail.com
Peter Saint-Andre
&yet
EMail: peter@andyet.com
URI: https://andyet.com/
Sheffer, et al. Best Current Practice [Page 27]
=========================================================================
Internet Engineering Task Force (IETF) K. Moriarty
Request for Comments: 8996 CIS
BCP: 195 S. Farrell
Obsoletes: 5469, 7507 Trinity College Dublin
Updates: 3261, 3329, 3436, 3470, 3501, 3552, March 2021
3568, 3656, 3749, 3767, 3856, 3871,
3887, 3903, 3943, 3983, 4097, 4111,
4162, 4168, 4217, 4235, 4261, 4279,
4497, 4513, 4531, 4540, 4582, 4616,
4642, 4680, 4681, 4712, 4732, 4743,
4744, 4785, 4791, 4823, 4851, 4964,
4975, 4976, 4992, 5018, 5019, 5023,
5024, 5049, 5054, 5091, 5158, 5216,
5238, 5263, 5281, 5364, 5415, 5422,
5456, 5734, 5878, 5953, 6012, 6042,
6083, 6084, 6176, 6347, 6353, 6367,
6460, 6614, 6739, 6749, 6750, 7030,
7465, 7525, 7562, 7568, 8261, 8422
Category: Best Current Practice
ISSN: 2070-1721
Deprecating TLS 1.0 and TLS 1.1
Abstract
This document formally deprecates Transport Layer Security (TLS)
versions 1.0 (RFC 2246) and 1.1 (RFC 4346). Accordingly, those
documents have been moved to Historic status. These versions lack
support for current and recommended cryptographic algorithms and
mechanisms, and various government and industry profiles of
applications using TLS now mandate avoiding these old TLS versions.
TLS version 1.2 became the recommended version for IETF protocols in
2008 (subsequently being obsoleted by TLS version 1.3 in 2018),
providing sufficient time to transition away from older versions.
Removing support for older versions from implementations reduces the
attack surface, reduces opportunity for misconfiguration, and
streamlines library and product maintenance.
This document also deprecates Datagram TLS (DTLS) version 1.0 (RFC
4347) but not DTLS version 1.2, and there is no DTLS version 1.1.
This document updates many RFCs that normatively refer to TLS version
1.0 or TLS version 1.1, as described herein. This document also
updates the best practices for TLS usage in RFC 7525; hence, it is
part of BCP 195.
Status of This Memo
This memo documents an Internet Best Current Practice.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
BCPs is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc8996.
Copyright Notice
Copyright (c) 2021 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction
1.1. RFCs Updated
1.2. Terminology
2. Support for Deprecation
3. SHA-1 Usage Problematic in TLS 1.0 and TLS 1.1
4. Do Not Use TLS 1.0
5. Do Not Use TLS 1.1
6. Updates to RFC 7525
7. Operational Considerations
8. Security Considerations
9. IANA Considerations
10. References
10.1. Normative References
10.2. Informative References
Acknowledgements
Authors' Addresses
1. Introduction
Transport Layer Security (TLS) versions 1.0 [RFC2246] and 1.1
[RFC4346] were superseded by TLS 1.2 [RFC5246] in 2008, which has now
itself been superseded by TLS 1.3 [RFC8446]. Datagram Transport
Layer Security (DTLS) version 1.0 [RFC4347] was superseded by DTLS
1.2 [RFC6347] in 2012. Therefore, it is timely to further deprecate
TLS 1.0, TLS 1.1, and DTLS 1.0. Accordingly, the aforementioned
documents have been moved to Historic status.
Technical reasons for deprecating these versions include:
* They require the implementation of older cipher suites that are no
longer desirable for cryptographic reasons, e.g., TLS 1.0 makes
TLS_DHE_DSS_WITH_3DES_EDE_CBC_SHA mandatory to implement.
* There is a lack of support for current recommended cipher suites,
especially authenticated encryption with associated data (AEAD)
ciphers, which were not supported prior to TLS 1.2. Note that
registry entries for no-longer-desirable ciphersuites remain in
the registries, but many TLS registries were updated by [RFC8447],
which indicates that such entries are not recommended by the IETF.
* The integrity of the handshake depends on SHA-1 hash.
* The authentication of the peers depends on SHA-1 signatures.
* Support for four TLS protocol versions increases the likelihood of
misconfiguration.
* At least one widely used library has plans to drop TLS 1.1 and TLS
1.0 support in upcoming releases; products using such libraries
would need to use older versions of the libraries to support TLS
1.0 and TLS 1.1, which is clearly undesirable.
Deprecation of these versions is intended to assist developers as
additional justification to no longer support older (D)TLS versions
and to migrate to a minimum of (D)TLS 1.2. Deprecation also assists
product teams with phasing out support for the older versions, to
reduce the attack surface and the scope of maintenance for protocols
in their offerings.
1.1. RFCs Updated
This document updates the following RFCs that normatively reference
TLS 1.0, TLS 1.1, or DTLS 1.0. The update is to obsolete usage of
these older versions. Fallback to these versions is prohibited
through this update. Specific references to mandatory minimum
protocol versions of TLS 1.0 or TLS 1.1 are replaced by TLS 1.2, and
references to minimum protocol version DTLS 1.0 are replaced by DTLS
1.2. Statements that "TLS 1.0 is the most widely deployed version
and will provide the broadest interoperability" are removed without
replacement.
[RFC3261] [RFC3329] [RFC3436] [RFC3470] [RFC3501] [RFC3552] [RFC3568]
[RFC3656] [RFC3749] [RFC3767] [RFC3856] [RFC3871] [RFC3887] [RFC3903]
[RFC3943] [RFC3983] [RFC4097] [RFC4111] [RFC4162] [RFC4168] [RFC4217]
[RFC4235] [RFC4261] [RFC4279] [RFC4497] [RFC4513] [RFC4531] [RFC4540]
[RFC4582] [RFC4616] [RFC4642] [RFC4680] [RFC4681] [RFC4712] [RFC4732]
[RFC4785] [RFC4791] [RFC4823] [RFC4851] [RFC4964] [RFC4975] [RFC4976]
[RFC4992] [RFC5018] [RFC5019] [RFC5023] [RFC5024] [RFC5049] [RFC5054]
[RFC5091] [RFC5158] [RFC5216] [RFC5238] [RFC5263] [RFC5281] [RFC5364]
[RFC5415] [RFC5422] [RFC5456] [RFC5734] [RFC5878] [RFC6012] [RFC6042]
[RFC6083] [RFC6084] [RFC6176] [RFC6353] [RFC6367] [RFC6739] [RFC6749]
[RFC6750] [RFC7030] [RFC7465] [RFC7525] [RFC7562] [RFC7568] [RFC8261]
[RFC8422]
The status of [RFC7562], [RFC6042], [RFC5456], [RFC5024], [RFC4540],
and [RFC3656] will be updated with permission of the Independent
Submissions Editor.
In addition, these RFCs normatively refer to TLS 1.0 or TLS 1.1 and
have already been obsoleted; they are still listed here and marked as
updated by this document in order to reiterate that any usage of the
obsolete protocol should use modern TLS: [RFC3316], [RFC3489],
[RFC3546], [RFC3588], [RFC3734], [RFC3920], [RFC4132], [RFC4244],
[RFC4347], [RFC4366], [RFC4492], [RFC4507], [RFC4572], [RFC4582],
[RFC4934], [RFC5077], [RFC5081], [RFC5101], and [RFC5953].
Note that [RFC4642] has already been updated by [RFC8143], which
makes an overlapping, but not quite identical, update as this
document.
[RFC6614] has a requirement for TLS 1.1 or later, although it only
makes an informative reference to [RFC4346]. This requirement is
updated to be for TLS 1.2 or later.
[RFC6460], [RFC4744], and [RFC4743] are already Historic; they are
still listed here and marked as updated by this document in order to
reiterate that any usage of the obsolete protocol should use modern
TLS.
This document updates DTLS [RFC6347]. [RFC6347] had allowed for
negotiating the use of DTLS 1.0, which is now forbidden.
The DES and International Data Encryption Algorithm (IDEA) cipher
suites specified in [RFC5469] were specifically removed from TLS 1.2
by [RFC5246]; since the only versions of TLS for which their usage is
defined are now Historic, [RFC5469] has been moved to Historic as
well.
The version-fallback Signaling Cipher Suite Value specified in
[RFC7507] was defined to detect when a given client and server
negotiate a lower version of (D)TLS than their highest shared
version. TLS 1.3 ([RFC8446]) incorporates a different mechanism that
achieves this purpose, via sentinel values in the ServerHello.Random
field. With (D)TLS versions prior to 1.2 fully deprecated, the only
way for (D)TLS implementations to negotiate a lower version than
their highest shared version would be to negotiate (D)TLS 1.2 while
supporting (D)TLS 1.3; supporting (D)TLS 1.3 implies support for the
ServerHello.Random mechanism. Accordingly, the functionality from
[RFC7507] has been superseded, and this document marks it as
Obsolete.
1.2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
2. Support for Deprecation
Specific details on attacks against TLS 1.0 and TLS 1.1, as well as
their mitigations, are provided in [NIST800-52r2], [RFC7457], and
other RFCs referenced therein. Although mitigations for the current
known vulnerabilities have been developed, any future issues
discovered in old protocol versions might not be mitigated in older
library versions when newer library versions do not support those old
protocols.
For example, NIST has provided the following rationale, copied with
permission from Section 1.1, "History of TLS", of [NIST800-52r2]:
| TLS 1.1, specified in RFC 4346 [24], was developed to address
| weaknesses discovered in TLS 1.0, primarily in the areas of
| initialization vector selection and padding error processing.
| Initialization vectors were made explicit to prevent a certain
| class of attacks on the Cipher Block Chaining (CBC) mode of
| operation used by TLS. The handling of padding errors was altered
| to treat a padding error as a bad message authentication code
| rather than a decryption failure. In addition, the TLS 1.1 RFC
| acknowledges attacks on CBC mode that rely on the time to compute
| the message authentication code (MAC). The TLS 1.1 specification
| states that to defend against such attacks, an implementation must
| process records in the same manner regardless of whether padding
| errors exist. Further implementation considerations for CBC modes
| (which were not included in RFC 4346 [24]) are discussed in
| Section 3.3.2.
|
| TLS 1.2, specified in RFC 5246 [25], made several cryptographic
| enhancements, particularly in the area of hash functions, with the
| ability to use or specify the SHA-2 family of algorithms for hash,
| MAC, and Pseudorandom Function (PRF) computations. TLS 1.2 also
| adds authenticated encryption with associated data (AEAD) cipher
| suites.
|
| TLS 1.3, specified in RFC 8446 [57], represents a significant
| change to TLS that aims to address threats that have arisen over
| the years. Among the changes are a new handshake protocol, a new
| key derivation process that uses the HMAC-based Extract-and-Expand
| Key Derivation Function (HKDF) [37], and the removal of cipher
| suites that use RSA key transport or static Diffie-Hellman ( DH)
| [sic] key exchanges, the CBC mode of operation, or SHA-1. Many
| extensions defined for use with TLS 1.2 and previous versions
| cannot be used with TLS 1.3.
3. SHA-1 Usage Problematic in TLS 1.0 and TLS 1.1
The integrity of both TLS 1.0 and TLS 1.1 depends on a running SHA-1
hash of the exchanged messages. This makes it possible to perform a
downgrade attack on the handshake by an attacker able to perform 2^77
operations, well below the acceptable modern security margin.
Similarly, the authentication of the handshake depends on signatures
made using a SHA-1 hash or a concatenation of MD5 and SHA-1 hashes
that is not appreciably stronger than a SHA-1 hash, allowing the
attacker to impersonate a server when it is able to break the
severely weakened SHA-1 hash.
Neither TLS 1.0 nor TLS 1.1 allows the peers to select a stronger
hash for signatures in the ServerKeyExchange or CertificateVerify
messages, making the only upgrade path the use of a newer protocol
version.
See [Bhargavan2016] for additional details.
4. Do Not Use TLS 1.0
TLS 1.0 MUST NOT be used. Negotiation of TLS 1.0 from any version of
TLS MUST NOT be permitted.
Any other version of TLS is more secure than TLS 1.0. While TLS 1.0
can be configured to prevent some types of interception, using the
highest version available is preferred.
Pragmatically, clients MUST NOT send a ClientHello with
ClientHello.client_version set to {03,01}. Similarly, servers MUST
NOT send a ServerHello with ServerHello.server_version set to
{03,01}. Any party receiving a Hello message with the protocol
version set to {03,01} MUST respond with a "protocol_version" alert
message and close the connection.
Historically, TLS specifications were not clear on what the record
layer version number (TLSPlaintext.version) could contain when
sending a ClientHello message. Appendix E of [RFC5246] notes that
TLSPlaintext.version could be selected to maximize interoperability,
though no definitive value is identified as ideal. That guidance is
still applicable; therefore, TLS servers MUST accept any value
{03,XX} (including {03,00}) as the record layer version number for
ClientHello, but they MUST NOT negotiate TLS 1.0.
5. Do Not Use TLS 1.1
TLS 1.1 MUST NOT be used. Negotiation of TLS 1.1 from any version of
TLS MUST NOT be permitted.
Pragmatically, clients MUST NOT send a ClientHello with
ClientHello.client_version set to {03,02}. Similarly, servers MUST
NOT send a ServerHello with ServerHello.server_version set to
{03,02}. Any party receiving a Hello message with the protocol
version set to {03,02} MUST respond with a "protocol_version" alert
message and close the connection.
Any newer version of TLS is more secure than TLS 1.1. While TLS 1.1
can be configured to prevent some types of interception, using the
highest version available is preferred. Support for TLS 1.1 is
dwindling in libraries and will impact security going forward if
mitigations for attacks cannot be easily addressed and supported in
older libraries.
Historically, TLS specifications were not clear on what the record
layer version number (TLSPlaintext.version) could contain when
sending a ClientHello message. Appendix E of [RFC5246] notes that
TLSPlaintext.version could be selected to maximize interoperability,
though no definitive value is identified as ideal. That guidance is
still applicable; therefore, TLS servers MUST accept any value
{03,XX} (including {03,00}) as the record layer version number for
ClientHello, but they MUST NOT negotiate TLS 1.1.
6. Updates to RFC 7525
"Recommendations for Secure Use of Transport Layer Security (TLS) and
Datagram Transport Layer Security (DTLS)" [RFC7525] is BCP 195, which
is the most recent Best Current Practice for implementing TLS and was
based on TLS 1.2. At the time of publication, TLS 1.0 and TLS 1.1
had not yet been deprecated. As such, BCP 195 is called out
specifically to update text implementing the deprecation
recommendations of this document.
This document updates Section 3.1.1 of [RFC7525] by changing SHOULD
NOT to MUST NOT as follows:
* Implementations MUST NOT negotiate TLS version 1.0 [RFC2246].
Rationale: TLS 1.0 (published in 1999) does not support many
modern, strong cipher suites. In addition, TLS 1.0 lacks a per-
record Initialization Vector (IV) for CBC-based cipher suites and
does not warn against common padding errors.
* Implementations MUST NOT negotiate TLS version 1.1 [RFC4346].
Rationale: TLS 1.1 (published in 2006) is a security improvement
over TLS 1.0 but still does not support certain stronger cipher
suites.
This document updates Section 3.1.2 of [RFC7525] by changing SHOULD
NOT to MUST NOT and adding a reference to RFC 6347 as follows:
* Implementations MUST NOT negotiate DTLS version 1.0 [RFC4347]
[RFC6347].
Version 1.0 of DTLS correlates to version 1.1 of TLS (see above).
7. Operational Considerations
This document is part of BCP 195 and, as such, reflects the
understanding of the IETF (at the time of this document's
publication) as to the best practices for TLS and DTLS usage.
Though TLS 1.1 has been obsolete since the publication of [RFC5246]
in 2008, and DTLS 1.0 has been obsolete since the publication of
[RFC6347] in 2012, there may remain some systems in operation that do
not support (D)TLS 1.2 or higher. Adopting the practices recommended
by this document for any systems that need to communicate with the
aforementioned class of systems will cause failure to interoperate.
