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RFC 6170

Updates RFC 3709

Internet Engineering Task Force (IETF)                      S. Santesson
Request for Comments: 6170                                  3xA Security
Updates: 3709                                                 R. Housley
Category: Standards Track                                 Vigil Security
ISSN: 2070-1721                                                 S. Bajaj
                                                          Symantec Corp.
                                                            L. Rosenthol
                                                                May 2011

     Internet X.509 Public Key Infrastructure -- Certificate Image


   This document specifies a method to bind a visual representation of a
   certificate in the form of a certificate image to a public key
   certificate as defined in RFC 5280, by defining a new "otherLogos"
   image type according to RFC 3709.

Status of This Memo

   This is an Internet Standards Track document.

   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
   Internet Standards 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

Copyright Notice

   Copyright (c) 2011 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
   ( 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.

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RFC 6170                    Certificate Image                   May 2011

Table of Contents

   1. Introduction ....................................................2
      1.1. Terminology ................................................3
   2. Certificate Image ...............................................3
   3. LogotypeImageInfo ...............................................4
   4. Embedded Images .................................................5
   5. Certificate Image Formats .......................................6
      5.1. PDF ........................................................6
      5.2. SVG ........................................................6
      5.3. PNG ........................................................7
   6. Security Considerations .........................................7
   7. Acknowledgements ................................................8
   8. References ......................................................9
      8.1. Normative References .......................................9
      8.2. Informative References .....................................9
   Appendix A.  ASN.1 Module .........................................10
   Appendix B.  Example ..............................................11

1.  Introduction

   This standard specifies how to bind a certificate image to a
   certificate (defined in [RFC5280]), providing a visual representation
   of that certificate using the Logotype extension defined in [RFC3709]
   and specifying the certificate image as a new "otherLogos" type.

   The purpose of the certificate image is to aid human interpretation
   of a certificate by providing meaningful visual information to the
   user interface (UI).

   Typical situations when a human needs to examine the visual
   representation of a certificate are:

   -  A person establishes a secured channel with an authenticated
      service.  The person needs to determine the identity of the
      service based on the authenticated credentials.

   -  A person validates the signature on critical information, such as
      signed executable code, and needs to determine the identity of the
      signer based on the signer's certificate.

   -  A person is required to select an appropriate certificate to be
      used when authenticating to a service or Identity Management
      infrastructure.  The person needs to see the available
      certificates in order to distinguish between them in the selection

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RFC 6170                    Certificate Image                   May 2011

   The display of certificate information to humans is challenging due
   to lack of well-defined semantics for critical identity attributes.
   Unless the application has out-of-band knowledge about a particular
   certificate, the application will not know the exact nature of the
   data stored in common identification attributes such as serialNumber,
   organizationName, country, etc.  Consequently, the application can
   display the actual data, but faces the problem of labeling that data
   in the UI and informing the human about the exact nature (semantics)
   of that data.  It is also challenging for the application to
   determine which identification attributes are important to display
   and how to organize them in a logical order.

   RFC 3709 [RFC3709] defines a certificate extension for binding images
   to a certificate, such as a community logo and issuer logo, enhancing
   the display of certificate information.  The syntax is extensible and
   allows inclusion of new image types using the otherLogos structure.
   This standard defines how to include a complete certificate image
   using the extensibility mechanism of RFC 3709.

1.1.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

2.  Certificate Image

   This section defines the certificate image as a new otherLogos type
   according to Section 4.1 of [RFC3709].

