PKIX Working Group S. Santesson (AddTrust)
INTERNET-DRAFT R. Housley (RSA Laboratories)
Expires December 2002 T. Freeman (Microsoft)
June 2002
Internet X.509 Public Key Infrastructure
Logotypes in X.509 certificates
<draft-ietf-pkix-logotypes-03.txt>
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering
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Copyright (C) The Internet Society (2002). All Rights Reserved.
Abstract
This document specifies a certificate extension for including
logotypes in public key certificates and attribute certificates.
Please send comments on this document to the ietf-pkix@imc.org
mailing list.
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Table of Contents
1 Introduction ................................................. 3
1.1 Certificate-based Identification ......................... 4
1.2 Selection of Certificates ................................ 4
1.3 Combination of Verification Techniques ................... 5
1.4 Terminology .............................................. 6
2 Different types of logotypes in Certificates ................. 6
3 Image formats ................................................ 6
4 Logotype extension ........................................... 7
5 Type of certificates ......................................... 9
6 Use in Clients ............................................... 10
7 Security considerations ...................................... 10
8 References ................................................... 12
A ASN.1 Module ................................................. 13
B Logotype placement ........................................... 15
B.1 Qualifier ................................................ 15
B.2 Issuer and Subject Alt Names ............................. 15
B.3 New extension ............................................ 16
B.4 Conclusion ............................................... 16
C Author Addresses ............................................. 17
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1. Introduction
The basic function of a certificate is to bind a public key to the
identity of an entity (the subject). From a strictly technical
viewpoint, this goal could be achieved by signing the identity of the
subject together with its public key. However, the art of PKI has
developed certificates far beyond this functionality in order to meet
the needs of modern global networks and heterogeneous IT structures.
Certificate users must be able to determine certificate policies,
appropriate key usage, assurance level, and name form constraints.
Before a relying party can make an informed decision whether a
particular certificate is trustworthy and relevant for its intended
usage, a certificate may be examined from several different
perspectives.
Systematic processing is necessary to determine whether a particular
certificate meets the predefined prerequisites for an intended usage.
Much of the information contained in certificates is appropriate and
effective for machine processing; however, this information is not
suitable for a corresponding human trust and recognition process.
Humans prefer to structure information into categories and symbols.
Most humans associate complex structures of reality with easy
recognizable logotypes and marks. Humans tend to trust things that
they recognize from previous experiences. Humans may examine
information to confirm their initial reaction. Very few consumers
actually read all terms and conditions they accept when accepting a
service, rather they commonly act on trust derived from previous
experience and recognition.
A big part of this process is branding. Service providers and product
vendors invest a lot of money and resources into creating a strong
relation between positive user experiences and easily recognizable
trademarks, servicemarks, and logotypes.
Branding is also pervasive in identification instruments, including
identification cards, passports, driver's licenses, credit cards,
gasoline cards, and loyalty cards. Identification instruments are
intended to identify the holder as a particular person or as member
of community. The community may represent the subscribers of a
service or any other group. Identification instruments, in physical
form, commonly use logotypes and symbols, solely to enhance human
recognition and trust in the identification instrument itself. They
may also include a registered trademark to allow legal recourse for
unauthorized duplication.
Since certificates play an equivalent role in electronic exchanges,
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we examine the inclusion of logotypes in certificates. We consider
certificate-based identification and certificate selection.
1.1. Certificate-based Identification
The need for human recognition depends on the manner in which
certificates are used and whether certificates need to be visible to
human users. If certificates are to be used in open environments and
in applications that bring the user in conscious contact with the
result of a certificate-based identification process, then human
recognition is highly relevant, and it may be a necessity.
Examples of such applications include:
- Web server identification where a user identifies the owner
of the web site.
- Peer e-mail exchange in B2B, B2C, and private communications.
- Exchange of medical records, and system for medical
prescriptions.
- Unstructured e-business applications (i.e., non-EDI
applications).
- Wireless client authenticating to a service provider.