However, disregarding the recommendations of this document in order
to continue to interoperate with the aforementioned class of systems
incurs some amount of risk. The nature of the risks incurred by
operating in contravention to the recommendations of this document
are discussed in Sections 2 and 3, and knowledge of those risks
should be used along with any potential mitigating factors and the
risks inherent to updating the systems in question when deciding how
quickly to adopt the recommendations specified in this document.
8. Security Considerations
This document deprecates two older TLS protocol versions and one
older DTLS protocol version for security reasons already described.
The attack surface is reduced when there are a smaller number of
supported protocols and fallback options are removed.
9. IANA Considerations
This document has no IANA actions.
10. References
10.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC2246] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
RFC 2246, DOI 10.17487/RFC2246, January 1999,
<https://www.rfc-editor.org/info/rfc2246>.
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
DOI 10.17487/RFC3261, June 2002,
<https://www.rfc-editor.org/info/rfc3261>.
[RFC3329] Arkko, J., Torvinen, V., Camarillo, G., Niemi, A., and T.
Haukka, "Security Mechanism Agreement for the Session
Initiation Protocol (SIP)", RFC 3329,
DOI 10.17487/RFC3329, January 2003,
<https://www.rfc-editor.org/info/rfc3329>.
[RFC3436] Jungmaier, A., Rescorla, E., and M. Tuexen, "Transport
Layer Security over Stream Control Transmission Protocol",
RFC 3436, DOI 10.17487/RFC3436, December 2002,
<https://www.rfc-editor.org/info/rfc3436>.
[RFC3470] Hollenbeck, S., Rose, M., and L. Masinter, "Guidelines for
the Use of Extensible Markup Language (XML) within IETF
Protocols", BCP 70, RFC 3470, DOI 10.17487/RFC3470,
January 2003, <https://www.rfc-editor.org/info/rfc3470>.
[RFC3501] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION
4rev1", RFC 3501, DOI 10.17487/RFC3501, March 2003,
<https://www.rfc-editor.org/info/rfc3501>.
[RFC3552] Rescorla, E. and B. Korver, "Guidelines for Writing RFC
Text on Security Considerations", BCP 72, RFC 3552,
DOI 10.17487/RFC3552, July 2003,
<https://www.rfc-editor.org/info/rfc3552>.
[RFC3568] Barbir, A., Cain, B., Nair, R., and O. Spatscheck, "Known
Content Network (CN) Request-Routing Mechanisms",
RFC 3568, DOI 10.17487/RFC3568, July 2003,
<https://www.rfc-editor.org/info/rfc3568>.
[RFC3656] Siemborski, R., "The Mailbox Update (MUPDATE) Distributed
Mailbox Database Protocol", RFC 3656,
DOI 10.17487/RFC3656, December 2003,
<https://www.rfc-editor.org/info/rfc3656>.
[RFC3749] Hollenbeck, S., "Transport Layer Security Protocol
Compression Methods", RFC 3749, DOI 10.17487/RFC3749, May
2004, <https://www.rfc-editor.org/info/rfc3749>.
[RFC3767] Farrell, S., Ed., "Securely Available Credentials
Protocol", RFC 3767, DOI 10.17487/RFC3767, June 2004,
<https://www.rfc-editor.org/info/rfc3767>.
[RFC3856] Rosenberg, J., "A Presence Event Package for the Session
Initiation Protocol (SIP)", RFC 3856,
DOI 10.17487/RFC3856, August 2004,
<https://www.rfc-editor.org/info/rfc3856>.
[RFC3871] Jones, G., Ed., "Operational Security Requirements for
Large Internet Service Provider (ISP) IP Network
Infrastructure", RFC 3871, DOI 10.17487/RFC3871, September
2004, <https://www.rfc-editor.org/info/rfc3871>.
[RFC3887] Hansen, T., "Message Tracking Query Protocol", RFC 3887,
DOI 10.17487/RFC3887, September 2004,
<https://www.rfc-editor.org/info/rfc3887>.
[RFC3903] Niemi, A., Ed., "Session Initiation Protocol (SIP)
Extension for Event State Publication", RFC 3903,
DOI 10.17487/RFC3903, October 2004,
<https://www.rfc-editor.org/info/rfc3903>.
[RFC3943] Friend, R., "Transport Layer Security (TLS) Protocol
Compression Using Lempel-Ziv-Stac (LZS)", RFC 3943,
DOI 10.17487/RFC3943, November 2004,
<https://www.rfc-editor.org/info/rfc3943>.
[RFC3983] Newton, A. and M. Sanz, "Using the Internet Registry
Information Service (IRIS) over the Blocks Extensible
Exchange Protocol (BEEP)", RFC 3983, DOI 10.17487/RFC3983,
January 2005, <https://www.rfc-editor.org/info/rfc3983>.
[RFC4097] Barnes, M., Ed., "Middlebox Communications (MIDCOM)
Protocol Evaluation", RFC 4097, DOI 10.17487/RFC4097, June
2005, <https://www.rfc-editor.org/info/rfc4097>.
[RFC4111] Fang, L., Ed., "Security Framework for Provider-
Provisioned Virtual Private Networks (PPVPNs)", RFC 4111,
DOI 10.17487/RFC4111, July 2005,
<https://www.rfc-editor.org/info/rfc4111>.
[RFC4162] Lee, H.J., Yoon, J.H., and J.I. Lee, "Addition of SEED
Cipher Suites to Transport Layer Security (TLS)",
RFC 4162, DOI 10.17487/RFC4162, August 2005,
<https://www.rfc-editor.org/info/rfc4162>.
[RFC4168] Rosenberg, J., Schulzrinne, H., and G. Camarillo, "The
Stream Control Transmission Protocol (SCTP) as a Transport
for the Session Initiation Protocol (SIP)", RFC 4168,
DOI 10.17487/RFC4168, October 2005,
<https://www.rfc-editor.org/info/rfc4168>.
[RFC4217] Ford-Hutchinson, P., "Securing FTP with TLS", RFC 4217,
DOI 10.17487/RFC4217, October 2005,
<https://www.rfc-editor.org/info/rfc4217>.
[RFC4235] Rosenberg, J., Schulzrinne, H., and R. Mahy, Ed., "An
INVITE-Initiated Dialog Event Package for the Session
Initiation Protocol (SIP)", RFC 4235,
DOI 10.17487/RFC4235, November 2005,
<https://www.rfc-editor.org/info/rfc4235>.
[RFC4261] Walker, J. and A. Kulkarni, Ed., "Common Open Policy
Service (COPS) Over Transport Layer Security (TLS)",
RFC 4261, DOI 10.17487/RFC4261, December 2005,
<https://www.rfc-editor.org/info/rfc4261>.
[RFC4279] Eronen, P., Ed. and H. Tschofenig, Ed., "Pre-Shared Key
Ciphersuites for Transport Layer Security (TLS)",
RFC 4279, DOI 10.17487/RFC4279, December 2005,
<https://www.rfc-editor.org/info/rfc4279>.
[RFC4346] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.1", RFC 4346,
DOI 10.17487/RFC4346, April 2006,
<https://www.rfc-editor.org/info/rfc4346>.
[RFC4497] Elwell, J., Derks, F., Mourot, P., and O. Rousseau,
"Interworking between the Session Initiation Protocol
(SIP) and QSIG", BCP 117, RFC 4497, DOI 10.17487/RFC4497,
May 2006, <https://www.rfc-editor.org/info/rfc4497>.
[RFC4513] Harrison, R., Ed., "Lightweight Directory Access Protocol
(LDAP): Authentication Methods and Security Mechanisms",
RFC 4513, DOI 10.17487/RFC4513, June 2006,
<https://www.rfc-editor.org/info/rfc4513>.
[RFC4531] Zeilenga, K., "Lightweight Directory Access Protocol
(LDAP) Turn Operation", RFC 4531, DOI 10.17487/RFC4531,
June 2006, <https://www.rfc-editor.org/info/rfc4531>.
[RFC4540] Stiemerling, M., Quittek, J., and C. Cadar, "NEC's Simple
Middlebox Configuration (SIMCO) Protocol Version 3.0",
RFC 4540, DOI 10.17487/RFC4540, May 2006,
<https://www.rfc-editor.org/info/rfc4540>.
[RFC4582] Camarillo, G., Ott, J., and K. Drage, "The Binary Floor
Control Protocol (BFCP)", RFC 4582, DOI 10.17487/RFC4582,
November 2006, <https://www.rfc-editor.org/info/rfc4582>.
[RFC4616] Zeilenga, K., Ed., "The PLAIN Simple Authentication and
Security Layer (SASL) Mechanism", RFC 4616,
DOI 10.17487/RFC4616, August 2006,
<https://www.rfc-editor.org/info/rfc4616>.
[RFC4642] Murchison, K., Vinocur, J., and C. Newman, "Using
Transport Layer Security (TLS) with Network News Transfer
Protocol (NNTP)", RFC 4642, DOI 10.17487/RFC4642, October
2006, <https://www.rfc-editor.org/info/rfc4642>.
[RFC4680] Santesson, S., "TLS Handshake Message for Supplemental
Data", RFC 4680, DOI 10.17487/RFC4680, October 2006,
<https://www.rfc-editor.org/info/rfc4680>.
[RFC4681] Santesson, S., Medvinsky, A., and J. Ball, "TLS User
Mapping Extension", RFC 4681, DOI 10.17487/RFC4681,
October 2006, <https://www.rfc-editor.org/info/rfc4681>.
[RFC4712] Siddiqui, A., Romascanu, D., Golovinsky, E., Rahman, M.,
and Y. Kim, "Transport Mappings for Real-time Application
Quality-of-Service Monitoring (RAQMON) Protocol Data Unit
(PDU)", RFC 4712, DOI 10.17487/RFC4712, October 2006,
<https://www.rfc-editor.org/info/rfc4712>.
[RFC4732] Handley, M., Ed., Rescorla, E., Ed., and IAB, "Internet
Denial-of-Service Considerations", RFC 4732,
DOI 10.17487/RFC4732, December 2006,
<https://www.rfc-editor.org/info/rfc4732>.
[RFC4743] Goddard, T., "Using NETCONF over the Simple Object Access
Protocol (SOAP)", RFC 4743, DOI 10.17487/RFC4743, December
2006, <https://www.rfc-editor.org/info/rfc4743>.
[RFC4744] Lear, E. and K. Crozier, "Using the NETCONF Protocol over
the Blocks Extensible Exchange Protocol (BEEP)", RFC 4744,
DOI 10.17487/RFC4744, December 2006,
<https://www.rfc-editor.org/info/rfc4744>.
[RFC4785] Blumenthal, U. and P. Goel, "Pre-Shared Key (PSK)
Ciphersuites with NULL Encryption for Transport Layer
Security (TLS)", RFC 4785, DOI 10.17487/RFC4785, January
2007, <https://www.rfc-editor.org/info/rfc4785>.
[RFC4791] Daboo, C., Desruisseaux, B., and L. Dusseault,
"Calendaring Extensions to WebDAV (CalDAV)", RFC 4791,
DOI 10.17487/RFC4791, March 2007,
<https://www.rfc-editor.org/info/rfc4791>.
[RFC4823] Harding, T. and R. Scott, "FTP Transport for Secure Peer-
to-Peer Business Data Interchange over the Internet",
RFC 4823, DOI 10.17487/RFC4823, April 2007,
<https://www.rfc-editor.org/info/rfc4823>.
[RFC4851] Cam-Winget, N., McGrew, D., Salowey, J., and H. Zhou, "The
Flexible Authentication via Secure Tunneling Extensible
Authentication Protocol Method (EAP-FAST)", RFC 4851,
DOI 10.17487/RFC4851, May 2007,
<https://www.rfc-editor.org/info/rfc4851>.
[RFC4964] Allen, A., Ed., Holm, J., and T. Hallin, "The P-Answer-
State Header Extension to the Session Initiation Protocol
for the Open Mobile Alliance Push to Talk over Cellular",
RFC 4964, DOI 10.17487/RFC4964, September 2007,
<https://www.rfc-editor.org/info/rfc4964>.
[RFC4975] Campbell, B., Ed., Mahy, R., Ed., and C. Jennings, Ed.,
"The Message Session Relay Protocol (MSRP)", RFC 4975,
DOI 10.17487/RFC4975, September 2007,
<https://www.rfc-editor.org/info/rfc4975>.
[RFC4976] Jennings, C., Mahy, R., and A. B. Roach, "Relay Extensions
for the Message Sessions Relay Protocol (MSRP)", RFC 4976,
DOI 10.17487/RFC4976, September 2007,
<https://www.rfc-editor.org/info/rfc4976>.
[RFC4992] Newton, A., "XML Pipelining with Chunks for the Internet
Registry Information Service", RFC 4992,
DOI 10.17487/RFC4992, August 2007,
<https://www.rfc-editor.org/info/rfc4992>.
[RFC5018] Camarillo, G., "Connection Establishment in the Binary
Floor Control Protocol (BFCP)", RFC 5018,
DOI 10.17487/RFC5018, September 2007,
<https://www.rfc-editor.org/info/rfc5018>.
[RFC5019] Deacon, A. and R. Hurst, "The Lightweight Online
Certificate Status Protocol (OCSP) Profile for High-Volume
Environments", RFC 5019, DOI 10.17487/RFC5019, September
2007, <https://www.rfc-editor.org/info/rfc5019>.
[RFC5023] Gregorio, J., Ed. and B. de hOra, Ed., "The Atom
Publishing Protocol", RFC 5023, DOI 10.17487/RFC5023,
October 2007, <https://www.rfc-editor.org/info/rfc5023>.
[RFC5024] Friend, I., "ODETTE File Transfer Protocol 2.0", RFC 5024,
DOI 10.17487/RFC5024, November 2007,
<https://www.rfc-editor.org/info/rfc5024>.
[RFC5049] Bormann, C., Liu, Z., Price, R., and G. Camarillo, Ed.,
"Applying Signaling Compression (SigComp) to the Session
Initiation Protocol (SIP)", RFC 5049,
DOI 10.17487/RFC5049, December 2007,
<https://www.rfc-editor.org/info/rfc5049>.
[RFC5054] Taylor, D., Wu, T., Mavrogiannopoulos, N., and T. Perrin,
"Using the Secure Remote Password (SRP) Protocol for TLS
Authentication", RFC 5054, DOI 10.17487/RFC5054, November
2007, <https://www.rfc-editor.org/info/rfc5054>.
[RFC5091] Boyen, X. and L. Martin, "Identity-Based Cryptography
Standard (IBCS) #1: Supersingular Curve Implementations of
the BF and BB1 Cryptosystems", RFC 5091,
DOI 10.17487/RFC5091, December 2007,
<https://www.rfc-editor.org/info/rfc5091>.
[RFC5158] Huston, G., "6to4 Reverse DNS Delegation Specification",
RFC 5158, DOI 10.17487/RFC5158, March 2008,
<https://www.rfc-editor.org/info/rfc5158>.
[RFC5216] Simon, D., Aboba, B., and R. Hurst, "The EAP-TLS
Authentication Protocol", RFC 5216, DOI 10.17487/RFC5216,
March 2008, <https://www.rfc-editor.org/info/rfc5216>.
[RFC5238] Phelan, T., "Datagram Transport Layer Security (DTLS) over
the Datagram Congestion Control Protocol (DCCP)",
RFC 5238, DOI 10.17487/RFC5238, May 2008,
<https://www.rfc-editor.org/info/rfc5238>.
[RFC5263] Lonnfors, M., Costa-Requena, J., Leppanen, E., and H.
Khartabil, "Session Initiation Protocol (SIP) Extension
for Partial Notification of Presence Information",
RFC 5263, DOI 10.17487/RFC5263, September 2008,
<https://www.rfc-editor.org/info/rfc5263>.
[RFC5281] Funk, P. and S. Blake-Wilson, "Extensible Authentication
Protocol Tunneled Transport Layer Security Authenticated
Protocol Version 0 (EAP-TTLSv0)", RFC 5281,
DOI 10.17487/RFC5281, August 2008,
<https://www.rfc-editor.org/info/rfc5281>.
