Traceable Anonymous Certificate
draft-ietf-pkix-tac-04
The information below is for an old version of the document that is already published as an RFC.
| Document | Type |
This is an older version of an Internet-Draft that was ultimately published as RFC 5636.
|
|
|---|---|---|---|
| Authors | Sanghwan Park , Yoojae Won , Jaeil Lee , Stephen Kent , Haeryong Park | ||
| Last updated | 2019-12-13 (Latest revision 2009-05-26) | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Intended RFC status | Experimental | ||
| Formats | |||
| Reviews | |||
| Additional resources | Mailing list discussion | ||
| Stream | WG state | (None) | |
| Document shepherd | (None) | ||
| IESG | IESG state | Became RFC 5636 (Experimental) | |
| Action Holders |
(None)
|
||
| Consensus boilerplate | Unknown | ||
| Telechat date | (None) | ||
| Responsible AD | Tim Polk | ||
| Send notices to | (None) |
draft-ietf-pkix-tac-04
Network Working Group S.Park
Internet Draft H.Park
Intended status: Experimental Y. Won
Expires: November 2009 J. Lee
KISA
S.Kent
BBN Technologies
May 22, 2009
Traceable Anonymous Certificate
draft-ietf-pkix-tac-04.txt
Status of this Memo
This Internet-Draft is submitted to IETF in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that other
groups may also distribute working documents as Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
This Internet-Draft will expire on November 22, 2009.
Copyright Notice
Copyright (c) 2009 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 (http://trustee.ietf.org/license-info).
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document.
Park Expires November 22, 2009 [Page 1]
Internet-Draft Traceable Anonymous Certificate May 2009
Abstract
Public Key Infrastructure (PKI) provides a powerful means of
authenticating individuals, organizations, and computers(e.g.,
web servers). However, when individuals use certificates to
access resources on the public Internet, there are legitimate
concerns about personal privacy, and thus there are increasing
demands for privacy enhancing techniques on the Internet.
In a PKI, an authorized entity such as a certification Authority
(CA) or a Registration Authority (RA) may be perceived, from a
privacy perspective, as a "big brother," even when a CA issues a
certificate containing a Subject name that is a pseudonym. This
is because such entities can always map a pseudonym in a
certificate they issued to the name of the real user to whom it
was issued. This document defines a practical architecture and
protocols for offering privacy for a user who requests and uses
an X.509 certificate containing a pseudonym, while still retaining
the ability to map such a certificate to the real user who
requested it. The architecture is compatible with IETF certificate
request formats such as PKCS10 [3]and CMC[4]. The architecture
separates the authorities involved in issuing a certificate: one
for verifying ownership of a private key (Blind Issuer) and the
other for validating the contents of a certificate (Anonymity
Issuer). The end-entity(EE) certificates issued under this model
are called Traceable Anonymous Certificates (TACs).
Conventions used in this document
In examples, "C:" and "S:" indicate lines sent by the client and
server respectively.
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 [1].
Table of Contents
1. Introduction..................................................3
2. General Overview..............................................5
3. Requirements..................................................5
4. Traceable Anonymous Certificate Model.........................6
5. Issuing a TAC.................................................7
Park Expires November 22, 2009 [Page 2]
Internet-Draft Traceable Anonymous Certificate May 2009
5.1. Steps in issuing a TAC...................................9
5.2. Mapping a TAC to a User's Real Identity.................16
5.3. TAC Request Message Format Profile......................18
5.3.1. PKCS10 Profile.....................................18
5.3.2. CMC Profile........................................19
6. Security Considerations......................................20
7. IANA Considerations..........................................23
8. Acknowledgments..............................................23
9. References...................................................23
9.1. Normative References....................................23
9.2. Informative References..................................23
APPENDIX A: Traceable Anonymous Certificate ASN.1 Modules.......25
APPENDIX B: TAC message exchanges over Transport Layer Security.27
Author's Addresses..............................................32
1. Introduction
A Public Key Infrastructure (PKI) typically serves to identify the
holder of a private key (to the corresponding public key in a
certificate), in a standard fashion. The public key, identity, and
related information, are signed by an entity acting as a
Certification Authority (CA) as specified in X.509 and as profiled
for use in the Internet [2]. During the past decade, PKIs have been
widely deployed to support various types of communications and
transactions over the Internet.