Most applications provide the human user with an opportunity to view
the results of a successful certificate-based identification process.
When the user takes the steps necessary to view these results, the
user is presented with a view of a certificate. This solution has two
major problems. First, the function to view a certificate is often
rather hard to find for a non-technical user. Second, the
presentation of the certificate is too technical and, it is not user
friendly. It contains no graphic symbols or logotypes to enhance
human recognition.
Many investigations have shown that users of today's applications do
not take the steps necessary to view certificates. This could be due
to poor user interfaces. Further, many applications are structured to
hide certificates from users. The application designers do not want
to expose certificates to users at all.
1.2. Selection of Certificates
One situation where software applications must expose human users to
certificates is when the user must select a single certificate from a
portfolio of certificates. In some cases, the software application
can use information within the certificates to filter the list for
suitability; however, the user must be queried if more than one
certificate is suitable. The human user must select one of them.
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This situation is comparable to a person selecting a suitable plastic
card from his wallet. In this situation, substantial assistance is
provided by card color, location, and branding.
In order to provide similar support for certificate selection, the
users need tools to easily recognize and distinguish certificates.
Introduction of logotypes into certificates provides the necessary
graphic.
1.3. Combination of Verification Techniques
The use of logotypes will in many cases affect the users decision to
trust and use a certificate. It is therefore important that there is
a distinct and clear architectural and functional distinction between
the processes and objectives of the systematic certificate
verification and human recognition.
Systematic certification path verification determines whether the
end-entity certificate can be verified according to defined policy.
The algorithm for this verification is specified in RFC 3280
[PKIX-1].
The systematic processing provides assurance that the certificate is
valid. It does not indicate whether the subject is entitled to any
particular information or whether the subject ought to be trusted to
perform a particular service. These are access control decisions.
Automatic processing will make some access control decisions, but
others, depending on the application context, involve the human user.
In some situations, where automated procedures have failed to
establish the suitability of the certificate to the task, the human
user is the final arbitrator of the post certificate verification
access control decisions. In the end, the human will decide whether
or not to accept an executable email attachment, to release personal
information, or follow the instructions displayed by a web browser.
This decision will often be based on recognition and previous
experience.
The distinction between systematic processing and human processing is
rather straightforward. They can be complementary. While the
systematic process is focused on certification path construction and
verification, the human acceptance process is focused on recognition
and related previous experience.
There are some situations where systematic processing and human
processing interfere with each other. These issues are discussed in
the Security Considerations section.
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1.4. 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 RFC 2119 [STDWORDS].
2. Different Types of Logotypes in Certificates
This specification defines the inclusion of three predefined logotype
types.
1) Community logotype
2) Issuer organization logotype
3) Subject organization logotype
The community logotype - is the general mark for a community. It
identifies a service concept for entity identification and
certificate issuance. Many issuers may use a community logotype to
co-brand with a global community in order to gain global recognition
of its local service provision. This type of community branding is
very common in the credit card business where local independent card
issuers include a globally recognized brand (such as VISA and
MasterCard).
Issuer organization logotype - is a logotype representing the
organization identified as part of the issuer name in the
certificate.
Subject organization logotype - is a logotype representing the
organization identified in the subject name in the certificate.
3. Logotype data
This specification defines two types of logotype data: image data and
audio data.
There is no need to significantly increase the size of the
certificate by including image and audio data of logotypes. Rather, a
URI identifying the location to the logotype data and a one-way hash
of the referenced data is included in the certificate.
Several image files, representing the same image in different
formats, sizes, and color palates, may represent each logotype image.
Several audio files may further represent the same audio sequence in
different formats and resolutions.
Each logotype present in a certificate MUST be represented by at
least one image data file.
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Applications SHOULD enhance processing and off-line functionality by
cashing logotype data.