[RFC5364] Garcia-Martin, M. and G. Camarillo, "Extensible Markup
Language (XML) Format Extension for Representing Copy
Control Attributes in Resource Lists", RFC 5364,
DOI 10.17487/RFC5364, October 2008,
<https://www.rfc-editor.org/info/rfc5364>.
[RFC5422] Cam-Winget, N., McGrew, D., Salowey, J., and H. Zhou,
"Dynamic Provisioning Using Flexible Authentication via
Secure Tunneling Extensible Authentication Protocol (EAP-
FAST)", RFC 5422, DOI 10.17487/RFC5422, March 2009,
<https://www.rfc-editor.org/info/rfc5422>.
[RFC5469] Eronen, P., Ed., "DES and IDEA Cipher Suites for Transport
Layer Security (TLS)", RFC 5469, DOI 10.17487/RFC5469,
February 2009, <https://www.rfc-editor.org/info/rfc5469>.
[RFC5734] Hollenbeck, S., "Extensible Provisioning Protocol (EPP)
Transport over TCP", STD 69, RFC 5734,
DOI 10.17487/RFC5734, August 2009,
<https://www.rfc-editor.org/info/rfc5734>.
[RFC5878] Brown, M. and R. Housley, "Transport Layer Security (TLS)
Authorization Extensions", RFC 5878, DOI 10.17487/RFC5878,
May 2010, <https://www.rfc-editor.org/info/rfc5878>.
[RFC5953] Hardaker, W., "Transport Layer Security (TLS) Transport
Model for the Simple Network Management Protocol (SNMP)",
RFC 5953, DOI 10.17487/RFC5953, August 2010,
<https://www.rfc-editor.org/info/rfc5953>.
[RFC6042] Keromytis, A., "Transport Layer Security (TLS)
Authorization Using KeyNote", RFC 6042,
DOI 10.17487/RFC6042, October 2010,
<https://www.rfc-editor.org/info/rfc6042>.
[RFC6176] Turner, S. and T. Polk, "Prohibiting Secure Sockets Layer
(SSL) Version 2.0", RFC 6176, DOI 10.17487/RFC6176, March
2011, <https://www.rfc-editor.org/info/rfc6176>.
[RFC6353] Hardaker, W., "Transport Layer Security (TLS) Transport
Model for the Simple Network Management Protocol (SNMP)",
STD 78, RFC 6353, DOI 10.17487/RFC6353, July 2011,
<https://www.rfc-editor.org/info/rfc6353>.
[RFC6367] Kanno, S. and M. Kanda, "Addition of the Camellia Cipher
Suites to Transport Layer Security (TLS)", RFC 6367,
DOI 10.17487/RFC6367, September 2011,
<https://www.rfc-editor.org/info/rfc6367>.
[RFC6739] Schulzrinne, H. and H. Tschofenig, "Synchronizing Service
Boundaries and <mapping> Elements Based on the Location-
to-Service Translation (LoST) Protocol", RFC 6739,
DOI 10.17487/RFC6739, October 2012,
<https://www.rfc-editor.org/info/rfc6739>.
[RFC6749] Hardt, D., Ed., "The OAuth 2.0 Authorization Framework",
RFC 6749, DOI 10.17487/RFC6749, October 2012,
<https://www.rfc-editor.org/info/rfc6749>.
[RFC6750] Jones, M. and D. Hardt, "The OAuth 2.0 Authorization
Framework: Bearer Token Usage", RFC 6750,
DOI 10.17487/RFC6750, October 2012,
<https://www.rfc-editor.org/info/rfc6750>.
[RFC7030] Pritikin, M., Ed., Yee, P., Ed., and D. Harkins, Ed.,
"Enrollment over Secure Transport", RFC 7030,
DOI 10.17487/RFC7030, October 2013,
<https://www.rfc-editor.org/info/rfc7030>.
[RFC7465] Popov, A., "Prohibiting RC4 Cipher Suites", RFC 7465,
DOI 10.17487/RFC7465, February 2015,
<https://www.rfc-editor.org/info/rfc7465>.
[RFC7507] Moeller, B. and A. Langley, "TLS Fallback Signaling Cipher
Suite Value (SCSV) for Preventing Protocol Downgrade
Attacks", RFC 7507, DOI 10.17487/RFC7507, April 2015,
<https://www.rfc-editor.org/info/rfc7507>.
[RFC7525] Sheffer, Y., Holz, R., and P. Saint-Andre,
"Recommendations for Secure Use of Transport Layer
Security (TLS) and Datagram Transport Layer Security
(DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC7525, May
2015, <https://www.rfc-editor.org/info/rfc7525>.
[RFC7562] Thakore, D., "Transport Layer Security (TLS) Authorization
Using Digital Transmission Content Protection (DTCP)
Certificates", RFC 7562, DOI 10.17487/RFC7562, July 2015,
<https://www.rfc-editor.org/info/rfc7562>.
[RFC7568] Barnes, R., Thomson, M., Pironti, A., and A. Langley,
"Deprecating Secure Sockets Layer Version 3.0", RFC 7568,
DOI 10.17487/RFC7568, June 2015,
<https://www.rfc-editor.org/info/rfc7568>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8422] Nir, Y., Josefsson, S., and M. Pegourie-Gonnard, "Elliptic
Curve Cryptography (ECC) Cipher Suites for Transport Layer
Security (TLS) Versions 1.2 and Earlier", RFC 8422,
DOI 10.17487/RFC8422, August 2018,
<https://www.rfc-editor.org/info/rfc8422>.
10.2. Informative References
[Bhargavan2016]
Bhargavan, K. and G. Leuren, "Transcript Collision
Attacks: Breaking Authentication in TLS, IKE, and SSH",
DOI 10.14722/ndss.2016.23418, February 2016,
<https://www.mitls.org/downloads/transcript-
collisions.pdf>.
[NIST800-52r2]
National Institute of Standards and Technology,
"Guidelines for the Selection, Configuration, and Use of
Transport Layer Security (TLS) Implementations NIST
SP800-52r2", DOI 10.6028/NIST.SP.800-52r2, August 2019,
<https://nvlpubs.nist.gov/nistpubs/SpecialPublications/
NIST.SP.800-52r2.pdf>.
[RFC3316] Arkko, J., Kuijpers, G., Soliman, H., Loughney, J., and J.
Wiljakka, "Internet Protocol Version 6 (IPv6) for Some
Second and Third Generation Cellular Hosts", RFC 3316,
DOI 10.17487/RFC3316, April 2003,
<https://www.rfc-editor.org/info/rfc3316>.
[RFC3489] Rosenberg, J., Weinberger, J., Huitema, C., and R. Mahy,
"STUN - Simple Traversal of User Datagram Protocol (UDP)
Through Network Address Translators (NATs)", RFC 3489,
DOI 10.17487/RFC3489, March 2003,
<https://www.rfc-editor.org/info/rfc3489>.
[RFC3546] Blake-Wilson, S., Nystrom, M., Hopwood, D., Mikkelsen, J.,
and T. Wright, "Transport Layer Security (TLS)
Extensions", RFC 3546, DOI 10.17487/RFC3546, June 2003,
<https://www.rfc-editor.org/info/rfc3546>.
[RFC3588] Calhoun, P., Loughney, J., Guttman, E., Zorn, G., and J.
Arkko, "Diameter Base Protocol", RFC 3588,
DOI 10.17487/RFC3588, September 2003,
<https://www.rfc-editor.org/info/rfc3588>.
[RFC3734] Hollenbeck, S., "Extensible Provisioning Protocol (EPP)
Transport Over TCP", RFC 3734, DOI 10.17487/RFC3734, March
2004, <https://www.rfc-editor.org/info/rfc3734>.
[RFC3920] Saint-Andre, P., Ed., "Extensible Messaging and Presence
Protocol (XMPP): Core", RFC 3920, DOI 10.17487/RFC3920,
October 2004, <https://www.rfc-editor.org/info/rfc3920>.
[RFC4132] Moriai, S., Kato, A., and M. Kanda, "Addition of Camellia
Cipher Suites to Transport Layer Security (TLS)",
RFC 4132, DOI 10.17487/RFC4132, July 2005,
<https://www.rfc-editor.org/info/rfc4132>.
[RFC4244] Barnes, M., Ed., "An Extension to the Session Initiation
Protocol (SIP) for Request History Information", RFC 4244,
DOI 10.17487/RFC4244, November 2005,
<https://www.rfc-editor.org/info/rfc4244>.
[RFC4347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security", RFC 4347, DOI 10.17487/RFC4347, April 2006,
<https://www.rfc-editor.org/info/rfc4347>.
[RFC4366] Blake-Wilson, S., Nystrom, M., Hopwood, D., Mikkelsen, J.,
and T. Wright, "Transport Layer Security (TLS)
Extensions", RFC 4366, DOI 10.17487/RFC4366, April 2006,
<https://www.rfc-editor.org/info/rfc4366>.
[RFC4492] Blake-Wilson, S., Bolyard, N., Gupta, V., Hawk, C., and B.
Moeller, "Elliptic Curve Cryptography (ECC) Cipher Suites
for Transport Layer Security (TLS)", RFC 4492,
DOI 10.17487/RFC4492, May 2006,
<https://www.rfc-editor.org/info/rfc4492>.
[RFC4507] Salowey, J., Zhou, H., Eronen, P., and H. Tschofenig,
"Transport Layer Security (TLS) Session Resumption without
Server-Side State", RFC 4507, DOI 10.17487/RFC4507, May
2006, <https://www.rfc-editor.org/info/rfc4507>.
[RFC4572] Lennox, J., "Connection-Oriented Media Transport over the
Transport Layer Security (TLS) Protocol in the Session
Description Protocol (SDP)", RFC 4572,
DOI 10.17487/RFC4572, July 2006,
<https://www.rfc-editor.org/info/rfc4572>.
[RFC4934] Hollenbeck, S., "Extensible Provisioning Protocol (EPP)
Transport Over TCP", RFC 4934, DOI 10.17487/RFC4934, May
2007, <https://www.rfc-editor.org/info/rfc4934>.
[RFC5077] Salowey, J., Zhou, H., Eronen, P., and H. Tschofenig,
"Transport Layer Security (TLS) Session Resumption without
Server-Side State", RFC 5077, DOI 10.17487/RFC5077,
January 2008, <https://www.rfc-editor.org/info/rfc5077>.
[RFC5081] Mavrogiannopoulos, N., "Using OpenPGP Keys for Transport
Layer Security (TLS) Authentication", RFC 5081,
DOI 10.17487/RFC5081, November 2007,
<https://www.rfc-editor.org/info/rfc5081>.
[RFC5101] Claise, B., Ed., "Specification of the IP Flow Information
Export (IPFIX) Protocol for the Exchange of IP Traffic
Flow Information", RFC 5101, DOI 10.17487/RFC5101, January
2008, <https://www.rfc-editor.org/info/rfc5101>.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246,
DOI 10.17487/RFC5246, August 2008,
<https://www.rfc-editor.org/info/rfc5246>.
[RFC5415] Calhoun, P., Ed., Montemurro, M., Ed., and D. Stanley,
Ed., "Control And Provisioning of Wireless Access Points
(CAPWAP) Protocol Specification", RFC 5415,
DOI 10.17487/RFC5415, March 2009,
<https://www.rfc-editor.org/info/rfc5415>.
[RFC5456] Spencer, M., Capouch, B., Guy, E., Ed., Miller, F., and K.
Shumard, "IAX: Inter-Asterisk eXchange Version 2",
RFC 5456, DOI 10.17487/RFC5456, February 2010,
<https://www.rfc-editor.org/info/rfc5456>.
[RFC6012] Salowey, J., Petch, T., Gerhards, R., and H. Feng,
"Datagram Transport Layer Security (DTLS) Transport
Mapping for Syslog", RFC 6012, DOI 10.17487/RFC6012,
October 2010, <https://www.rfc-editor.org/info/rfc6012>.
[RFC6083] Tuexen, M., Seggelmann, R., and E. Rescorla, "Datagram
Transport Layer Security (DTLS) for Stream Control
Transmission Protocol (SCTP)", RFC 6083,
DOI 10.17487/RFC6083, January 2011,
<https://www.rfc-editor.org/info/rfc6083>.
[RFC6084] Fu, X., Dickmann, C., and J. Crowcroft, "General Internet
Signaling Transport (GIST) over Stream Control
Transmission Protocol (SCTP) and Datagram Transport Layer
Security (DTLS)", RFC 6084, DOI 10.17487/RFC6084, January
2011, <https://www.rfc-editor.org/info/rfc6084>.
[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
January 2012, <https://www.rfc-editor.org/info/rfc6347>.
[RFC6460] Salter, M. and R. Housley, "Suite B Profile for Transport
Layer Security (TLS)", RFC 6460, DOI 10.17487/RFC6460,
January 2012, <https://www.rfc-editor.org/info/rfc6460>.
[RFC6614] Winter, S., McCauley, M., Venaas, S., and K. Wierenga,
"Transport Layer Security (TLS) Encryption for RADIUS",
RFC 6614, DOI 10.17487/RFC6614, May 2012,
<https://www.rfc-editor.org/info/rfc6614>.
[RFC7457] Sheffer, Y., Holz, R., and P. Saint-Andre, "Summarizing
Known Attacks on Transport Layer Security (TLS) and
Datagram TLS (DTLS)", RFC 7457, DOI 10.17487/RFC7457,
February 2015, <https://www.rfc-editor.org/info/rfc7457>.
[RFC8143] Elie, J., "Using Transport Layer Security (TLS) with
Network News Transfer Protocol (NNTP)", RFC 8143,
DOI 10.17487/RFC8143, April 2017,
<https://www.rfc-editor.org/info/rfc8143>.
[RFC8261] Tuexen, M., Stewart, R., Jesup, R., and S. Loreto,
"Datagram Transport Layer Security (DTLS) Encapsulation of
SCTP Packets", RFC 8261, DOI 10.17487/RFC8261, November
2017, <https://www.rfc-editor.org/info/rfc8261>.
[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/info/rfc8446>.
[RFC8447] Salowey, J. and S. Turner, "IANA Registry Updates for TLS
and DTLS", RFC 8447, DOI 10.17487/RFC8447, August 2018,
<https://www.rfc-editor.org/info/rfc8447>.
Acknowledgements
Thanks to those that provided usage data and reviewed and/or improved
this document, including: Michael Ackermann, David Benjamin, David
Black, Deborah Brungard, Alan DeKok, Viktor Dukhovni, Julien Élie,
Adrian Farrelll, Gary Gapinski, Alessandro Ghedini, Peter Gutmann,
Jeremy Harris, Nick Hilliard, James Hodgkinson, Russ Housley, Hubert
Kario, Benjamin Kaduk, John Klensin, Watson Ladd, Eliot Lear, Ted
Lemon, John Mattsson, Keith Moore, Tom Petch, Eric Mill, Yoav Nir,
Andrei Popov, Michael Richardson, Eric Rescorla, Rich Salz, Mohit
Sethi, Yaron Sheffer, Rob Sayre, Robert Sparks, Barbara Stark, Martin
Thomson, Sean Turner, Loganaden Velvindron, Jakub Wilk, and
Christopher Wood.
Authors' Addresses
Kathleen Moriarty
Center for Internet Security (CIS)
East Greenbush, NY
United States of America
Email: Kathleen.Moriarty.ietf@gmail.com
Stephen Farrell
Trinity College Dublin
Dublin
2
Ireland
Phone: +353-1-896-2354
Email: stephen.farrell@cs.tcd.ie
=========================================================================
Internet Engineering Task Force (IETF) Y. Sheffer
Request for Comments: 9325 Intuit
BCP: 195 P. Saint-Andre
Obsoletes: 7525 Independent
Updates: 5288, 6066 T. Fossati
Category: Best Current Practice ARM Limited
ISSN: 2070-1721 November 2022
Recommendations for Secure Use of Transport Layer Security (TLS) and
Datagram Transport Layer Security (DTLS)
Abstract
Transport Layer Security (TLS) and Datagram Transport Layer Security
(DTLS) are used to protect data exchanged over a wide range of
application protocols and can also form the basis for secure
transport protocols. Over the years, the industry has witnessed
several serious attacks on TLS and DTLS, including attacks on the
most commonly used cipher suites and their modes of operation. This
document provides the latest recommendations for ensuring the
security of deployed services that use TLS and DTLS. These
recommendations are applicable to the majority of use cases.
