INTERNET-DRAFT Stefan Santesson (3xA Security)
Intended Status: Proposed Standard Russ Housley (Vigil Security)
Updates: 3709 (once approved) Siddharth Bajaj (VeriSign)
Expires: January 27, 2011 Leonard Rosenthol (Adobe)
July 26, 2010
Internet X.509 Public Key Infrastructure - Certificate Image
<draft-ietf-pkix-certimage-10>
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Abstract
This document specifies a method to bind a visual representation of a
certificate in the form of a certificate image to a [RFC5280] public
key certificate by defining a new otherLogos image type according to
[RFC3709].
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Certificate Image . . . . . . . . . . . . . . . . . . . . . . . 4
3. LogotypeImageInfo . . . . . . . . . . . . . . . . . . . . . . . 5
4. Embedded images . . . . . . . . . . . . . . . . . . . . . . . . 6
5. Certificate Image Formats . . . . . . . . . . . . . . . . . . . 7
5.1. PDF . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
5.2. SVG . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
5.3. PNG . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
6. Security Considerations . . . . . . . . . . . . . . . . . . . . 9
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 10
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
9.1. Normative References . . . . . . . . . . . . . . . . . . 10
9.2. Informative References . . . . . . . . . . . . . . . . . 11
Appendix A - ASN.1 Module . . . . . . . . . . . . . . . . . . . . 12
Appendix B - Example . . . . . . . . . . . . . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 14
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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], 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.
Typical situations when a human needs to examine the visual
representation of a certificate are:
- A person establishes 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.
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 problem to label that data in the
UI, informing the human about the exact nature (semantics) of that
data. It is also challenging for the application to determine which
identification attribute that 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 community logo and issuer logo, enhancing
display of certificate information. The syntax is extensible and
allows inclusion of new image types using the other-Logos structure.
This standard defines how to include a complete certificate image
using the extensibility mechanism of RFC 3709.
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1.2. 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
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:
- Certificate Context
- Certificate Issuer
- Certificate Subject
Certificate Context information is visual marks and/or textual
information which 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
and graphical symbols and elements that represents a complete visual
representation of the certificate. However, The visual representation
of Subject and Issuer information from the certificate MUST have the
same meaning as the textual representation of that information in the
certificate itself.
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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
When the optional LogotypeImageInfo is included with a certificate
image, the parameters shall be used with the following semantics and
restrictions.
xSize and ySize represents recommended display size for the image.
When a value of 0 (zero) is present, no recommended display size
specified. When non-zero values are present and these values differ
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 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.
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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
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 setting that otherwise apply to URLs in
general.
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 to be used as
intended.
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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 PDF, SVG
and 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] 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 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
Graphic (SVG) image, which MUST follow the SVG Tiny profile [SVGT]
with the following amendments:
- The SVG image MUST NOT contain any 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 end-of-line (EOL) character when calculating a hash over the
SVG image.
The referenced SVG file MAY be provided in GZIP [RFC1952] compressed
form as an SVGZ file according to section 1.2 in SVG 1.1 [SVG]. Hash
over the SVGZ file is calculated over the decompressed SVG content
with canonicalized EOL characters (<LF>) as specified above.
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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 5, SVG image data SHOULD be provided in SVGZ (GZIP
compressed) form and MAY be provided in uncompressed SVG form.
Compliant implementations that process embedded SVG images MUST be
able to handle both compressed and uncompressed image data.
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 bit mapped image, the PNG
[ISO15948] format SHOULD be used.
PNG images are identified by the following mediaType in
LogotypeDetails:
image/png
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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 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. A bad performing 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.
This underlines the necessity for CAs to provide reliable services
and the relying party's responsibility and need to carefully select
which CA that is 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 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
meets all requirements of section 5.2 and reject images that contain
references to external data.
CAs issuing certificate 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 case the
accepted image may contain data that could help an attacker to obtain
colliding certificates with identical certificate signatures.
Certificates, and hence their cert images, are commonly public
objects and as such usually will not contain privacy sensitive
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information. However, when a cert 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 work group, the CA Browser Forum and James Manger
for review and sample data.
8. IANA Considerations
This document requires no actions from IANA.
9. References
9.1. Normative References
[RFC1952] P. Deutsch, "GZIP file format specification version 4.3",
RFC 1952, May 1996
[RFC2119] S. Bradner, "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997
[RFC2397] L. Masinter, 'The "data" URL scheme' RFC 2397, August 1998
[RFC3709] S. Santesson, R. Housley, 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
[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
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[SVG] W3C Recommendation, "Scalable Vector Graphics (SVG) 1.1
Specification", January 2003
[SVGT] W3C Recommendation, "Scalable Vector Graphics (SVG) Tiny
1.2 Specification", December 2008
9.2. Informative References
[RFC3778] E. Taft, J. Pravetz, S. Zilles, L. Masinter "The
application/pdf Media Type", RFC 3778, May 2004
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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,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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
VeriSign
685 East Middlefield rd
Mountain view, CA 94043
USA
Email: sbajaj@verisign.com
Leonard Rosenthol
3533 Sunset Way
Huntingdon Valley, PA 19006
USA
Email: leonardr@adobe.com
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