4. Logotype extension
The logotype extension MAY be included in public key certificates
[PKIX-1] or attribute certificates [PKIX-AC]. The logotype extension
MUST be identified by the following object identifier:
id-pe-logotype OBJECT IDENTIFIER ::=
{ iso(1) identified-organization(3) dod(6) internet(1)
security(5) mechanisms(5) pkix(7) id-pe(1) 12 }
Logotype data may be referenced through either direct or indirect
addressing.
The direct addressing scheme includes information about each
logotype, but not the image and audio data files themselves, in the
certificate. This option is intended for cases where just one or a
few images and audio files are referenced.
The indirect addressing scheme includes one reference to an external
hashed data structure that contains all information on the type,
content and location of each image and audio file. This option is
intended for cases where each logotype is represented by many audio
or image files.
The logotype extension MUST have the following syntax:
LogotypeExtn ::= SEQUENCE {
communityLogo [0] LogotypeInfo OPTIONAL,
issuerLogo [1] LogotypeInfo OPTIONAL,
subjectLogo [2] LogotypeInfo OPTIONAL,
otherLogos [3] SEQUENCE OF OtherLogotypeInfo OPTIONAL }
LogotypeInfo ::= CHOICE {
direct LogotypeData,
indirect LogotypeReference }
LogotypeData ::= SEQUENCE {
image SEQUENCE OF LogotypeImage OPTIONAL,
audio SEQUENCE OF LogotypeAudio OPTIONAL }
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LogotypeImage ::= SEQUENCE {
imageFormat IA5String, -- MIME image subtype
hashAlg AlgorithmIdentifier,
imageHash OCTET STRING,
imageURI SEQUENCE OF IA5String,
imageInfo LogotypeImageInfo OPTIONAL }
LogotypeAudio ::= SEQUENCE {
audioFormat IA5String, -- MIME audio subtype
hashAlg AlgorithmIdentifier,
audioHash OCTET STRING,
audioURI SEQUENCE OF IA5String,
audioInfo LogotypeAudioInfo OPTIONAL }
LogotypeImageInfo ::= SEQUENCE {
fileSize INTEGER, -- In octets
xSize INTEGER, -- In pixels
ySize INTEGER, -- In pixels
numColors INTEGER } -- In bits
LogotypeAudioInfo ::= SEQUENCE {
fileSize INTEGER, -- In octets
playTime INTEGER, -- In milliseconds
sampleRate INTEGER, -- Samples per second
channels INTEGER, -- 1=mono, 2=stereo, 4=quad
language IA5String OPTIONAL } -- RFC 3066 Language Tag
OtherLogotypeInfo ::= SEQUENCE {
logotypeType OBJECT IDENTIFIER,
info LogotypeInfo }
LogotypeReference ::= SEQUENCE {
hashAlg AlgorithmIdentifier,
refStructHash OCTET STRING,
refStructURI SEQUENCE OF IA5String }
This extension MUST NOT be marked critical.
When using the indirect addressing scheme, the URI (refStructURI)
pointing to the external data structure MUST point to a file
containing data with the syntax LogotypeData as defined above. The
referenced file name SHOULD include a file extension of "LTD".
At least one of the optional elements in the LogotypeExtn structure
MUST be present. Avoid the use of otherLogos whenever possible.
The LogotypeReference, LogotypeImage and LogotypeAudio structures
explicitly identify the one-way hash functions employed.
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Implementations MUST support the SHA-1 [SHS] algorithm, and
implementations MAY support other one-way hash functions.
A MIME type is used to specify the format of the file containing the
logotype data. Implementations MUST support both the JPEG and GIF
image formats (with MIME types of "image/jpeg" and "image/gif",
respectively). Implementations that support audio MUST support the
MP3 audio format (with a MIME type of "audio/mpeg").
When language is specified within the LogotypeAudioInfo structure,
the language tag MUST use the RFC 3066 [LANGCODES] syntax.
The predefined logotype types are:
Community Logotype. If communityLogo is present, the logotype MUST
represent the community to which the certificate issuer is a
member. The communityLogo MAY be present in an end entity
certificate or an attribute certificate. The communityLogo MUST
NOT be present in a CA certificate.