RFC 7525, an earlier version of the TLS recommendations, was
published when the industry was transitioning to TLS 1.2. Years
later, this transition is largely complete, and TLS 1.3 is widely
available. This document updates the guidance given the new
environment and obsoletes RFC 7525. In addition, this document
updates RFCs 5288 and 6066 in view of recent attacks.
Status of This Memo
This memo documents an Internet Best Current Practice.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
BCPs is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9325.
Copyright Notice
Copyright (c) 2022 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Revised BSD License text as described in Section 4.e of the
Trust Legal Provisions and are provided without warranty as described
in the Revised BSD License.
Table of Contents
1. Introduction
2. Terminology
3. General Recommendations
3.1. Protocol Versions
3.1.1. SSL/TLS Protocol Versions
3.1.2. DTLS Protocol Versions
3.1.3. Fallback to Lower Versions
3.2. Strict TLS
3.3. Compression
3.3.1. Certificate Compression
3.4. TLS Session Resumption
3.5. Renegotiation in TLS 1.2
3.6. Post-Handshake Authentication
3.7. Server Name Indication (SNI)
3.8. Application-Layer Protocol Negotiation (ALPN)
3.9. Multi-Server Deployment
3.10. Zero Round-Trip Time (0-RTT) Data in TLS 1.3
4. Recommendations: Cipher Suites
4.1. General Guidelines
4.2. Cipher Suites for TLS 1.2
4.2.1. Implementation Details
4.3. Cipher Suites for TLS 1.3
4.4. Limits on Key Usage
4.5. Public Key Length
4.6. Truncated HMAC
5. Applicability Statement
5.1. Security Services
5.2. Opportunistic Security
6. IANA Considerations
7. Security Considerations
7.1. Host Name Validation
7.2. AES-GCM
7.2.1. Nonce Reuse in TLS 1.2
7.3. Forward Secrecy
7.4. Diffie-Hellman Exponent Reuse
7.5. Certificate Revocation
8. References
8.1. Normative References
8.2. Informative References
Appendix A. Differences from RFC 7525
Acknowledgments
Authors' Addresses
1. Introduction
Transport Layer Security (TLS) and Datagram Transport Layer Security
(DTLS) are used to protect data exchanged over a wide variety of
application protocols, including HTTP [RFC9112] [RFC9113], IMAP
[RFC9051], Post Office Protocol (POP) [STD53], SIP [RFC3261], SMTP
[RFC5321], and the Extensible Messaging and Presence Protocol (XMPP)
[RFC6120]. Such protocols use both the TLS or DTLS handshake
protocol and the TLS or DTLS record layer. Although the TLS
handshake protocol can also be used with different record layers to
define secure transport protocols (the most prominent example is QUIC
[RFC9000]), such transport protocols are not directly in scope for
this document; nevertheless, many of the recommendations here might
apply insofar as such protocols use the TLS handshake protocol.
Over the years leading to 2015, the industry had witnessed serious
attacks on TLS and DTLS, including attacks on the most commonly used
cipher suites and their modes of operation. For instance, both the
AES-CBC [RFC3602] and RC4 [RFC7465] encryption algorithms, which
together were once the most widely deployed ciphers, were attacked in
the context of TLS. Detailed information about the attacks known
prior to 2015 is provided in a companion document [RFC7457] to the
previous version of the TLS recommendations [RFC7525], which will
help the reader understand the rationale behind the recommendations
provided here. That document has not been updated in concert with
this one; instead, newer attacks are described in this document, as
are mitigations for those attacks.
The TLS community reacted to the attacks described in [RFC7457] in
several ways:
* Detailed guidance was published on the use of TLS 1.2 [RFC5246]
and DTLS 1.2 [RFC6347] along with earlier protocol versions. This
guidance is included in the original [RFC7525] and mostly retained
in this revised version; note that this guidance was mostly
adopted by the industry since the publication of RFC 7525 in 2015.
* Versions of TLS earlier than 1.2 were deprecated [RFC8996].
* Version 1.3 of TLS [RFC8446] was released, followed by version 1.3
of DTLS [RFC9147]; these versions largely mitigate or resolve the
described attacks.
Those who implement and deploy TLS and TLS-based protocols need
guidance on how they can be used securely. This document provides
guidance for deployed services as well as for software
implementations, assuming the implementer expects their code to be
deployed in the environments defined in Section 5. Concerning
deployment, this document targets a wide audience, namely all
deployers who wish to add authentication (be it one-way only or
mutual), confidentiality, and data integrity protection to their
communications.
The recommendations herein take into consideration the security of
various mechanisms, their technical maturity and interoperability,
and their prevalence in implementations at the time of writing.
Unless it is explicitly called out that a recommendation applies to
TLS alone or to DTLS alone, each recommendation applies to both TLS
and DTLS.
This document attempts to minimize new guidance to TLS 1.2
implementations, and the overall approach is to encourage systems to
move to TLS 1.3. However, this is not always practical. Newly
discovered attacks, as well as ecosystem changes, necessitated some
new requirements that apply to TLS 1.2 environments. Those are
summarized in Appendix A.
Naturally, future attacks are likely, and this document cannot
address them. Those who implement and deploy TLS/DTLS and protocols
based on TLS/DTLS are strongly advised to pay attention to future
developments. In particular, although it is known that the creation
of quantum computers will have a significant impact on the security
of cryptographic primitives and the technologies that use them,
currently post-quantum cryptography is a work in progress and it is
too early to make recommendations; once the relevant specifications
are standardized in the IETF or elsewhere, this document should be
updated to reflect best practices at that time.
As noted, the TLS 1.3 specification resolves many of the
vulnerabilities listed in this document. A system that deploys TLS
1.3 should have fewer vulnerabilities than TLS 1.2 or below.
Therefore, this document replaces [RFC7525], with an explicit goal to
encourage migration of most uses of TLS 1.2 to TLS 1.3.
These are minimum recommendations for the use of TLS in the vast
majority of implementation and deployment scenarios, with the
exception of unauthenticated TLS (see Section 5). Other
specifications that reference this document can have stricter
requirements related to one or more aspects of the protocol, based on
their particular circumstances (e.g., for use with a specific
application protocol); when that is the case, implementers are
advised to adhere to those stricter requirements. Furthermore, this
document provides a floor, not a ceiling: where feasible,
administrators of services are encouraged to go beyond the minimum
support available in implementations to provide the strongest
security possible. For example, based on knowledge about the
deployed base for an existing application protocol and a cost-benefit
analysis regarding security strength vs. interoperability, a given
service provider might decide to disable TLS 1.2 entirely and offer
only TLS 1.3.
Community knowledge about the strength of various algorithms and
feasible attacks can change quickly, and experience shows that a Best
Current Practice (BCP) document about security is a point-in-time
statement. Readers are advised to seek out any errata or updates
that apply to this document.
This document updates [RFC5288] in view of the [Boeck2016] attack.
See Section 7.2.1 for the details.
This document updates [RFC6066] in view of the [ALPACA] attack. See
Section 3.7 for the details.
2. Terminology
A number of security-related terms in this document are used in the
sense defined in [RFC4949], including "attack", "authentication",
"certificate", "cipher", "compromise", "confidentiality",
"credential", "data integrity", "encryption", "forward secrecy",
"key", "key length", "self-signed certificate", "strength", and
"strong".
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
3. General Recommendations
This section provides general recommendations on the secure use of
TLS. Recommendations related to cipher suites are discussed in the
following section.
3.1. Protocol Versions
3.1.1. SSL/TLS Protocol Versions
It is important both to stop using old, less secure versions of SSL/
TLS and to start using modern, more secure versions; therefore, the
following are the recommendations concerning TLS/SSL protocol
versions:
* Implementations MUST NOT negotiate SSL version 2.
Rationale: Today, SSLv2 is considered insecure [RFC6176].
* Implementations MUST NOT negotiate SSL version 3.
Rationale: SSLv3 [RFC6101] was an improvement over SSLv2 and
plugged some significant security holes but did not support strong
cipher suites. SSLv3 does not support TLS extensions, some of
which (e.g., renegotiation_info [RFC5746]) are security critical.
In addition, with the emergence of the Padding Oracle On
Downgraded Legacy Encryption (POODLE) attack [POODLE], SSLv3 is
now widely recognized as fundamentally insecure. See [RFC7568]
for further details.
* Implementations MUST NOT negotiate TLS version 1.0 [RFC2246].
Rationale: TLS 1.0 (published in 1999) does not support many
modern, strong cipher suites. In addition, TLS 1.0 lacks a per-
record Initialization Vector (IV) for cipher suites based on
cipher block chaining (CBC) and does not warn against common
padding errors. This and other recommendations in this section
are in line with [RFC8996].
* Implementations MUST NOT negotiate TLS version 1.1 [RFC4346].
Rationale: TLS 1.1 (published in 2006) is a security improvement
over TLS 1.0 but still does not support certain stronger cipher
suites that were introduced with the standardization of TLS 1.2 in
2008, including the cipher suites recommended for TLS 1.2 by this
document (see Section 4.2 below).
* Implementations MUST support TLS 1.2 [RFC5246].
Rationale: TLS 1.2 is implemented and deployed more widely than
TLS 1.3 at this time, and when the recommendations in this
document are followed to mitigate known attacks, the use of TLS
1.2 is as safe as the use of TLS 1.3. In most application
protocols that reuse TLS and DTLS, there is no immediate need to
migrate solely to TLS 1.3. Indeed, because many application
clients are dependent on TLS libraries or operating systems that
do not yet support TLS 1.3, proactively deprecating TLS 1.2 would
introduce significant interoperability issues, thus harming
security more than helping it. Nevertheless, it is expected that
a future version of this BCP will deprecate the use of TLS 1.2
when it is appropriate to do so.
* Implementations SHOULD support TLS 1.3 [RFC8446] and, if
implemented, MUST prefer to negotiate TLS 1.3 over earlier
versions of TLS.
Rationale: TLS 1.3 is a major overhaul to the protocol and
resolves many of the security issues with TLS 1.2. To the extent
that an implementation supports TLS 1.2 (even if it defaults to
TLS 1.3), it MUST follow the recommendations regarding TLS 1.2
specified in this document.
* New transport protocols that integrate the TLS/DTLS handshake
protocol and/or record layer MUST use only TLS/DTLS 1.3 (for
instance, QUIC [RFC9001] took this approach). New application
protocols that employ TLS/DTLS for channel or session encryption
MUST integrate with both TLS/DTLS versions 1.2 and 1.3;
nevertheless, in rare cases where broad interoperability is not a
concern, application protocol designers MAY choose to forego TLS
1.2.
Rationale: Secure deployment of TLS 1.3 is significantly easier
and less error prone than secure deployment of TLS 1.2. When
designing a new secure transport protocol such as QUIC, there is
no reason to support TLS 1.2. By contrast, new application
protocols that reuse TLS need to support both TLS 1.3 and TLS 1.2
in order to take advantage of underlying library or operating
system support for both versions.
This BCP applies to TLS 1.3, TLS 1.2, and earlier versions. It is
not safe for readers to assume that the recommendations in this BCP
apply to any future version of TLS.
3.1.2. DTLS Protocol Versions
DTLS, an adaptation of TLS for UDP datagrams, was introduced when TLS
1.1 was published. The following are the recommendations with
respect to DTLS:
* Implementations MUST NOT negotiate DTLS version 1.0 [RFC4347].
Version 1.0 of DTLS correlates to version 1.1 of TLS (see above).
* Implementations MUST support DTLS 1.2 [RFC6347].
Version 1.2 of DTLS correlates to version 1.2 of TLS (see above).
(There is no version 1.1 of DTLS.)
* Implementations SHOULD support DTLS 1.3 [RFC9147] and, if
implemented, MUST prefer to negotiate DTLS version 1.3 over
earlier versions of DTLS.
Version 1.3 of DTLS correlates to version 1.3 of TLS (see above).
3.1.3. Fallback to Lower Versions
TLS/DTLS 1.2 clients MUST NOT fall back to earlier TLS versions,
since those versions have been deprecated [RFC8996]. As a result,
the downgrade-protection Signaling Cipher Suite Value (SCSV)
mechanism [RFC7507] is no longer needed for clients. In addition,
TLS 1.3 implements a new version-negotiation mechanism.
3.2. Strict TLS
The following recommendations are provided to help prevent "SSL
Stripping" and STARTTLS command injection (attacks that are
summarized in [RFC7457]):
* Many existing application protocols were designed before the use
of TLS became common. These protocols typically support TLS in
one of two ways: either via a separate port for TLS-only
communication (e.g., port 443 for HTTPS) or via a method for
dynamically upgrading a channel from unencrypted to TLS protected
(e.g., STARTTLS, which is used in protocols such as IMAP and
XMPP). Regardless of the mechanism for protecting the
communication channel (TLS-only port or dynamic upgrade), what
matters is the end state of the channel. When a protocol defines
both a dynamic upgrade method and a separate TLS-only method, then
the separate TLS-only method MUST be supported by implementations
and MUST be configured by administrators to be used in preference
to the dynamic upgrade method. When a protocol supports only a
dynamic upgrade method, implementations MUST provide a way for
administrators to set a strict local policy that forbids use of
plaintext in the absence of a negotiated TLS channel, and
administrators MUST use this policy.
* HTTP client and server implementations intended for use in the
World Wide Web (see Section 5) MUST support the HTTP Strict
Transport Security (HSTS) header field [RFC6797] so that web
servers can advertise that they are willing to accept TLS-only
clients. Web servers SHOULD use HSTS to indicate that they are
willing to accept TLS-only clients, unless they are deployed in
such a way that using HSTS would in fact weaken overall security
(e.g., it can be problematic to use HSTS with self-signed
certificates, as described in Section 11.3 of [RFC6797]). Similar
technologies exist for non-HTTP application protocols, such as
Mail Transfer Agent Strict Transport Security (MTA-STS) for mail
transfer agents [RFC8461] and methods based on DNS-Based
Authentication of Named Entities (DANE) [RFC6698] for SMTP
[RFC7672] and XMPP [RFC7712].
Rationale: Combining unprotected and TLS-protected communication
opens the way to SSL Stripping and similar attacks, since an initial
part of the communication is not integrity protected and therefore
can be manipulated by an attacker whose goal is to keep the
communication in the clear.
3.3. Compression
In order to help prevent compression-related attacks (summarized in
Section 2.6 of [RFC7457]) when using TLS 1.2, implementations and
deployments SHOULD NOT support TLS-level compression (Section 6.2.2
of [RFC5246]); the only exception is when the application protocol in
question has been proven not to be open to such attacks. However,
even in this case, extreme caution is warranted because of the
potential for future attacks related to TLS compression. More
specifically, the HTTP protocol is known to be vulnerable to
compression-related attacks. (This recommendation applies to TLS 1.2
only, because compression has been removed from TLS 1.3.)
Rationale: TLS compression has been subject to security attacks such
as the Compression Ratio Info-leak Made Easy (CRIME) attack.
Implementers should note that compression at higher protocol levels
can allow an active attacker to extract cleartext information from
the connection. The Browser Reconnaissance and Exfiltration via
Adaptive Compression of Hypertext (BREACH) attack is one such case.
These issues can only be mitigated outside of TLS and are thus
outside the scope of this document. See Section 2.6 of [RFC7457] for
further details.
3.3.1. Certificate Compression
Certificate chains often take up most of the bytes transmitted during
the handshake. In order to manage their size, some or all of the
following methods can be employed (see also Section 4 of [RFC9191]
for further suggestions):
* Limit the number of names or extensions.
* Use keys with small public key representations, like the Elliptic
Curve Digital Signature Algorithm (ECDSA).
* Use certificate compression.
To achieve the latter, TLS 1.3 defines the compress_certificate
extension in [RFC8879]. See also Section 5 of [RFC8879] for security
and privacy considerations associated with its use. For the
avoidance of doubt, CRIME-style attacks on TLS compression do not
apply to certificate compression.