   The certificate image otherLogos type is identified by the Object
   Identifier (OID) id-logo-certimage.

      id-pkix  OBJECT IDENTIFIER  ::=
           { iso(1) identified-organization(3) dod(6) internet(1)
           security(5) mechanisms(5) pkix(7) }

      id-logo OBJECT IDENTIFIER ::= { id-pkix 20 }

      id-logo-certimage OBJECT IDENTIFIER ::= { id-logo 3 }

   When present, the certificate image MUST be a complete visual
   representation of the certificate.  This means that the display of
   this certificate image represents all information about the
   certificate that the issuer subjectively defines as relevant to show
   to a typical human user within the typical intended use of the
   certificate, giving adequate information about at least the following
   three aspects of the certificate:

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RFC 6170                    Certificate Image                   May 2011

   -  Certificate Context

   -  Certificate Issuer

   -  Certificate Subject

   Certificate Context information is visual marks and/or textual
   information that helps the typical user to understand the typical
   usage and/or purpose of the certificate.

   It is up to the issuer to decide what information -- in the form of
   text, graphical symbols, and elements -- represents a complete visual
   representation of the certificate.  However, the visual
   representation of Certificate Subject and Certificate Issuer
   information from the certificate MUST have the same meaning as the
   textual representation of that information in the certificate itself.

   Applications providing a Graphical User Interface (GUI) to the
   certificate user MAY present a certificate image according to this
   standard in any given application interface, as the only visual
   representation of a certificate.

3.  LogotypeImageInfo

   The optional LogotypeImageInfo structure is defined in [RFC3709] and
   is included here for convenience:

     LogotypeImageInfo ::= SEQUENCE {
        type          [0] LogotypeImageType DEFAULT color,
        fileSize      INTEGER,  -- In octets
        xSize         INTEGER,  -- Horizontal size in pixels
        ySize         INTEGER,  -- Vertical size in pixels
        resolution    LogotypeImageResolution OPTIONAL,
        language      [4] IA5String OPTIONAL }  -- RFC 3066 Language Tag

   NOTE: The referenced RFC 3066 in the structure above (from RFC 3709)
         is obsolete and is currently replaced by RFC 5646 [RFC5646].
         The language tag may carry information about the language used
         to express any textual elements within the image as well as any
         audio information associated with the image.

   When the optional LogotypeImageInfo is included with a certificate
   image, the parameters shall be used with the following semantics and

   xSize and ySize represent the recommended display size for the image.
   When a value of 0 (zero) is present, no recommended display size is
   specified.  When non-zero values are present and these values differ

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RFC 6170                    Certificate Image                   May 2011

   from corresponding size values in the referenced image file, then the
   referenced image SHOULD be scaled to fit within the size parameters
   of LogotypeImageInfo, while keeping the x and y ratio intact.

   The resolution parameter is redundant for all image formats that are
   relevant for certificate images and MUST NOT be specified.

4.  Embedded Images

   The certificate image otherLogos type defined in this specification
   and all logotype types defined in RFC 3709 [RFC3709] MAY be stored
   within the logotype extension using the "data" URL scheme defined in
   RFC 2397 [RFC2397] if the logotype image is provided through direct
   addressing, i.e., the image is referenced using the LogotypeDetails

   The syntax of Logotype details defined in RFC 3709 is included here
   for convenience:

      LogotypeDetails ::= SEQUENCE {
         mediaType       IA5String, -- MIME media type name and optional
                                    -- parameters (see Section 5)
         logotypeHash    SEQUENCE SIZE (1..MAX) OF HashAlgAndValue,
         logotypeURI     SEQUENCE SIZE (1..MAX) OF IA5String }

   The syntax of the "data" URL scheme defined in RFC 2397 is included
   here for convenience:

      dataurl    := "data:" [ mediatype ] [ ";base64" ] "," data
      mediatype  := [ type "/" subtype ] *( ";" parameter )
      data       := *urlchar
      parameter  := attribute "=" value

   When including the image data in the logotype extension using the
   "data" URL scheme, the following conventions apply.

   -  The value of mediaType in LogotypeDetails MUST be identical to the
      media type value in the "data" URL.

   -  The hash of the image MUST be included in logotypeHash and MUST be
      calculated over the same data as it would have been, had the image
      been referenced through a link to an external resource.