Issuer Organization Logotype. If issuerLogo is present, the
logotype MUST represent the issuer's organization. The logotype
MUST be consistent with, and require the presence of, an
organization name stored in the organization attribute in the
issuer field (for either a public key certificate or attribute
certificate). The issuerLogo MAY be present in an end entity
certificate, a CA certificate, or an attribute certificate.
Subject Organization Logotype. If subjectLogo is present, the
logotype MUST represent the subject's organization. The logotype
MUST be consistent with, and require the presence of, an
organization name stored in the organization attribute in the
subject field (for either a public key certificate or attribute
certificate). The subjectLogo MAY be present in an end entity
certificate, a CA certificate, or an attribute certificate.
The relationship between the subject organization and the subject
organization logotype and the relationship between the issuer and
either the issuer organization logotype or the community logotype,
are relationships claimed by the issuer. The policy under which the
issuer checks these logotypes is outside the scope of this standard.
5. Type of certificates
Logotypes MAY be present in three types of certificates:
- CA certificates
- End-entity certificates
- Attribute certificates
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CA certificates include self-signed certificates (often used to
represent trust anchors) or Intermediate CA certificates.
Some types of logotypes are not permitted in CA certificates. This
ensures that logotypes are excludes from all aspects of certification
path processing. As discussed above, logotypes are not intended to be
part of certification path validation or any type of systematic
processing. The sole purpose of logotypes is to enhance display of a
particular certificate, regardless of its position in a certification
path.
Logotypes MUST NOT be an active component in certification path
processing, and they are included in public key certificates and
attribute certificates at the discretion of the certificate issuer.
6. Use in Clients
All PKI implementations require relying party software to have some
mechanism to determine whether a trusted CA issues a particular
certificate. This is an issue for certification path validation,
including consistent policy and name checking.
After a certification path is successfully validated, the replying
party trusts the information that the CA includes in the certificate,
including any certificate extensions. The client software can choose
to make use of such information, or the client software can ignore
it. Current standards do not provide any mechanism for cross-
certifying CAs to constrain subordinate CAs from including private
extensions (see the security considerations section).
Consequently, if relying party software accepts a CA, then it should
be prepared to (unquestioningly) display the associated logotypes to
its human user, given that it is configured to do so.
If the relying party software is unable to successfully validate a
particular certificate, then it MUST NOT display any logotype data
associated with that certificate.
7. Security considerations
Logotypes are very difficult to securely and accurately define. Names
are also difficult in this regard, but logotypes are even worse. It
is quite difficult to specify what is, and what is not, a legitimate
logotype of an organization. There is a whole legal structure around
this issue, and it will not be repeated here. However, issuers should
be aware of the implications of including images associated with a
trademark or servicemark before doing so.
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As logotypes can be difficult (and sometimes expensive) to verify,
this increases the possibility of errors related to assigning wrong
logotypes to organizations.
This is not a new issue for electronic identification instruments.
It is already dealt with in numerous of similar situations in the
physical world, including physical employee identification cards.
Secondly, there are situations where identification of logotypes is
rather simple and straightforward, such as logotypes for well-known
industries and institutes. These issues should not stop those service
providers who want to issue logotypes from doing so, where relevant.
The premise used for the logotype work is that logotype graphics in a
certificate are trusted only if the certificate is successfully
validated within a valid path. It is however impossible to prevent
fraudulent creation of certificates by non-validated issuers,
containing names and logotypes that the issuer has no claim to. Such
certificates could be created in an attempt to socially engineer a
user into accepting a certificate. It is thus imperative that the
representation of any certificate that fails to validate is not
enhanced in any way by using the logotype graphic.
Certification paths may also impose name constraints that are
systematically checked during certification path processing, which,
in theory, may be circumvented by logotypes.