Due to the strong likelihood of middlebox interference, compression
in the style of [RFC8879] has not been made available in TLS 1.2. In
theory, the cached_info extension defined in [RFC7924] could be used,
but it is not supported widely enough to be considered a practical
alternative.
3.4. TLS Session Resumption
Session resumption drastically reduces the number of full TLS
handshakes and thus is an essential performance feature for most
deployments.
Stateless session resumption with session tickets is a popular
strategy. For TLS 1.2, it is specified in [RFC5077]. For TLS 1.3, a
more secure mechanism based on the use of a pre-shared key (PSK) is
described in Section 4.6.1 of [RFC8446]. See [Springall16] for a
quantitative study of the risks induced by TLS cryptographic
"shortcuts", including session resumption.
When it is used, the resumption information MUST be authenticated and
encrypted to prevent modification or eavesdropping by an attacker.
Further recommendations apply to session tickets:
* A strong cipher MUST be used when encrypting the ticket (at least
as strong as the main TLS cipher suite).
* Ticket-encryption keys MUST be changed regularly, e.g., once every
week, so as not to negate the benefits of forward secrecy (see
Section 7.3 for details on forward secrecy). Old ticket-
encryption keys MUST be destroyed at the end of the validity
period.
* For similar reasons, session ticket validity MUST be limited to a
reasonable duration (e.g., half as long as ticket-encryption key
validity).
* TLS 1.2 does not roll the session key forward within a single
session. Thus, to prevent an attack where the server's ticket-
encryption key is stolen and used to decrypt the entire content of
a session (negating the concept of forward secrecy), a TLS 1.2
server SHOULD NOT resume sessions that are too old, e.g., sessions
that have been open longer than two ticket-encryption key rotation
periods.
Rationale: Session resumption is another kind of TLS handshake and
therefore must be as secure as the initial handshake. This document
(Section 4) recommends the use of cipher suites that provide forward
secrecy, i.e., that prevent an attacker who gains momentary access to
the TLS endpoint (either client or server) and its secrets from
reading either past or future communication. The tickets must be
managed so as not to negate this security property.
TLS 1.3 provides the powerful option of forward secrecy even within a
long-lived connection that is periodically resumed. Section 2.2 of
[RFC8446] recommends that clients SHOULD send a "key_share" when
initiating session resumption. In order to gain forward secrecy,
this document recommends that server implementations SHOULD select
the "psk_dhe_ke" PSK key exchange mode and respond with a "key_share"
to complete an Ephemeral Elliptic Curve Diffie-Hellman (ECDHE)
exchange on each session resumption. As a more performant
alternative, server implementations MAY refrain from responding with
a "key_share" until a certain amount of time (e.g., measured in
hours) has passed since the last ECDHE exchange; this implies that
the "key_share" operation would not occur for the presumed majority
of session resumption requests (which would occur within a few hours)
while still ensuring forward secrecy for longer-lived sessions.
TLS session resumption introduces potential privacy issues where the
server is able to track the client, in some cases indefinitely. See
[Sy2018] for more details.
3.5. Renegotiation in TLS 1.2
The recommendations in this section apply to TLS 1.2 only, because
renegotiation has been removed from TLS 1.3.
Renegotiation in TLS 1.2 is a handshake that establishes new
cryptographic parameters for an existing session. The mechanism
existed in TLS 1.2 and in earlier protocol versions and was improved
following several major attacks including a plaintext injection
attack, CVE-2009-3555 [CVE].
TLS 1.2 clients and servers MUST implement the renegotiation_info
extension, as defined in [RFC5746].
TLS 1.2 clients MUST send renegotiation_info in the Client Hello. If
the server does not acknowledge the extension, the client MUST
generate a fatal handshake_failure alert prior to terminating the
connection.
Rationale: It is not safe for a client to connect to a TLS 1.2 server
that does not support renegotiation_info regardless of whether either
endpoint actually implements renegotiation. See also Section 4.1 of
[RFC5746].
A related attack resulting from TLS session parameters not being
properly authenticated is a Triple Handshake [Triple-Handshake]. To
address this attack, TLS 1.2 implementations MUST support the
extended_master_secret extension defined in [RFC7627].
3.6. Post-Handshake Authentication
Renegotiation in TLS 1.2 was (partially) replaced in TLS 1.3 by
separate post-handshake authentication and key update mechanisms. In
the context of protocols that multiplex requests over a single
connection (such as HTTP/2 [RFC9113]), post-handshake authentication
has the same problems as TLS 1.2 renegotiation. Multiplexed
protocols SHOULD follow the advice provided for HTTP/2 in
Section 9.2.3 of [RFC9113].
3.7. Server Name Indication (SNI)
TLS implementations MUST support the Server Name Indication (SNI)
extension defined in Section 3 of [RFC6066] for those higher-level
protocols that would benefit from it, including HTTPS. However, the
actual use of SNI in particular circumstances is a matter of local
policy. At the time of writing, a technology for encrypting the SNI
(called Encrypted Client Hello) is being worked on in the TLS Working
Group [TLS-ECH]. Once that method has been standardized and widely
implemented, it will likely be appropriate to recommend its usage in
a future version of this BCP.
Rationale: SNI supports deployment of multiple TLS-protected virtual
servers on a single address, and therefore enables fine-grained
security for these virtual servers, by allowing each one to have its
own certificate. However, SNI also leaks the target domain for a
given connection; this information leak will be closed by use of TLS
Encrypted Client Hello once that method has been standardized.
In order to prevent the attacks described in [ALPACA], a server that
does not recognize the presented server name SHOULD NOT continue the
handshake and instead SHOULD fail with a fatal-level
unrecognized_name(112) alert. Note that this recommendation updates
Section 3 of [RFC6066], which stated:
| If the server understood the ClientHello extension but does not
| recognize the server name, the server SHOULD take one of two
| actions: either abort the handshake by sending a fatal-level
| unrecognized_name(112) alert or continue the handshake.
Clients SHOULD abort the handshake if the server acknowledges the SNI
extension but presents a certificate with a different hostname than
the one sent by the client.
3.8. Application-Layer Protocol Negotiation (ALPN)
TLS implementations (both client- and server-side) MUST support the
Application-Layer Protocol Negotiation (ALPN) extension [RFC7301].
In order to prevent "cross-protocol" attacks resulting from failure
to ensure that a message intended for use in one protocol cannot be
mistaken for a message for use in another protocol, servers are
advised to strictly enforce the behavior prescribed in Section 3.2 of
[RFC7301]:
| In the event that the server supports no protocols that the client
| advertises, then the server SHALL respond with a fatal
| 'no_application_protocol' alert.
Clients SHOULD abort the handshake if the server acknowledges the
ALPN extension but does not select a protocol from the client list.
Failure to do so can result in attacks such those described in
[ALPACA].
Protocol developers are strongly encouraged to register an ALPN
identifier for their protocols. This applies both to new protocols
and to well-established protocols; however, because the latter might
have a large deployed base, strict enforcement of ALPN usage may not
be feasible when an ALPN identifier is registered for a well-
established protocol.
3.9. Multi-Server Deployment
Deployments that involve multiple servers or services can increase
the size of the attack surface for TLS. Two scenarios are of
interest:
1. Deployments in which multiple services handle the same domain
name via different protocols (e.g., HTTP and IMAP). In this
case, an attacker might be able to direct a connecting endpoint
to the service offering a different protocol and mount a cross-
protocol attack. In a cross-protocol attack, the client and
server believe they are using different protocols, which the
attacker might exploit if messages sent in one protocol are
interpreted as messages in the other protocol with undesirable
effects (see [ALPACA] for more detailed information about this
class of attacks). To mitigate this threat, service providers
SHOULD deploy ALPN (see Section 3.8). In addition, to the extent
possible, they SHOULD ensure that multiple services handling the
same domain name provide equivalent levels of security that are
consistent with the recommendations in this document; such
measures SHOULD include the handling of configurations across
multiple TLS servers and protections against compromise of
credentials held by those servers.
2. Deployments in which multiple servers providing the same service
have different TLS configurations. In this case, an attacker
might be able to direct a connecting endpoint to a server with a
TLS configuration that is more easily exploitable (see [DROWN]
for more detailed information about this class of attacks). To
mitigate this threat, service providers SHOULD ensure that all
servers providing the same service provide equivalent levels of
security that are consistent with the recommendations in this
document.
3.10. Zero Round-Trip Time (0-RTT) Data in TLS 1.3
The 0-RTT early data feature is new in TLS 1.3. It provides reduced
latency when TLS connections are resumed, at the potential cost of
certain security properties. As a result, it requires special
attention from implementers on both the server and the client side.
Typically, this extends to the TLS library as well as protocol layers
above it.
For HTTP over TLS, refer to [RFC8470] for guidance.
For QUIC on TLS, refer to Section 9.2 of [RFC9001].
For other protocols, generic guidance is given in Section 8 and
Appendix E.5 of [RFC8446]. To paraphrase Appendix E.5, applications
MUST avoid this feature unless an explicit specification exists for
the application protocol in question to clarify when 0-RTT is
appropriate and secure. This can take the form of an IETF RFC, a
non-IETF standard, or documentation associated with a non-standard
protocol.
4. Recommendations: Cipher Suites
TLS 1.2 provided considerable flexibility in the selection of cipher
suites. Unfortunately, the security of some of these cipher suites
has degraded over time to the point where some are known to be
insecure (this is one reason why TLS 1.3 restricted such
flexibility). Incorrectly configuring a server leads to no or
reduced security. This section includes recommendations on the
selection and negotiation of cipher suites.
4.1. General Guidelines
Cryptographic algorithms weaken over time as cryptanalysis improves:
algorithms that were once considered strong become weak.
Consequently, cipher suites using weak algorithms need to be phased
out and replaced with more secure cipher suites. This helps to
ensure that the desired security properties still hold. SSL/TLS has
been in existence for well over 20 years and many of the cipher
suites that have been recommended in various versions of SSL/TLS are
now considered weak or at least not as strong as desired. Therefore,
this section modernizes the recommendations concerning cipher suite
selection.
* Implementations MUST NOT negotiate the cipher suites with NULL
encryption.
Rationale: The NULL cipher suites do not encrypt traffic and so
provide no confidentiality services. Any entity in the network
with access to the connection can view the plaintext of contents
being exchanged by the client and server. Nevertheless, this
document does not discourage software from implementing NULL
cipher suites, since they can be useful for testing and debugging.
* Implementations MUST NOT negotiate RC4 cipher suites.
Rationale: The RC4 stream cipher has a variety of cryptographic
weaknesses, as documented in [RFC7465]. Note that DTLS
specifically forbids the use of RC4 already.
* Implementations MUST NOT negotiate cipher suites offering less
than 112 bits of security, including so-called "export-level"
encryption (which provides 40 or 56 bits of security).
Rationale: Based on [RFC3766], at least 112 bits of security is
needed. 40-bit and 56-bit security (found in so-called "export
ciphers") are considered insecure today.
* Implementations SHOULD NOT negotiate cipher suites that use
algorithms offering less than 128 bits of security.
Rationale: Cipher suites that offer 112 or more bits but less than
128 bits of security are not considered weak at this time;
however, it is expected that their useful lifespan is short enough
to justify supporting stronger cipher suites at this time.
128-bit ciphers are expected to remain secure for at least several
years and 256-bit ciphers until the next fundamental technology
breakthrough. Note that, because of so-called "meet-in-the-
middle" attacks [Multiple-Encryption], some legacy cipher suites
(e.g., 168-bit Triple DES (3DES)) have an effective key length
that is smaller than their nominal key length (112 bits in the
case of 3DES). Such cipher suites should be evaluated according
to their effective key length.
* Implementations SHOULD NOT negotiate cipher suites based on RSA
key transport, a.k.a. "static RSA".
Rationale: These cipher suites, which have assigned values
starting with the string "TLS_RSA_WITH_*", have several drawbacks,
especially the fact that they do not support forward secrecy.
* Implementations SHOULD NOT negotiate cipher suites based on non-
ephemeral (static) finite-field Diffie-Hellman (DH) key agreement.
Similarly, implementations SHOULD NOT negotiate non-ephemeral
Elliptic Curve DH key agreement.
Rationale: The former cipher suites, which have assigned values
prefixed by "TLS_DH_*", have several drawbacks, especially the
fact that they do not support forward secrecy. The latter
("TLS_ECDH_*") also lack forward secrecy and are subject to
invalid curve attacks [Jager2015].
* Implementations MUST support and prefer to negotiate cipher suites
offering forward secrecy. However, TLS 1.2 implementations SHOULD
NOT negotiate cipher suites based on ephemeral finite-field
Diffie-Hellman key agreement (i.e., "TLS_DHE_*" suites). This is
justified by the known fragility of the construction (see
[RACCOON]) and the limitation around negotiation, including using
[RFC7919], which has seen very limited uptake.
Rationale: Forward secrecy (sometimes called "perfect forward
secrecy") prevents the recovery of information that was encrypted
with older session keys, thus limiting how far back in time data
can be decrypted when an attack is successful. See Sections 7.3
and 7.4 for a detailed discussion.
4.2. Cipher Suites for TLS 1.2
Given the foregoing considerations, implementation and deployment of
the following cipher suites is RECOMMENDED:
* TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256
* TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384
* TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256
* TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384
As these are Authenticated Encryption with Associated Data (AEAD)
algorithms [RFC5116], these cipher suites are supported only in TLS
1.2 and not in earlier protocol versions.
Typically, to prefer these suites, the order of suites needs to be
explicitly configured in server software. It would be ideal if
server software implementations were to prefer these suites by
default.
Some devices have hardware support for AES Counter Mode with CBC-MAC
(AES-CCM) but not AES Galois/Counter Mode (AES-GCM), so they are
unable to follow the foregoing recommendations regarding cipher
suites. There are even devices that do not support public key
cryptography at all, but these are out of scope entirely.
A cipher suite that operates in CBC (cipher block chaining) mode
(e.g., TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA256) SHOULD NOT be used
unless the encrypt_then_mac extension [RFC7366] is also successfully
negotiated. This requirement applies to both client and server
implementations.
When using ECDSA signatures for authentication of TLS peers, it is
RECOMMENDED that implementations use the NIST curve P-256. In
addition, to avoid predictable or repeated nonces (which could reveal
the long-term signing key), it is RECOMMENDED that implementations
implement "deterministic ECDSA" as specified in [RFC6979] and in line
with the recommendations in [RFC8446].
Note that implementations of "deterministic ECDSA" may be vulnerable
to certain side-channel and fault injection attacks precisely because
of their determinism. While most fault injection attacks described
in the literature assume physical access to the device (and therefore
are more relevant in Internet of Things (IoT) deployments with poor
or non-existent physical security), some can be carried out remotely
[Poddebniak2017], e.g., as Rowhammer [Kim2014] variants. In
deployments where side-channel attacks and fault injection attacks
are a concern, implementation strategies combining both randomness
and determinism (for example, as described in [CFRG-DET-SIGS]) can be
used to avoid the risk of successful extraction of the signing key.
4.2.1. Implementation Details
Clients SHOULD include TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256 as the
first proposal to any server. Servers MUST prefer this cipher suite
over weaker cipher suites whenever it is proposed, even if it is not
the first proposal. Clients are of course free to offer stronger
cipher suites, e.g., using AES-256; when they do, the server SHOULD
prefer the stronger cipher suite unless there are compelling reasons
(e.g., seriously degraded performance) to choose otherwise.
The previous version of the TLS recommendations [RFC7525] implicitly
allowed the old RFC 5246 mandatory-to-implement cipher suite,
TLS_RSA_WITH_AES_128_CBC_SHA. At the time of writing, this cipher
suite does not provide additional interoperability, except with very
old clients. As with other cipher suites that do not provide forward
secrecy, implementations SHOULD NOT support this cipher suite. Other
application protocols specify other cipher suites as mandatory to
implement (MTI).
[RFC8422] allows clients and servers to negotiate ECDH parameters
(curves). Both clients and servers SHOULD include the "Supported
Elliptic Curves Extension" [RFC8422]. Clients and servers SHOULD
support the NIST P-256 (secp256r1) [RFC8422] and X25519 (x25519)
[RFC7748] curves. Note that [RFC8422] deprecates all but the
uncompressed point format. Therefore, if the client sends an
ec_point_formats extension, the ECPointFormatList MUST contain a
single element, "uncompressed".