   NOTE: As the "data" URL scheme is processed as a data source rather
         than as a URL, the image data is typically not limited by any
         URL length limit settings that otherwise apply to URLs in

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   NOTE: Implementations need to be cautious about the size of images
         included in a certificate in order to ensure that the size of
         the certificate does not prevent the certificate from being
         used as intended.

5.  Certificate Image Formats

   Implementations of this specification MUST support JPEG and GIF as
   defined in RFC 3709 [RFC3709].  In addition to these mandatory-to-
   implement formats, this specification specifies the use of the
   Portable Document Format (PDF), Scalable Vector Graphics (SVG), and
   Portable Network Graphics (PNG) as image formats.

5.1.  PDF

   A certificate image MAY be provided in the form of a Portable
   Document Format (PDF) document according to [ISO32000] and following
   the conventions defined in this section.  When a certificate image is
   formatted as a PDF document, it MUST also be formatted according to
   the profile PDF/A [ISO19005].

   When including a PDF document as a certificate image, the following
   MIME media type as specified in [RFC3778] MUST be used as mediaType
   in LogotypeDetails:


5.2.  SVG

   A certificate image MAY be provided in the form of a Scalable Vector
   Graphics (SVG) image, which MUST follow the SVG Tiny profile [SVGT]
   with the following amendments:

   -  The SVG image MUST NOT contain any Internationalized Resource
      Identifier (IRI) references to information stored outside of the
      SVG image of type B, C, or D, according to Section 14.1.4 of SVG
      Tiny 1.2 [SVGT].

   -  The SVG image MUST NOT contain any 'script' element, according to
      Section 15.2 of SVG Tiny 1.2 [SVGT].

   -  The XML structure in the SVG file MUST use <LF> (linefeed 0x0A) as
      the end-of-line (EOL) character when calculating a hash over the
      SVG image.

   The referenced SVG file MAY be provided in GZIP-compressed [RFC1952]
   form as an SVGZ file.  In this case, the extension 'svgz' is used as
   an alias for 'svg.gz' [RFC1952], i.e., octet streams of type

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   image/svg+xml, subsequently compressed with gzip as specified in
   [SVGR].  The hash over the SVGZ file is calculated over the
   decompressed SVG content with canonicalized EOL characters (<LF>) as
   specified above.

   The following MIME media type, defined in Appendix M of [SVGT], MUST
   be included as mediaType in LogotypeDetails for all SVG and SVGZ


   When the SVG image is embedded using the "data" URL scheme as defined
   in Section 4, SVG image data MUST be provided in SVGZ (GZIP
   compressed) form (i.e., it MUST NOT be provided in uncompressed SVG

   Compliant implementations of this specification SHOULD be able to
   process SVG images that are formatted according to this section.

5.3.  PNG

   If a certificate image is provided as a bitmapped image, the PNG
   [ISO15948] format SHOULD be used.

   PNG images are identified by the following mediaType in


6.  Security Considerations

   This document is based on and inherits all security considerations
   from RFC 3709 [RFC3709].  In particular, RFC 3709 discusses several
   issues a Certificate Authority (CA) should take into consideration
   when evaluating a request to issue a certificate with a certificate

   Images incorporated according to RFC 3709 provide an additional
   possibility for a CA with bad intentions or bad security procedures
   to include false, conflicting, or malicious information to relying
   parties.  Such a CA may, for example:

   -  include information in graphical form that is in conflict with
      information in provided text-based attributes or other name forms,

   -  include malicious data that could exploit known security bugs in
      common software libraries used to render graphical images.

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   This underlines the necessity for CAs to provide reliable services,
   and the relying party's responsibility and need to carefully select
   which CAs are trusted to provide public key certificates.

   This also underlines the general necessity for relying parties to use
   up-to-date software libraries to render or dereference data from
   external sources (such as certificates), to minimize risks related to
   processing potentially malicious data before the data has been
   adequately verified and validated.