Certificate path processing does not constrain the inclusion of
logotype data in certificates. A parent CA can constrain
certification path validation such that subordinate CAs cannot issue
valid certificates to end-entities outside a limited name space or
outside specific certificate polices. A malicious CA can comply with
these name and policy requirements and still include inappropriate
logotypes in the certificates that it issues. These certificates will
pass the certification path validation algorithm, which means the
client will trust the logotypes in the certificates. Since there is
no technical mechanism to prevent or control subordinate CAs from
including the logotype extension or its contents, where appropriate,
a parent CA could employ a legal agreement to impose a suitable
restriction on the subordinate CA. This situation is not unique to
the logotype extension.
The controls available to a parent CA to protect itself from rogue
subordinate CAs are non-technical. They include:
- Contractual agreements of suitable behavior, including
terms of liability and severance pay in case of material
breach.
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- Control mechanisms and procedures to monitor and
follow-up behavior of subordinate CAs.
- Use of certificate policies to declare assurance level
of logotype data as well as to guide applications on how
to treat and display logotypes.
- Use of revocation functions to revoke any misbehaving CA.
There is not a simple, straightforward, and absolute technical
solution. Rather, involved parties must settle some aspects of PKI
outside the scope of technical controls. As such, issuers need to
clearly identify and communicate the associated risks.
8. References
[PKIX-1] R. Housley, W. Polk, W. Ford, and D. Solo, "Internet
X.509 Public Key Infrastructure: Certificate and
Certificate Revocation List (CRL) Profile", RFC 3280,
April 2002.
[SHS] Federal Information Processing Standards Publication
(FIPS PUB) 180-1, Secure Hash Standard, 17 April 1995.
[Supersedes FIPS PUB 180 dated 11 May 1993.]
[STDWORDS] S. Bradner, "Key words for use in RFCs to Indicate
Requirement Levels", RFC 2119, March 1997.
[LANGCODES] H. T. Alvestrand, "Tags for Identification of Languages",
RFC 3066, January 2001.
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APPENDIX A. ASN.1 Module
LogotypeCertExtn
{ iso(1) identified-organization(3) dod(6) internet(1)
security(5) mechanisms(5) pkix(7) id-mod(0)
id-mod-logotype(22) }
DEFINITIONS IMPLICIT TAGS ::=
BEGIN
IMPORTS
AlgorithmIdentifier FROM PKIX1Explicit88
{ iso(1) identified-organization(3) dod(6) internet(1)
security(5) mechanisms(5) pkix(7) id-mod(0)
id-pkix1-explicit(18) };
-- Logotype Extension OID
id-pe-logotype OBJECT IDENTIFIER ::=
{ iso(1) identified-organization(3) dod(6) internet(1)
security(5) mechanisms(5) pkix(7) id-pe(1) 12 }
-- Logotype Extension Syntax
LogotypeExtn ::= SEQUENCE {
communityLogo [0] LogotypeInfo OPTIONAL,
issuerLogo [1] LogotypeInfo OPTIONAL,
subjectLogo [2] LogotypeInfo OPTIONAL,
otherLogos [3] SEQUENCE OF OtherLogotypeInfo OPTIONAL }
LogotypeInfo ::= CHOICE {
direct LogotypeData,
indirect LogotypeReference }
LogotypeData ::= SEQUENCE {
image SEQUENCE OF LogotypeImage OPTIONAL,
audio SEQUENCE OF LogotypeAudio OPTIONAL }
LogotypeImage ::= SEQUENCE {
imageFormat IA5String, -- MIME image subtype
hashAlg AlgorithmIdentifier,
imageHash OCTET STRING,
imageURI SEQUENCE OF IA5String,
imageInfo LogotypeImageInfo OPTIONAL }
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LogotypeAudio ::= SEQUENCE {
audioFormat IA5String, -- MIME audio subtype
hashAlg AlgorithmIdentifier,
audioHash OCTET STRING,
audioURI SEQUENCE OF IA5String,
audioInfo LogotypeAudioInfo OPTIONAL }
LogotypeImageInfo ::= SEQUENCE {
fileSize INTEGER, -- In octets
xSize INTEGER, -- In pixels
ySize INTEGER, -- In pixels
numColors INTEGER } -- In bits
LogotypeAudioInfo ::= SEQUENCE {
fileSize INTEGER, -- In octets
playTime INTEGER, -- In milliseconds
sampleRate INTEGER, -- Samples per second
channels INTEGER, -- 1=mono, 2=stereo, 4=quad
language IA5String OPTIONAL } -- RFC 3066 Language Tag
OtherLogotypeInfo ::= SEQUENCE {
logotypeType OBJECT IDENTIFIER,
info LogotypeInfo }
LogotypeReference ::= SEQUENCE {
hashAlg AlgorithmIdentifier,
refStructHash OCTET STRING,
refStructURI SEQUENCE OF IA5String }
-- Places to get the same "LTD" file
-- Note: The content of referenced "LTD" files is defined by the
-- LogotypeData
END
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APPENDIX B. Logotype Placement
This Appendix documents reasons and rationales behind the technical
solution selected in this standard.