4.3. Cipher Suites for TLS 1.3
This document does not specify any cipher suites for TLS 1.3.
Readers are referred to Section 9.1 of [RFC8446] for cipher suite
recommendations.
4.4. Limits on Key Usage
All ciphers have an upper limit on the amount of traffic that can be
securely protected with any given key. In the case of AEAD cipher
suites, two separate limits are maintained for each key:
1. Confidentiality limit (CL), i.e., the number of records that can
be encrypted.
2. Integrity limit (IL), i.e., the number of records that are
allowed to fail authentication.
The latter applies to DTLS (and also to QUIC) but not to TLS itself,
since TLS connections are torn down on the first decryption failure.
When a sender is approaching CL, the implementation SHOULD initiate a
new handshake (in TLS 1.3, this can be achieved by sending a
KeyUpdate message on the established session) to rotate the session
key. When a receiver has reached IL, the implementation SHOULD close
the connection. Although these recommendations are a best practice,
implementers need to be aware that it is not always easy to
accomplish them in protocols that are built on top of TLS/DTLS
without introducing coordination across layer boundaries. See
Section 6 of [RFC9001] for an example of the cooperation that was
necessary in QUIC between the crypto and transport layers to support
key updates. Note that in general, application protocols might not
be able to emulate that method given their more constrained
interaction with TLS/DTLS. As a result of these complexities, these
recommendations are not mandatory.
For all TLS 1.3 cipher suites, readers are referred to Section 5.5 of
[RFC8446] for the values of CL and IL. For all DTLS 1.3 cipher
suites, readers are referred to Section 4.5.3 of [RFC9147].
For all AES-GCM cipher suites recommended for TLS 1.2 and DTLS 1.2 in
this document, CL can be derived by plugging the corresponding
parameters into the inequalities in Section 6.1 of [AEAD-LIMITS] that
apply to random, partially implicit nonces, i.e., the nonce
construction used in TLS 1.2. Although the obtained figures are
slightly higher than those for TLS 1.3, it is RECOMMENDED that the
same limit of 2^24.5 records is used for both versions.
For all AES-GCM cipher suites recommended for DTLS 1.2, IL (obtained
from the same inequalities referenced above) is 2^28.
4.5. Public Key Length
When using the cipher suites recommended in this document, two public
keys are normally used in the TLS handshake: one for the Diffie-
Hellman key agreement and one for server authentication. Where a
client certificate is used, a third public key is added.
With a key exchange based on modular exponential (MODP) Diffie-
Hellman groups ("DHE" cipher suites), DH key lengths of at least 2048
bits are REQUIRED.
Rationale: For various reasons, in practice, DH keys are typically
generated in lengths that are powers of two (e.g., 2^10 = 1024 bits,
2^11 = 2048 bits, 2^12 = 4096 bits). Because a DH key of 1228 bits
would be roughly equivalent to only an 80-bit symmetric key
[RFC3766], it is better to use keys longer than that for the "DHE"
family of cipher suites. A DH key of 1926 bits would be roughly
equivalent to a 100-bit symmetric key [RFC3766]. A DH key of 2048
bits (equivalent to a 112-bit symmetric key) is the minimum allowed
by the latest revision of [NIST.SP.800-56A] as of this writing (see
in particular Appendix D of that document).
As noted in [RFC3766], correcting for the emergence of The Weizmann
Institute Relation Locator (TWIRL) machine [TWIRL] would imply that
1024-bit DH keys yield about 61 bits of equivalent strength and that
a 2048-bit DH key would yield about 92 bits of equivalent strength.
The Logjam attack [Logjam] further demonstrates that 1024-bit Diffie-
Hellman parameters should be avoided.
With regard to ECDH keys, implementers are referred to the IANA "TLS
Supported Groups" registry (formerly known as the "EC Named Curve
Registry") within the "Transport Layer Security (TLS) Parameters"
registry [IANA_TLS] and in particular to the "recommended" groups.
Curves of less than 224 bits MUST NOT be used. This recommendation
is in line with the latest revision of [NIST.SP.800-56A].
When using RSA, servers MUST authenticate using certificates with at
least a 2048-bit modulus for the public key. In addition, the use of
the SHA-256 hash algorithm is RECOMMENDED and SHA-1 or MD5 MUST NOT
be used [RFC9155] (for more details, see also [CAB-Baseline], for
which the current version at the time of writing is 1.8.4). Clients
MUST indicate to servers that they request SHA-256 by using the
"Signature Algorithms" extension defined in TLS 1.2. For TLS 1.3,
the same requirement is already specified by [RFC8446].
4.6. Truncated HMAC
Implementations MUST NOT use the Truncated HMAC Extension, defined in
Section 7 of [RFC6066].
Rationale: The extension does not apply to the AEAD cipher suites
recommended above. However, it does apply to most other TLS cipher
suites. Its use has been shown to be insecure in [PatersonRS11].
5. Applicability Statement
The recommendations of this document primarily apply to the
implementation and deployment of application protocols that are most
commonly used with TLS and DTLS on the Internet today. Examples
include, but are not limited to:
* Web software and services that wish to protect HTTP traffic with
TLS.
* Email software and services that wish to protect IMAP, Post Office
Protocol version 3 (POP3), or SMTP traffic with TLS.
* Instant-messaging software and services that wish to protect
Extensible Messaging and Presence Protocol (XMPP) or Internet
Relay Chat (IRC) traffic with TLS.
* Realtime media software and services that wish to protect Secure
Realtime Transport Protocol (SRTP) traffic with DTLS.
This document does not modify the implementation and deployment
recommendations (e.g., mandatory-to-implement cipher suites)
prescribed by existing application protocols that employ TLS or DTLS.
If the community that uses such an application protocol wishes to
modernize its usage of TLS or DTLS to be consistent with the best
practices recommended here, it needs to explicitly update the
existing application protocol definition (one example is [RFC7590],
which updates [RFC6120]).
Designers of new application protocols developed through the Internet
Standards Process [RFC2026] are expected at minimum to conform to the
best practices recommended here, unless they provide documentation of
compelling reasons that would prevent such conformance (e.g.,
widespread deployment on constrained devices that lack support for
the necessary algorithms).
Although many of the recommendations provided here might also apply
to QUIC insofar that it uses the TLS 1.3 handshake protocol, QUIC and
other such secure transport protocols are out of scope of this
document. For QUIC specifically, readers are referred to Section 9.2
of [RFC9001].
This document does not address the use of TLS in constrained-node
networks [RFC7228]. For recommendations regarding the profiling of
TLS and DTLS for small devices with severe constraints on power,
memory, and processing resources, the reader is referred to [RFC7925]
and [IOT-PROFILE].
5.1. Security Services
This document provides recommendations for an audience that wishes to
secure their communication with TLS to achieve the following:
Confidentiality: all application-layer communication is encrypted
with the goal that no party should be able to decrypt it except
the intended receiver.
Data integrity: any changes made to the communication in transit are
detectable by the receiver.
Authentication: an endpoint of the TLS communication is
authenticated as the intended entity to communicate with.
With regard to authentication, TLS enables authentication of one or
both endpoints in the communication. In the context of opportunistic
security [RFC7435], TLS is sometimes used without authentication. As
discussed in Section 5.2, considerations for opportunistic security
are not in scope for this document.
If deployers deviate from the recommendations given in this document,
they need to be aware that they might lose access to one of the
foregoing security services.
This document applies only to environments where confidentiality is
required. It requires algorithms and configuration options that
enforce secrecy of the data in transit.
This document also assumes that data integrity protection is always
one of the goals of a deployment. In cases where integrity is not
required, it does not make sense to employ TLS in the first place.
There are attacks against confidentiality-only protection that
utilize the lack of integrity to also break confidentiality (see, for
instance, [DegabrieleP07] in the context of IPsec).
This document addresses itself to application protocols that are most
commonly used on the Internet with TLS and DTLS. Typically, all
communication between TLS clients and TLS servers requires all three
of the above security services. This is particularly true where TLS
clients are user agents like web browsers or email clients.
This document does not address the rarer deployment scenarios where
one of the above three properties is not desired, such as the use
case described in Section 5.2. As another scenario where
confidentiality is not needed, consider a monitored network where the
authorities in charge of the respective traffic domain require full
access to unencrypted (plaintext) traffic and where users collaborate
and send their traffic in the clear.
5.2. Opportunistic Security
There are several important scenarios in which the use of TLS is
optional, i.e., the client decides dynamically ("opportunistically")
whether to use TLS with a particular server or to connect in the
clear. This practice, often called "opportunistic security", is
described at length in [RFC7435] and is often motivated by a desire
for backward compatibility with legacy deployments.
In these scenarios, some of the recommendations in this document
might be too strict, since adhering to them could cause fallback to
cleartext, a worse outcome than using TLS with an outdated protocol
version or cipher suite.
6. IANA Considerations
This document has no IANA actions.
7. Security Considerations
This entire document discusses the security practices directly
affecting applications using the TLS protocol. This section contains
broader security considerations related to technologies used in
conjunction with or by TLS. The reader is referred to the Security
Considerations sections of TLS 1.3 [RFC8446], DTLS 1.3 [RFC9147], TLS
1.2 [RFC5246], and DTLS 1.2 [RFC6347] for further context.
7.1. Host Name Validation
Application authors should take note that some TLS implementations do
not validate host names. If the TLS implementation they are using
does not validate host names, authors might need to write their own
validation code or consider using a different TLS implementation.
It is noted that the requirements regarding host name validation
(and, in general, binding between the TLS layer and the protocol that
runs above it) vary between different protocols. For HTTPS, these
requirements are defined by Sections 4.3.3, 4.3.4, and 4.3.5 of
[RFC9110].
Host name validation is security-critical for all common TLS use
cases. Without it, TLS ensures that the certificate is valid and
guarantees possession of the private key but does not ensure that the
connection terminates at the desired endpoint. Readers are referred
to [RFC6125] for further details regarding generic host name
validation in the TLS context. In addition, that RFC contains a long
list of application protocols, some of which implement a policy very
different from HTTPS.
If the host name is discovered indirectly and insecurely (e.g., by a
cleartext DNS query for an SRV or Mail Exchange (MX) record), it
SHOULD NOT be used as a reference identifier [RFC6125] even when it
matches the presented certificate. This proviso does not apply if
the host name is discovered securely (for further discussion, see
[RFC7673] and [RFC7672]).
Host name validation typically applies only to the leaf "end entity"
certificate. Naturally, in order to ensure proper authentication in
the context of the PKI, application clients need to verify the entire
certification path in accordance with [RFC5280].
7.2. AES-GCM
Section 4.2 recommends the use of the AES-GCM authenticated
encryption algorithm. Please refer to Section 6 of [RFC5288] for
security considerations that apply specifically to AES-GCM when used
with TLS.
7.2.1. Nonce Reuse in TLS 1.2
The existence of deployed TLS stacks that mistakenly reuse the AES-
GCM nonce is documented in [Boeck2016], showing there is an actual
risk of AES-GCM getting implemented insecurely and thus making TLS
sessions that use an AES-GCM cipher suite vulnerable to attacks such
as [Joux2006]. (See [CVE] records: CVE-2016-0270, CVE-2016-10213,
CVE-2016-10212, and CVE-2017-5933.)
While this problem has been fixed in TLS 1.3, which enforces a
deterministic method to generate nonces from record sequence numbers
and shared secrets for all its AEAD cipher suites (including AES-
GCM), TLS 1.2 implementations could still choose their own
(potentially insecure) nonce generation methods.
It is therefore RECOMMENDED that TLS 1.2 implementations use the
64-bit sequence number to populate the nonce_explicit part of the GCM
nonce, as described in the first two paragraphs of Section 5.3 of
[RFC8446]. This stronger recommendation updates Section 3 of
[RFC5288], which specifies that the use of 64-bit sequence numbers to
populate the nonce_explicit field is optional.
We note that at the time of writing, there are no cipher suites
defined for nonce-reuse-resistant algorithms such as AES-GCM-SIV
[RFC8452].
7.3. Forward Secrecy
Forward secrecy (also called "perfect forward secrecy" or "PFS" and
defined in [RFC4949]) is a defense against an attacker who records
encrypted conversations where the session keys are only encrypted
with the communicating parties' long-term keys.
Should the attacker be able to obtain these long-term keys at some
point later in time, the session keys and thus the entire
conversation could be decrypted.
In the context of TLS and DTLS, such compromise of long-term keys is
not entirely implausible. It can happen, for example, due to:
* A client or server being attacked by some other attack vector, and
the private key retrieved.
* A long-term key retrieved from a device that has been sold or
otherwise decommissioned without prior wiping.
* A long-term key used on a device as a default key [Heninger2012].
* A key generated by a trusted third party like a CA and later
retrieved from it by either extortion or compromise
[Soghoian2011].
* A cryptographic breakthrough or the use of asymmetric keys with
insufficient length [Kleinjung2010].
* Social engineering attacks against system administrators.
* Collection of private keys from inadequately protected backups.
Forward secrecy ensures in such cases that it is not feasible for an
attacker to determine the session keys even if the attacker has
obtained the long-term keys some time after the conversation. It
also protects against an attacker who is in possession of the long-
term keys but remains passive during the conversation.
Forward secrecy is generally achieved by using the Diffie-Hellman
scheme to derive session keys. The Diffie-Hellman scheme has both
parties maintain private secrets and send parameters over the network
as modular powers over certain cyclic groups. The properties of the
so-called Discrete Logarithm Problem (DLP) allow the parties to
derive the session keys without an eavesdropper being able to do so.
There is currently no known attack against DLP if sufficiently large
parameters are chosen. A variant of the Diffie-Hellman scheme uses
elliptic curves instead of the originally proposed modular
arithmetic. Given the current state of the art, Elliptic Curve
Diffie-Hellman appears to be more efficient, permits shorter key
lengths, and allows less freedom for implementation errors than
finite-field Diffie-Hellman.
Unfortunately, many TLS/DTLS cipher suites were defined that do not
feature forward secrecy, e.g., TLS_RSA_WITH_AES_256_CBC_SHA256. This
document therefore advocates strict use of forward-secrecy-only
ciphers.
7.4. Diffie-Hellman Exponent Reuse
For performance reasons, it is not uncommon for TLS implementations
to reuse Diffie-Hellman and Elliptic Curve Diffie-Hellman exponents
across multiple connections. Such reuse can result in major security
issues:
* If exponents are reused for too long (in some cases, even as
little as a few hours), an attacker who gains access to the host
can decrypt previous connections. In other words, exponent reuse
negates the effects of forward secrecy.
* TLS implementations that reuse exponents should test the DH public
key they receive for group membership, in order to avoid some
known attacks. These tests are not standardized in TLS at the
time of writing, although general guidance in this area is
provided by [NIST.SP.800-56A] and available in many protocol
implementations.
* Under certain conditions, the use of static finite-field DH keys,
or of ephemeral finite-field DH keys that are reused across
multiple connections, can lead to timing attacks (such as those
described in [RACCOON]) on the shared secrets used in Diffie-
Hellman key exchange.
* An "invalid curve" attack can be mounted against Elliptic Curve DH
if the victim does not verify that the received point lies on the
correct curve. If the victim is reusing the DH secrets, the
attacker can repeat the probe varying the points to recover the
full secret (see [Antipa2003] and [Jager2015]).
To address these concerns:
* TLS implementations SHOULD NOT use static finite-field DH keys and
SHOULD NOT reuse ephemeral finite-field DH keys across multiple
connections.
* Server implementations that want to reuse Elliptic Curve DH keys
SHOULD either use a "safe curve" [SAFECURVES] (e.g., X25519) or
perform the checks described in [NIST.SP.800-56A] on the received
points.
7.5. Certificate Revocation
The following considerations and recommendations represent the
current state of the art regarding certificate revocation, even
though no complete and efficient solution exists for the problem of
checking the revocation status of common public key certificates
[RFC5280]:
* Certificate revocation is an important tool when recovering from
attacks on the TLS implementation as well as cases of misissued
certificates. TLS implementations MUST implement a strategy to
distrust revoked certificates.
