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RFC 6170

Internet X.509 Public Key Infrastructure -- Certificate Image

Pages: 12
Proposed Standard
Updates:  3709

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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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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 process.
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   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", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 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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   -  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 restrictions. 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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   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 structure. 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 general.
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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: application/pdf

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 LogotypeDetails: image/png

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 image. 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, and - 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 1998. [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) } DEFINITIONS EXPLICIT TAGS ::= BEGIN 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 } END
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Appendix B. Example

The following example stores an embedded svgz-encoded SVG image using the "data" URL scheme. data:image/svg+xml;base64,H4sICLXutU0AA0NlcnRJbWFnZURlbW8uc3ZnANVa W2/bOBZ+n19BqBigwdoS7xK9jmeapB0EWHQHzez2WZZoR1tZMiQ5jvvr95CSL7Gl1E m8C9d9iERSPOd85+O5EB3+9jhL0YMuyiTPLh3iYgfpLMrjJJteOv/661M/cFBZhVkc pnmmL50sd34b/TIsH6YoiS+da11UySSJwkqj21k41Q6CDbNyUMSTS+e+quYDz1sul+ 6SuXkx9YhSysPUo7QPK/rlKqvCx35Wvmu+a/uGYow9EOigh0Qvr/LHSwcjjDjGiGHQ 914n0/sKlMf4Vwctk7i6X7/sGEYdNA5L/WeRT5IUDKmSbLVWNoo2cqNCh1XyoKN8Ns uz0iqwVW8Qb1fOF0Vqp+PI06me6awqPeISzxn9goYzXYVxWIUWpfWLCMwcGoLpgy83 n8wzGkbR4GtefENmMBznC7DEroKpOBpM8mIWVqPEYGtA+BvoMfS2E5uF1Wqu7R6FLv NFEelWReNolpiV3l2VpGntMW9nk6RKdf0+9BrFrMbeVuWhtzbHvMR6UlobPyVpBWjX Bk7six2vH5nCwY6nXCo5xb7YusvFVPqCOGh16fSxSxglmPkScLfvmDDmC4FlDc1wov 8IF2WZhNlVumgEPRliimDD3PhGPyTgUUMC6lKqKAjxaptq1boUJvQFsvi+LOJyxZkP E/vCwHuAmXmoj1AarnRBatzqkbv7cK5Ls2ORfwM/vsOG5lURZqXxOnDXPKZw5t5jVz IhFKO0B6D6hARSXDR6Fzqq7H7mQeJAOQiUSPvFIrUHOfuui3zrFI5dYVeAmpcOcOb9 u63vLjae4kYX4yRifYPrTa2SlMigYdO+cEWeGADMLZLH96SH4R9xRYApl6q3Y02f+N zlRAl+cZSKhB6qSIVa80fsqMnWOqZJpmsXwAPoyNaQ95uNIGasKPwhxGzQzOXzMIIz BKabmLIil470zfSjWWn+kvpvLQ9g1l3yRIc8gukz0uysEcakcDfy3KMk+l0SOXlOop ltJL7EPtUlzZfP4tnM70k8xkKCySt92MwfIXPoTe0pnu4dYbp7hJ/kxWySN0ey0o/1 qbiCsxDXJMWWo37QekBcAUFPSGkPCnUJF5wwBacDK5cGlEp4BC2lYoJcrNNGVc7DzI qxT4CKsPlrAG8mL8whRejiQe9EmImIAoz3sds9NxP4RZEzugqzb7c3Q89u3WQKY9ae gbsA/AUJB/bJs6pfJt9BHFEuk5DWITzOH5uZSThLUsDjQ5GE6RMsyihMTaQLfA6BIi AQMAhnHHN1sd61WtUhDVJiuhkrdBXd740+hLB9Vm1HjQe4ywLOBLWOMMiyQAXNB8sm 9Gx2qdGgGkMG6wY8aLfqgH4dfnmrVc+pPrE/Z/QnZOs8C1Okb2/ggwLdxlDC1D6DFP ZDD98txv8xQf5TEc7Ax6ZyaDf6BC4SylWKCMqtizp80+UMchATal63qHq0M3ZTs83O b/XO6LYsFzpGVY5+iLxdWvwY+NaKoR/0iJIXL3dBjT2hG+wO+NXm53XStSh1eogfeo jV35BTOaqh/cmPUe2Mdp91pQp2CjWOO2k7OamhjU1HB3DLGm66n6iajz4bqn2oICmN FxDR/x2mC5s+rKhlkUA3Ne3P8lgP0qJfjf9uvu+HWXSfFwNoH4uqGUmTadYMtOc7yj EEd9EUhkwEEOcDSHKQ+yhnSvUYRH8miQo2FK5TCjWZZGWKB8iHPud16wApnCvTOzjI FAj9TQdCxa+ddOTizaa1xJvD0qMrKx+Ydaj6iwJQG0vaSdYWpTv4HwVRAP3Z6ONjOJ unEIeKRVmhujpA2+wPmQR9WFQAFhh9bGQzFEXX+WwOnXq8pV35P2Acdn0pGebcMg7O gQKaEdOKEAkFlk/9HuEKGBVwucc4AjnJ/LBYU09hVwWY1F0HlBUC2lbyIuYF58O8p+ adMwUt9YAoX/IwRtAC9NAdBAyGuEB3VR59u8/TGYx9/Xjz8bPB/Z/F9B0SghBK+4xx fiwtr0GXECqedQQ9PRVpEAQ+26MidbGSmPm8RwRzcQsT17EPSmoorH3+av4Jcj78O/ vIp/uzMEkHKAE6/F7VHHSj8HddR0Q3ymcGZfRVjwfmOnNn3GuWR+FzhcPmPqiptHca yacT28T8j3Cs0/LQCwo6J2iYxP4R58AsobjFegusoJhuq7VNS2evRPcqASvQki+gbk BYwETNPt/1A2pT6UErR1zMzUITZRvF5Lp5basO1fk2U4aBSjkji8quL3cDyW7TpI3u nxezMcSTNhQJhfpGctKgKN2Amo7/7ShSev4oXicPSYS+6GkCm9a1Qw3VEchCUA+z5H tTcbQhK6F14YFUp+Yn7WgmzwpZCDf5DDiXT9B7U6RdHAHpdb7IqmLVjqZSLnTW61zj Q7/G7D3hm9E846uTDZoNMADmLlm7IG2ieXfUtu1US9TeNGUHibE9Nv//2jRJGZfQmK 3v7ykJJOv1IXjBsDCPpmgWppe6sHxR3KVSQKqp+WIqammuJbtqkxZmMHry4oS/9pLh dCXKq8uR0R+LDEqCKRxqc5VXdvPvIP+ggwR0RkyBfO9iKZvrWGAKVdz31cuocvoO/q emClFMYEFEH7oI+vpkek4s4bCMBqK+5mHQUlDpE/oylpy+2/6pWXK31PEYagP04epV 1cE50UMy6IQZeQM7+Ol74Z+eHfpHNc7OjffQ/HeV0X8BopoDkGEkAAA=
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Authors' Addresses

Stefan Santesson 3xA Security ( Bjornstorp 744 247 98 Genarp Sweden EMail: Russell Housley Vigil Security, LLC 918 Spring Knoll Drive Herndon, VA 20170 USA EMail: Siddharth Bajaj Symantec Corp. 350 Ellis Street Mountain View, CA 94043 USA EMail: Leonard Rosenthol 3533 Sunset Way Huntingdon Valley, PA 19006 USA EMail: