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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
Adobe
May 2011
Internet X.509 Public Key Infrastructure -- Certificate Image
Abstract
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
http://www.rfc-editor.org/info/rfc6170.
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
(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.
Santesson, et al. Standards Track [Page 1]
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
process.
Santesson, et al. Standards Track [Page 2]
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",
"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:
Santesson, et al. Standards Track [Page 3]
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
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
Santesson, et al. Standards Track [Page 4]
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
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.
Santesson, et al. Standards Track [Page 5]
RFC 6170 Certificate Image May 2011
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
Santesson, et al. Standards Track [Page 6]
RFC 6170 Certificate Image May 2011
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
images:
image/svg+xml
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
form).
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.
Santesson, et al. Standards Track [Page 7]
RFC 6170 Certificate Image May 2011
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.
Santesson, et al. Standards Track [Page 8]
RFC 6170 Certificate Image May 2011
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",
http://dev.w3.org/SVG/profiles/1.1F2/master/mimereg.html.
Santesson, et al. Standards Track [Page 9]
RFC 6170 Certificate Image May 2011
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
Santesson, et al. Standards Track [Page 10]
RFC 6170 Certificate Image May 2011
Appendix B. Example
The following example stores an embedded svgz-encoded SVG image using
the "data" URL scheme.
data:image/svg+xml;base64,H4sICLXutU0AA0NlcnRJbWFnZURlbW8uc3ZnANVa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Santesson, et al. Standards Track [Page 11]
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Authors' Addresses
Stefan Santesson
3xA Security (AAA-sec.com)
Bjornstorp 744
247 98 Genarp
Sweden
EMail: sts@aaa-sec.com
Russell Housley
Vigil Security, LLC
918 Spring Knoll Drive
Herndon, VA 20170
USA
EMail: housley@vigilsec.com
Siddharth Bajaj
Symantec Corp.
350 Ellis Street
Mountain View, CA 94043
USA
EMail: siddharthietf@gmail.com
Leonard Rosenthol
3533 Sunset Way
Huntingdon Valley, PA 19006
USA
EMail: leonardr@adobe.com
Santesson, et al. Standards Track [Page 12]