* Although Certificate Revocation Lists (CRLs) are the most widely
supported mechanism for distributing revocation information, they
have known scaling challenges that limit their usefulness, despite
workarounds such as partitioned CRLs and delta CRLs. The more
modern [CRLite] and the follow-on Let's Revoke [LetsRevoke] build
on the availability of Certificate Transparency [RFC9162] logs and
aggressive compression to allow practical use of the CRL
infrastructure, but at the time of writing, neither solution is
deployed for client-side revocation processing at scale.
* Proprietary mechanisms that embed revocation lists in the web
browser's configuration database cannot scale beyond the few most
heavily used web servers.
* The Online Certification Status Protocol (OCSP) [RFC6960] in its
basic form presents both scaling and privacy issues. In addition,
clients typically "soft-fail", meaning that they do not abort the
TLS connection if the OCSP server does not respond. (However,
this might be a workaround to avoid denial-of-service attacks if
an OCSP responder is taken offline.) For a recent survey of the
status of OCSP deployment in the web PKI, see [Chung18].
* The TLS Certificate Status Request extension (Section 8 of
[RFC6066]), commonly called "OCSP stapling", resolves the
operational issues with OCSP. However, it is still ineffective in
the presence of an active on-path attacker because the attacker
can simply ignore the client's request for a stapled OCSP
response.
* [RFC7633] defines a certificate extension that indicates that
clients must expect stapled OCSP responses for the certificate and
must abort the handshake ("hard-fail") if such a response is not
available.
* OCSP stapling as used in TLS 1.2 does not extend to intermediate
certificates within a certificate chain. The Multiple Certificate
Status extension [RFC6961] addresses this shortcoming, but it has
seen little deployment and had been deprecated by [RFC8446]. As a
result, although this extension was recommended for TLS 1.2 in
[RFC7525], it is no longer recommended by this document.
* TLS 1.3 (Section 4.4.2.1 of [RFC8446]) allows the association of
OCSP information with intermediate certificates by using an
extension to the CertificateEntry structure. However, using this
facility remains impractical because many certification
authorities (CAs) either do not publish OCSP for CA certificates
or publish OCSP reports with a lifetime that is too long to be
useful.
* Both CRLs and OCSP depend on relatively reliable connectivity to
the Internet, which might not be available to certain kinds of
nodes. A common example is newly provisioned devices that need to
establish a secure connection in order to boot up for the first
time.
For the common use cases of public key certificates in TLS, servers
SHOULD support the following as a best practice given the current
state of the art and as a foundation for a possible future solution:
OCSP [RFC6960] and OCSP stapling using the status_request extension
defined in [RFC6066]. Note that the exact mechanism for embedding
the status_request extension differs between TLS 1.2 and 1.3. As a
matter of local policy, server operators MAY request that CAs issue
must-staple [RFC7633] certificates for the server and/or for client
authentication, but we recommend reviewing the operational conditions
before deciding on this approach.
The considerations in this section do not apply to scenarios where
the DNS-Based Authentication of Named Entities (DANE) TLSA resource
record [RFC6698] is used to signal to a client which certificate a
server considers valid and good to use for TLS connections.
8. References
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC3766] Orman, H. and P. Hoffman, "Determining Strengths For
Public Keys Used For Exchanging Symmetric Keys", BCP 86,
RFC 3766, DOI 10.17487/RFC3766, April 2004,
<https://www.rfc-editor.org/info/rfc3766>.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246,
DOI 10.17487/RFC5246, August 2008,
<https://www.rfc-editor.org/info/rfc5246>.
[RFC5288] Salowey, J., Choudhury, A., and D. McGrew, "AES Galois
Counter Mode (GCM) Cipher Suites for TLS", RFC 5288,
DOI 10.17487/RFC5288, August 2008,
<https://www.rfc-editor.org/info/rfc5288>.
[RFC5746] Rescorla, E., Ray, M., Dispensa, S., and N. Oskov,
"Transport Layer Security (TLS) Renegotiation Indication
Extension", RFC 5746, DOI 10.17487/RFC5746, February 2010,
<https://www.rfc-editor.org/info/rfc5746>.
[RFC6066] Eastlake 3rd, D., "Transport Layer Security (TLS)
Extensions: Extension Definitions", RFC 6066,
DOI 10.17487/RFC6066, January 2011,
<https://www.rfc-editor.org/info/rfc6066>.
[RFC6125] Saint-Andre, P. and J. Hodges, "Representation and
Verification of Domain-Based Application Service Identity
within Internet Public Key Infrastructure Using X.509
(PKIX) Certificates in the Context of Transport Layer
Security (TLS)", RFC 6125, DOI 10.17487/RFC6125, March
2011, <https://www.rfc-editor.org/info/rfc6125>.
[RFC6176] Turner, S. and T. Polk, "Prohibiting Secure Sockets Layer
(SSL) Version 2.0", RFC 6176, DOI 10.17487/RFC6176, March
2011, <https://www.rfc-editor.org/info/rfc6176>.
[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
January 2012, <https://www.rfc-editor.org/info/rfc6347>.
[RFC6979] Pornin, T., "Deterministic Usage of the Digital Signature
Algorithm (DSA) and Elliptic Curve Digital Signature
Algorithm (ECDSA)", RFC 6979, DOI 10.17487/RFC6979, August
2013, <https://www.rfc-editor.org/info/rfc6979>.
[RFC7301] Friedl, S., Popov, A., Langley, A., and E. Stephan,
"Transport Layer Security (TLS) Application-Layer Protocol
Negotiation Extension", RFC 7301, DOI 10.17487/RFC7301,
July 2014, <https://www.rfc-editor.org/info/rfc7301>.
[RFC7366] Gutmann, P., "Encrypt-then-MAC for Transport Layer
Security (TLS) and Datagram Transport Layer Security
(DTLS)", RFC 7366, DOI 10.17487/RFC7366, September 2014,
<https://www.rfc-editor.org/info/rfc7366>.
[RFC7465] Popov, A., "Prohibiting RC4 Cipher Suites", RFC 7465,
DOI 10.17487/RFC7465, February 2015,
<https://www.rfc-editor.org/info/rfc7465>.
[RFC7627] Bhargavan, K., Ed., Delignat-Lavaud, A., Pironti, A.,
Langley, A., and M. Ray, "Transport Layer Security (TLS)
Session Hash and Extended Master Secret Extension",
RFC 7627, DOI 10.17487/RFC7627, September 2015,
<https://www.rfc-editor.org/info/rfc7627>.
[RFC7748] Langley, A., Hamburg, M., and S. Turner, "Elliptic Curves
for Security", RFC 7748, DOI 10.17487/RFC7748, January
2016, <https://www.rfc-editor.org/info/rfc7748>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8422] Nir, Y., Josefsson, S., and M. Pegourie-Gonnard, "Elliptic
Curve Cryptography (ECC) Cipher Suites for Transport Layer
Security (TLS) Versions 1.2 and Earlier", RFC 8422,
DOI 10.17487/RFC8422, August 2018,
<https://www.rfc-editor.org/info/rfc8422>.
[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/info/rfc8446>.
[RFC8996] Moriarty, K. and S. Farrell, "Deprecating TLS 1.0 and TLS
1.1", BCP 195, RFC 8996, DOI 10.17487/RFC8996, March 2021,
<https://www.rfc-editor.org/info/rfc8996>.
[RFC9147] Rescorla, E., Tschofenig, H., and N. Modadugu, "The
Datagram Transport Layer Security (DTLS) Protocol Version
1.3", RFC 9147, DOI 10.17487/RFC9147, April 2022,
<https://www.rfc-editor.org/info/rfc9147>.
[RFC9155] Velvindron, L., Moriarty, K., and A. Ghedini, "Deprecating
MD5 and SHA-1 Signature Hashes in TLS 1.2 and DTLS 1.2",
RFC 9155, DOI 10.17487/RFC9155, December 2021,
<https://www.rfc-editor.org/info/rfc9155>.
8.2. Informative References
[AEAD-LIMITS]
Günther, F., Thomson, M., and C. A. Wood, "Usage Limits on
AEAD Algorithms", Work in Progress, Internet-Draft, draft-
irtf-cfrg-aead-limits-05, 11 July 2022,
<https://datatracker.ietf.org/doc/html/draft-irtf-cfrg-
aead-limits-05>.
[ALPACA] Brinkmann, M., Dresen, C., Merget, R., Poddebniak, D.,
Müller, J., Somorovsky, J., Schwenk, J., and S. Schinzel,
"ALPACA: Application Layer Protocol Confusion - Analyzing
and Mitigating Cracks in TLS Authentication", 30th USENIX
Security Symposium (USENIX Security 21), August 2021,
<https://www.usenix.org/conference/usenixsecurity21/
presentation/brinkmann>.
[Antipa2003]
Antipa, A., Brown, D. R. L., Menezes, A., Struik, R., and
S. Vanstone, "Validation of Elliptic Curve Public Keys",
Public Key Cryptography - PKC 2003, December 2003,
<https://doi.org/10.1007/3-540-36288-6_16>.
[Boeck2016]
Böck, H., Zauner, A., Devlin, S., Somorovsky, J., and P.
Jovanovic, "Nonce-Disrespecting Adversaries: Practical
Forgery Attacks on GCM in TLS", May 2016,
<https://eprint.iacr.org/2016/475.pdf>.
[CAB-Baseline]
CA/Browser Forum, "Baseline Requirements for the Issuance
and Management of Publicly-Trusted Certificates",
Version 1.8.4, April 2022,
<https://cabforum.org/documents/>.
[CFRG-DET-SIGS]
Preuß Mattsson, J., Thormarker, E., and S. Ruohomaa,
"Deterministic ECDSA and EdDSA Signatures with Additional
Randomness", Work in Progress, Internet-Draft, draft-irtf-
cfrg-det-sigs-with-noise-00, 8 August 2022,
<https://datatracker.ietf.org/doc/html/draft-irtf-cfrg-
det-sigs-with-noise-00>.
[Chung18] Chung, T., Lok, J., Chandrasekaran, B., Choffnes, D.,
Levin, D., Maggs, B., Mislove, A., Rula, J., Sullivan, N.,
and C. Wilson, "Is the Web Ready for OCSP Must-Staple?",
Proceedings of the Internet Measurement Conference 2018,
DOI 10.1145/3278532.3278543, October 2018,
<https://doi.org/10.1145/3278532.3278543>.
[CRLite] Larisch, J., Choffnes, D., Levin, D., Maggs, B., Mislove,
A., and C. Wilson, "CRLite: A Scalable System for Pushing
All TLS Revocations to All Browsers", 2017 IEEE Symposium
on Security and Privacy (SP), DOI 10.1109/sp.2017.17, May
2017, <https://doi.org/10.1109/sp.2017.17>.
[CVE] MITRE, "Common Vulnerabilities and Exposures",
<https://cve.mitre.org>.
[DegabrieleP07]
Degabriele, J. and K. Paterson, "Attacking the IPsec
Standards in Encryption-only Configurations", 2007 IEEE
Symposium on Security and Privacy (SP '07),
DOI 10.1109/sp.2007.8, May 2007,
<https://doi.org/10.1109/sp.2007.8>.
[DROWN] Aviram, N., Schinzel, S., Somorovsky, J., Heninger, N.,
Dankel, M., Steube, J., Valenta, L., Adrian, D.,
Halderman, J., Dukhovni, V., Käsper, E., Cohney, S.,
Engels, S., Paar, C., and Y. Shavitt, "DROWN: Breaking TLS
using SSLv2", 25th USENIX Security Symposium (USENIX
Security 16), August 2016,
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technical-sessions/presentation/aviram>.
[Heninger2012]
Heninger, N., Durumeric, Z., Wustrow, E., and J. A.
Halderman, "Mining Your Ps and Qs: Detection of Widespread
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[IANA_TLS] IANA, "Transport Layer Security (TLS) Parameters",
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[IOT-PROFILE]
Tschofenig, H. and T. Fossati, "TLS/DTLS 1.3 Profiles for
the Internet of Things", Work in Progress, Internet-Draft,
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tls13-iot-profile-05>.
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Jager, T., Schwenk, J., and J. Somorovsky, "Practical
Invalid Curve Attacks on TLS-ECDH", Computer Security --
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[Joux2006] Joux, A., "Authentication Failures in NIST version of
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[Kim2014] Kim, Y., Daly, R., Kim, J., Fallin, C., Lee, J. H., Lee,
D., Wilkerson, C., Lai, K., and O. Mutlu, "Flipping Bits
in Memory Without Accessing Them: An Experimental Study of
DRAM Disturbance Errors", DOI 10.1109/ISCA.2014.6853210,
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isca14.pdf>.
[Kleinjung2010]
Kleinjung, T., Aoki, K., Franke, J., Lenstra, A., Thomé,
E., Bos, J., Gaudry, P., Kruppa, A., Montgomery, P.,
Osvik, D., te Riele, H., Timofeev, A., and P. Zimmermann,
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DOI 10.1007/978-3-642-14623-7_18, 2010,
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[LetsRevoke]
Smith, T., Dickinson, L., and K. Seamons, "Let's Revoke:
Scalable Global Certificate Revocation", Proceedings 2020
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<https://doi.org/10.14722/ndss.2020.24084>.
[Logjam] Adrian, D., Bhargavan, K., Durumeric, Z., Gaudry, P.,
Green, M., Halderman, J., Heninger, N., Springall, D.,
Thomé, E., Valenta, L., VanderSloot, B., Wustrow, E.,
Zanella-Béguelin, S., and P. Zimmermann, "Imperfect
Forward Secrecy: How Diffie-Hellman Fails in Practice",
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Merkle, R. and M. Hellman, "On the security of multiple
encryption", Communications of the ACM, Vol. 24, Issue 7,
pp. 465-467, DOI 10.1145/358699.358718, July 1981,
<https://doi.org/10.1145/358699.358718>.
[NIST.SP.800-56A]
National Institute of Standards and Technology,
"Recommendation for Pair-Wise Key-Establishment Schemes
Using Discrete Logarithm Cryptography", Revision 3, NIST
Special Publication 800-56A,
DOI 10.6028/NIST.SP.800-56Ar3, April 2018,
<https://doi.org/10.6028/NIST.SP.800-56Ar3>.
[PatersonRS11]
Paterson, K., Ristenpart, T., and T. Shrimpton, "Tag Size
Does Matter: Attacks and Proofs for the TLS Record
Protocol", Proceedings of the 17th International
conference on The Theory and Application of Cryptology and
Information Security, pp. 372-389,
DOI 10.1007/978-3-642-25385-0_20, December 2011,
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[Poddebniak2017]
Poddebniak, D., Somorovsky, J., Schinzel, S., Lochter, M.,
and P. Rösler, "Attacking Deterministic Signature Schemes
using Fault Attacks", Conference: 2018 IEEE European
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DOI 10.1109/EuroSP.2018.00031, April 2018,
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[POODLE] US-CERT, "SSL 3.0 Protocol Vulnerability and POODLE
Attack", October 2014,
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[RACCOON] Merget, R., Brinkmann, M., Aviram, N., Somorovsky, J.,
Mittmann, J., and J. Schwenk, "Raccoon Attack: Finding and
Exploiting Most-Significant-Bit-Oracles in TLS-DH(E)",
30th USENIX Security Symposium (USENIX Security 21), 2021,
<https://www.usenix.org/conference/usenixsecurity21/
presentation/merget>.
[RFC2026] Bradner, S., "The Internet Standards Process -- Revision
3", BCP 9, RFC 2026, DOI 10.17487/RFC2026, October 1996,
<https://www.rfc-editor.org/info/rfc2026>.
[RFC2246] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
RFC 2246, DOI 10.17487/RFC2246, January 1999,
<https://www.rfc-editor.org/info/rfc2246>.
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
DOI 10.17487/RFC3261, June 2002,
<https://www.rfc-editor.org/info/rfc3261>.
[RFC3602] Frankel, S., Glenn, R., and S. Kelly, "The AES-CBC Cipher
Algorithm and Its Use with IPsec", RFC 3602,
DOI 10.17487/RFC3602, September 2003,
<https://www.rfc-editor.org/info/rfc3602>.