Three alternatives for the placement of the logotypes in a
certificate have been considered. They are:
1. Inclusion in a policy qualifier;
2. Inclusion in Issuer and Subject Alternative names extensions;
and
3. Inclusion in a separate certificate extension.
B.1 Qualifier
This alternative would include logotype data as a newly defined
policy qualifier.
Pros:
- This solution provides a mechanism to directly control the use and
display of logotypes under a particular policy.
Cons:
- RFC 3280 [PKIX-1] recommends against use of qualifiers.
- This is generally considered to be a major hack and stretch of
semantics, since this type of data doesn't qualify a policy in any
way.
B.2 Issuer and Subject Alt Names
This solution would use the other name form to include the issuer and
community logotypes in the issuer alt name extension, and subject
organization logo in the subject alt name extension.
Pros:
- This mechanism could possibly enable cross-certifying CAs to deny
any subordinate CA the right to include logotypes in descending end
entity certificates by listing the logotypes name form in
excludedSubtrees.
Cons:
- Logotypes are not a name form and should not be treated as a
displayable name.
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- It is generally understood that it should be possible to apply
general name constraint mechanisms (as described in RFC 2459 as
well as RFC <TBD> [PKIX-1]) to names in the subject and issuer
alt name extension. This is not possible to do with logotypes
since it is not a name form.
- This split storage of logotype data into 2 different locations,
which may make life worse for applications with no interest in
logotypes.
- It is generally agreed that inclusion of logotype data by no means
should be regarded as critical data. This may interfere with the
criticality policy of the alt name extensions, especially if the
certificate has no attributes in the subject field, forcing the
subject alt name to be set to critical.
- This usage would possibly interfere with the resolution between
IETF and ITU-T regarding use of permitted subtrees.
- Since this solution may break current implementations it would
possibly block adoption of logotypes.
B.3 New extension
This solution places logotype data in a new extension.
Pros:
- This is the cleanest solution.
- This does not impact on legacy implementations.
Cons:
- This solution activates the issue whether this extension may be
abused by a CA who include logotypes (in EE certificates) that
violates the intention of a name constraints set by a chaining CA.
This issue is addressed in the security consideration section
below.
B.4 Conclusion
We must not destroy current structures. We must not create problems
or confusion.
Only the private extension solution satisfies both of these criteria.
Therefore, the private extension was selected to carry logotype
information.
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While the syntax and semantics of the X.509 public key certificate
were used in this analysis, the logotype private extension can also
be included in an X.509 attribute certificate.
APPENDIX C. Author Addresses
Stefan Santesson
AddTrust AB
P.O. Box 465
S-201 24 Malmoe
Sweden
stefan@addtrust.com
Russell Housley
RSA Laboratories
918 Spring Knoll Drive
Herndon, VA 20170
USA
rhousley@rsasecurity.com
Trevor Freeman
Microsoft Corporation
One Microsoft Way
Redmond WA 98052
USA
trevorf@microsoft.com
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Full Copyright Statement
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