   Referenced image files are hashed in order to bind the image to the
   signature of the certificate.  Some image types, such as SVG, allow
   part of the image to be collected from an external source by
   incorporating a reference to an external image file.  If this feature
   were used within a certificate image file, the hash of the image file
   would only cover the URI reference to the external image file, but
   not the referenced image data.  Clients SHOULD verify that SVGT
   images meet all requirements listed in Section 5.2 and reject images
   that contain references to external data.

   CAs issuing certificates with embedded certificate images should be
   cautious when accepting graphics from the certificate requestor for
   inclusion in the certificate if the hash algorithm used to sign the
   certificate is vulnerable to collision attacks.  In such a case, the
   accepted image may contain data that could help an attacker to obtain
   colliding certificates with identical certificate signatures.

   Certificates, and hence their certificate images, are commonly public
   objects and as such usually will not contain privacy-sensitive
   information.  However, when a certificate image that is referenced
   from a certificate contains privacy-sensitive information,
   appropriate security controls should be in place to protect the
   privacy of that information.  Details of such controls are outside
   the scope of this document.

7.  Acknowledgements

   The authors recognize valuable contributions from members of the PKIX
   working group, the CA Browser Forum, and James Manger, for their
   review and sample data.

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8.  References

8.1.  Normative References

   [RFC1952]   Deutsch, P., "GZIP file format specification version
               4.3", RFC 1952, May 1996.

   [RFC2119]   Bradner, S., "Key words for use in RFCs to Indicate
               Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2397]   Masinter, L., "The "data" URL scheme", RFC 2397, August

   [RFC3709]   Santesson, S., Housley, R., and T. Freeman, "Internet
               X.509 Public Key Infrastructure: Logotypes in X.509
               Certificates", RFC 3709, February 2004.

   [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.

   [RFC5646]   Phillips, A., Ed., and M. Davis, Ed., "Tags for
               Identifying Languages", BCP 47, RFC 5646, September 2009.

   [ISO15948]  ISO/IEC 15948:2004, "Information technology -- Computer
               graphics and image processing -- Portable Network
               Graphics (PNG): Functional specification", 2004.

   [ISO19005]  ISO 19005-1:2005, "Document management -- Electronic
               document file format for long-term preservation -- Part
               1: Use of PDF 1.4 (PDF/A-1)", 2005.

   [ISO32000]  ISO 32000-1:2008, "Document management -- Portable
               document format -- Part 1: PDF 1.7", April 2008.

   [SVGT]      W3C Recommendation, "Scalable Vector Graphics (SVG) Tiny
               1.2 Specification", December 2008.

8.2.  Informative References

   [RFC3778]   Taft, E., Pravetz, J., Zilles, S., and L. Masinter, "The
               application/pdf Media Type", RFC 3778, May 2004.

   [SVGR]      "Media Type Registration for image/svg+xml",

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Appendix A.  ASN.1 Module

   CERT-IMAGE-MODULE { iso(1) identified-organization(3) dod(6)
       internet(1) security(5) mechanisms(5) pkix(7) id-mod(0)
       id-mod-logotype-certimage(68) }


     EXPORTS ALL;   -- export all items from this module

       id-logo-certImage  OBJECT IDENTIFIER  ::=
             { iso(1) identified-organization(3) dod(6) internet(1)
             security(5) mechanisms(5) pkix(7) id-logo(20) 3 }


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Appendix B.  Example

   The following example stores an embedded svgz-encoded SVG image using
   the "data" URL scheme.


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Authors' Addresses

   Stefan Santesson
   3xA Security (
   Bjornstorp 744
   247 98 Genarp

   Russell Housley
   Vigil Security, LLC
   918 Spring Knoll Drive
   Herndon, VA  20170

   Siddharth Bajaj
   Symantec Corp.
   350 Ellis Street
   Mountain View, CA  94043

   Leonard Rosenthol
   3533 Sunset Way
   Huntingdon Valley, PA  19006

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