[RFC4346] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.1", RFC 4346,
DOI 10.17487/RFC4346, April 2006,
<https://www.rfc-editor.org/info/rfc4346>.
[RFC4347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security", RFC 4347, DOI 10.17487/RFC4347, April 2006,
<https://www.rfc-editor.org/info/rfc4347>.
[RFC4949] Shirey, R., "Internet Security Glossary, Version 2",
FYI 36, RFC 4949, DOI 10.17487/RFC4949, August 2007,
<https://www.rfc-editor.org/info/rfc4949>.
[RFC5077] Salowey, J., Zhou, H., Eronen, P., and H. Tschofenig,
"Transport Layer Security (TLS) Session Resumption without
Server-Side State", RFC 5077, DOI 10.17487/RFC5077,
January 2008, <https://www.rfc-editor.org/info/rfc5077>.
[RFC5116] McGrew, D., "An Interface and Algorithms for Authenticated
Encryption", RFC 5116, DOI 10.17487/RFC5116, January 2008,
<https://www.rfc-editor.org/info/rfc5116>.
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
<https://www.rfc-editor.org/info/rfc5280>.
[RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
DOI 10.17487/RFC5321, October 2008,
<https://www.rfc-editor.org/info/rfc5321>.
[RFC6101] Freier, A., Karlton, P., and P. Kocher, "The Secure
Sockets Layer (SSL) Protocol Version 3.0", RFC 6101,
DOI 10.17487/RFC6101, August 2011,
<https://www.rfc-editor.org/info/rfc6101>.
[RFC6120] Saint-Andre, P., "Extensible Messaging and Presence
Protocol (XMPP): Core", RFC 6120, DOI 10.17487/RFC6120,
March 2011, <https://www.rfc-editor.org/info/rfc6120>.
[RFC6698] Hoffman, P. and J. Schlyter, "The DNS-Based Authentication
of Named Entities (DANE) Transport Layer Security (TLS)
Protocol: TLSA", RFC 6698, DOI 10.17487/RFC6698, August
2012, <https://www.rfc-editor.org/info/rfc6698>.
[RFC6797] Hodges, J., Jackson, C., and A. Barth, "HTTP Strict
Transport Security (HSTS)", RFC 6797,
DOI 10.17487/RFC6797, November 2012,
<https://www.rfc-editor.org/info/rfc6797>.
[RFC6960] Santesson, S., Myers, M., Ankney, R., Malpani, A.,
Galperin, S., and C. Adams, "X.509 Internet Public Key
Infrastructure Online Certificate Status Protocol - OCSP",
RFC 6960, DOI 10.17487/RFC6960, June 2013,
<https://www.rfc-editor.org/info/rfc6960>.
[RFC6961] Pettersen, Y., "The Transport Layer Security (TLS)
Multiple Certificate Status Request Extension", RFC 6961,
DOI 10.17487/RFC6961, June 2013,
<https://www.rfc-editor.org/info/rfc6961>.
[RFC7228] Bormann, C., Ersue, M., and A. Keranen, "Terminology for
Constrained-Node Networks", RFC 7228,
DOI 10.17487/RFC7228, May 2014,
<https://www.rfc-editor.org/info/rfc7228>.
[RFC7435] Dukhovni, V., "Opportunistic Security: Some Protection
Most of the Time", RFC 7435, DOI 10.17487/RFC7435,
December 2014, <https://www.rfc-editor.org/info/rfc7435>.
[RFC7457] Sheffer, Y., Holz, R., and P. Saint-Andre, "Summarizing
Known Attacks on Transport Layer Security (TLS) and
Datagram TLS (DTLS)", RFC 7457, DOI 10.17487/RFC7457,
February 2015, <https://www.rfc-editor.org/info/rfc7457>.
[RFC7507] Moeller, B. and A. Langley, "TLS Fallback Signaling Cipher
Suite Value (SCSV) for Preventing Protocol Downgrade
Attacks", RFC 7507, DOI 10.17487/RFC7507, April 2015,
<https://www.rfc-editor.org/info/rfc7507>.
[RFC7525] Sheffer, Y., Holz, R., and P. Saint-Andre,
"Recommendations for Secure Use of Transport Layer
Security (TLS) and Datagram Transport Layer Security
(DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC7525, May
2015, <https://www.rfc-editor.org/info/rfc7525>.
[RFC7568] Barnes, R., Thomson, M., Pironti, A., and A. Langley,
"Deprecating Secure Sockets Layer Version 3.0", RFC 7568,
DOI 10.17487/RFC7568, June 2015,
<https://www.rfc-editor.org/info/rfc7568>.
[RFC7590] Saint-Andre, P. and T. Alkemade, "Use of Transport Layer
Security (TLS) in the Extensible Messaging and Presence
Protocol (XMPP)", RFC 7590, DOI 10.17487/RFC7590, June
2015, <https://www.rfc-editor.org/info/rfc7590>.
[RFC7633] Hallam-Baker, P., "X.509v3 Transport Layer Security (TLS)
Feature Extension", RFC 7633, DOI 10.17487/RFC7633,
October 2015, <https://www.rfc-editor.org/info/rfc7633>.
[RFC7672] Dukhovni, V. and W. Hardaker, "SMTP Security via
Opportunistic DNS-Based Authentication of Named Entities
(DANE) Transport Layer Security (TLS)", RFC 7672,
DOI 10.17487/RFC7672, October 2015,
<https://www.rfc-editor.org/info/rfc7672>.
[RFC7673] Finch, T., Miller, M., and P. Saint-Andre, "Using DNS-
Based Authentication of Named Entities (DANE) TLSA Records
with SRV Records", RFC 7673, DOI 10.17487/RFC7673, October
2015, <https://www.rfc-editor.org/info/rfc7673>.
[RFC7712] Saint-Andre, P., Miller, M., and P. Hancke, "Domain Name
Associations (DNA) in the Extensible Messaging and
Presence Protocol (XMPP)", RFC 7712, DOI 10.17487/RFC7712,
November 2015, <https://www.rfc-editor.org/info/rfc7712>.
[RFC7919] Gillmor, D., "Negotiated Finite Field Diffie-Hellman
Ephemeral Parameters for Transport Layer Security (TLS)",
RFC 7919, DOI 10.17487/RFC7919, August 2016,
<https://www.rfc-editor.org/info/rfc7919>.
[RFC7924] Santesson, S. and H. Tschofenig, "Transport Layer Security
(TLS) Cached Information Extension", RFC 7924,
DOI 10.17487/RFC7924, July 2016,
<https://www.rfc-editor.org/info/rfc7924>.
[RFC7925] Tschofenig, H., Ed. and T. Fossati, "Transport Layer
Security (TLS) / Datagram Transport Layer Security (DTLS)
Profiles for the Internet of Things", RFC 7925,
DOI 10.17487/RFC7925, July 2016,
<https://www.rfc-editor.org/info/rfc7925>.
[RFC8452] Gueron, S., Langley, A., and Y. Lindell, "AES-GCM-SIV:
Nonce Misuse-Resistant Authenticated Encryption",
RFC 8452, DOI 10.17487/RFC8452, April 2019,
<https://www.rfc-editor.org/info/rfc8452>.
[RFC8461] Margolis, D., Risher, M., Ramakrishnan, B., Brotman, A.,
and J. Jones, "SMTP MTA Strict Transport Security (MTA-
STS)", RFC 8461, DOI 10.17487/RFC8461, September 2018,
<https://www.rfc-editor.org/info/rfc8461>.
[RFC8470] Thomson, M., Nottingham, M., and W. Tarreau, "Using Early
Data in HTTP", RFC 8470, DOI 10.17487/RFC8470, September
2018, <https://www.rfc-editor.org/info/rfc8470>.
[RFC8879] Ghedini, A. and V. Vasiliev, "TLS Certificate
Compression", RFC 8879, DOI 10.17487/RFC8879, December
2020, <https://www.rfc-editor.org/info/rfc8879>.
[RFC9000] Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
Multiplexed and Secure Transport", RFC 9000,
DOI 10.17487/RFC9000, May 2021,
<https://www.rfc-editor.org/info/rfc9000>.
[RFC9001] Thomson, M., Ed. and S. Turner, Ed., "Using TLS to Secure
QUIC", RFC 9001, DOI 10.17487/RFC9001, May 2021,
<https://www.rfc-editor.org/info/rfc9001>.
[RFC9051] Melnikov, A., Ed. and B. Leiba, Ed., "Internet Message
Access Protocol (IMAP) - Version 4rev2", RFC 9051,
DOI 10.17487/RFC9051, August 2021,
<https://www.rfc-editor.org/info/rfc9051>.
[RFC9110] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
Ed., "HTTP Semantics", STD 97, RFC 9110,
DOI 10.17487/RFC9110, June 2022,
<https://www.rfc-editor.org/info/rfc9110>.
[RFC9112] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
Ed., "HTTP/1.1", STD 99, RFC 9112, DOI 10.17487/RFC9112,
June 2022, <https://www.rfc-editor.org/info/rfc9112>.
[RFC9113] Thomson, M., Ed. and C. Benfield, Ed., "HTTP/2", RFC 9113,
DOI 10.17487/RFC9113, June 2022,
<https://www.rfc-editor.org/info/rfc9113>.
[RFC9162] Laurie, B., Messeri, E., and R. Stradling, "Certificate
Transparency Version 2.0", RFC 9162, DOI 10.17487/RFC9162,
December 2021, <https://www.rfc-editor.org/info/rfc9162>.
[RFC9191] Sethi, M., Preuß Mattsson, J., and S. Turner, "Handling
Large Certificates and Long Certificate Chains in TLS-
Based EAP Methods", RFC 9191, DOI 10.17487/RFC9191,
February 2022, <https://www.rfc-editor.org/info/rfc9191>.
[SAFECURVES]
Bernstein, D. J. and T. Lange, "SafeCurves: choosing safe
curves for elliptic-curve cryptography", December 2014,
<https://safecurves.cr.yp.to>.
[Soghoian2011]
Soghoian, C. and S. Stamm, "Certified Lies: Detecting and
Defeating Government Interception Attacks Against SSL",
SSRN Electronic Journal, DOI 10.2139/ssrn.1591033, April
2010, <https://doi.org/10.2139/ssrn.1591033>.
[Springall16]
Springall, D., Durumeric, Z., and J. Halderman, "Measuring
the Security Harm of TLS Crypto Shortcuts", Proceedings of
the 2016 Internet Measurement Conference, pp. 33-47,
DOI 10.1145/2987443.2987480, November 2016,
<https://doi.org/10.1145/2987443.2987480>.
[STD53] Myers, J. and M. Rose, "Post Office Protocol - Version 3",
STD 53, RFC 1939, May 1996.
<https://www.rfc-editor.org/info/std53>
[Sy2018] Sy, E., Burkert, C., Federrath, H., and M. Fischer,
"Tracking Users across the Web via TLS Session
Resumption", Proceedings of the 34th Annual Computer
Security Applications Conference, pp. 289-299,
DOI 10.1145/3274694.3274708, December 2018,
<https://doi.org/10.1145/3274694.3274708>.
[TLS-ECH] Rescorla, E., Oku, K., Sullivan, N., and C. A. Wood, "TLS
Encrypted Client Hello", Work in Progress, Internet-Draft,
draft-ietf-tls-esni-15, 3 October 2022,
<https://datatracker.ietf.org/doc/html/draft-ietf-tls-
esni-15>.
[Triple-Handshake]
Bhargavan, K., Lavaud, A., Fournet, C., Pironti, A., and
P. Strub, "Triple Handshakes and Cookie Cutters: Breaking
and Fixing Authentication over TLS", 2014 IEEE Symposium
on Security and Privacy, DOI 10.1109/sp.2014.14, May 2014,
<https://doi.org/10.1109/sp.2014.14>.
[TWIRL] Shamir, A. and E. Tromer, "Factoring Large Numbers with
the TWIRL Device", 2014 IEEE Symposium on Security and
Privacy, DOI 10.1007/978-3-540-45146-4_1, 2004,
<https://cs.tau.ac.il/~tromer/papers/twirl.pdf>.
Appendix A. Differences from RFC 7525
This revision of the Best Current Practices contains numerous
changes, and this section is focused on the normative changes.
* High-level differences:
- Described the expectations from new TLS-incorporating transport
protocols and from new application protocols layered on TLS.
- Clarified items (e.g., renegotiation) that only apply to TLS
1.2.
- Changed the status of TLS 1.0 and 1.1 from "SHOULD NOT" to
"MUST NOT".
- Added TLS 1.3 at a "SHOULD" level.
- Made similar changes to DTLS.
- Included specific guidance for multiplexed protocols.
- MUST-level implementation requirement for ALPN and more
specific SHOULD-level guidance for ALPN and SNI.
- Clarified discussion of strict TLS policies, including MUST-
level recommendations.
- Limits on key usage.
- New attacks since [RFC7457]: ALPACA, Raccoon, Logjam, and
"Nonce-Disrespecting Adversaries".
- RFC 6961 (OCSP status_request_v2) has been deprecated.
- MUST-level requirement for server-side RSA certificates to have
a 2048-bit modulus at a minimum, replacing a "SHOULD".
* Differences specific to TLS 1.2:
- SHOULD-level guidance on AES-GCM nonce generation.
- SHOULD NOT use (static or ephemeral) finite-field DH key
agreement.
- SHOULD NOT reuse ephemeral finite-field DH keys across multiple
connections.
- SHOULD NOT use static Elliptic Curve DH key exchange.
- 2048-bit DH is now a "MUST" and ECDH minimal curve size is 224
(vs. 192 previously).
- Support for extended_master_secret is now a "MUST" (previously
it was a soft recommendation, as the RFC had not been published
at the time). Also removed other, more complicated, related
mitigations.
- MUST-level restriction on session ticket validity, replacing a
"SHOULD".
- SHOULD-level restriction on the TLS session duration, depending
on the rotation period of an [RFC5077] ticket key.
- Dropped TLS_DHE_RSA_WITH_AES from the recommended ciphers.
- Added TLS_ECDHE_ECDSA_WITH_AES to the recommended ciphers.
- SHOULD NOT use the old MTI cipher suite,
TLS_RSA_WITH_AES_128_CBC_SHA.
- Recommended curve X25519 alongside NIST P-256.
* Differences specific to TLS 1.3:
- New TLS 1.3 capabilities: 0-RTT.
- Removed capabilities: renegotiation and compression.
- Added mention of TLS Encrypted Client Hello, but no
recommendation for use until it is finalized.
- SHOULD-level requirement for forward secrecy in TLS 1.3 session
resumption.
- Generic MUST-level guidance to avoid 0-RTT unless it is
documented for the particular protocol.
Acknowledgments
Thanks to Alexey Melnikov, Alvaro Retana, Andrei Popov, Ben Kaduk,
Christian Huitema, Corey Bonnell, Cullen Jennings, Daniel Kahn
Gillmor, David Benjamin, Eric Rescorla, Éric Vyncke, Francesca
Palombini, Hannes Tschofenig, Hubert Kario, Ilari Liusvaara, John
Preuß Mattsson, John R. Levine, Julien Élie, Lars Eggert, Leif
Johansson, Magnus Westerlund, Martin Duke, Martin Thomson, Mohit
Sahni, Nick Sullivan, Nimrod Aviram, Paul Wouters, Peter Gutmann,
Rich Salz, Robert Sayre, Robert Wilton, Roman Danyliw, Ryan Sleevi,
Sean Turner, Stephen Farrell, Tim Evans, Valery Smyslov, Viktor
Dukhovni, and Warren Kumari for helpful comments and discussions that
have shaped this document.
The authors gratefully acknowledge the contribution of Ralph Holz,
who was a coauthor of RFC 7525, the previous version of the TLS
recommendations.
See RFC 7525 for additional acknowledgments specific to the previous
version of the TLS recommendations.
Authors' Addresses
Yaron Sheffer
Intuit
Email: yaronf.ietf@gmail.com
Peter Saint-Andre
Independent
Email: stpeter@stpeter.im
Thomas Fossati
ARM Limited
Email: thomas.fossati@arm.com