However, with regard to privacy on the Internet, a PKI is generally
not supportive of privacy, at least in part because of the
following issues:
- A certificate typically contains, in the Subject field the
true identity of the user to whom it was issued. This identity is
disclosed to a relying party (e.g., a web site or the recipient
of an SMIME message [20]) whenever the certificate holder presents
it in a security protocol that requires a user to present a
certificate. In some protocols, e.g., TLS, a user's certificate
is sent via an unencrypted channel, prior to establishing a
secure communication capability.
- A certificate often is published by the CA, for example in a
directory system, which may be widely accessible.
- An anonymous (end entity) certificate [9] is one that
indicates that the holder's true identity is not represented in the
subject field. (Such a certificate might more accurately be called
Park Expires November 22, 2009 [Page 3]
Internet-Draft Traceable Anonymous Certificate May 2009
pseudonymous since an X.509 certificate must contain an identifier
to comply with PKI format standards, and a CA must not issue
multiple certificates with the same Subject name to different
entities. However, we use the more common term "anonymous"
throughout this document to refer to such certificates.)
Issuance of anonymous certificates could enhance user privacy.
There is however, a need to balance privacy and accountability
when issuing anonymous certificates. If a CA/RA is unable to map
an anonymous certificate to the real user to whom it was issued,
the user might abuse the anonymity afforded by the certificate,
because there would be no recourse for relying parties.
A CA or RA generally would be able to map an anonymous certificate
to the user to whom it was issued, to avoid such problems. To do
so the CA/RA would initially identify the user and maintain a
database that relates the user's true identity to the pseudonym
carried in the certificate's Subject field.
In a traditional PKI, there is a nominal separation of functions
between a RA and a CA, but in practice these roles are often
closely coordinated. Thus either the RA or CA could, in principle,
unilaterally map an autonomous certificate to the real user
identity.
The architecture, syntax, and protocol conventions described in
this document allow anonymous certificates to be issued and used
in existing PKIs in a way that provides a balance between privacy
and a conditional ability to map an anonymous certificate to the
individual to whom it was issued.
An anonymous certificate (Traceable Anonymous Certificate) in this
document is issued by a pair of entities that operate in a split
responsibility mode: a Blind Issuer (BI) and an Anonymity Issuer (AI).
The conditional traceability offered by this model assumes strong
separation between the RA and CA roles, and employs technical means
(threshold cryptography and "blinded" signatures), to facilitate that
separation. (A blinded signature is one in which the value being
signed is not made visible to the signer, via cryptographic
means. Additional details are provided later.)
The AI has knowledge of the certificate issued to the user, but no
knowledge of the user's real identity. The BI knows the user's real
identity, but has no knowledge of the certificate issued to that user.
Only if the AI and BI collaborate can they map the TAC issued to a
user to the real identity of that user.
Park Expires November 22, 2009 [Page 4]
Internet-Draft Traceable Anonymous Certificate May 2009
2. General Overview
This section defines the notion of a traceable anonymous
certificate(briefly TAC or anonymous certificate in this document).
It is distinguished from a conventional pseudonymous certificate
[8, 9] in that a TAC containing a pseudonym in the Subject
field will be conditionally traceable (as defined that it is not
trivial to design a system that issues anonymous certificates,
consistent with Internet PKI standards, when additional
constraints are imposed, as illustrated by the following scenarios.
- If a CA issues an anonymous certificate without verifying a
true identity, it is untraceable, which provides inadequate
recourse if the user to whom the certificate was issued abuses the
anonymity it provides. (Even without the ability to trace an
anonymous certificate to the corresponding user, the certificate
can always be revoked, but this may not be a sufficient response
to abuse.)
- If a CA issues an anonymous certificate but verifies the
real identity and maintains a record of that identity, the CA can
link the pseudonym in the Subject field to the real identity, hence
a potential "big brother" problem.
- If the CA issues a certificate with a certificate containing
a user-selected Subject name, and does not verify the user's
identity, the certificate is effectively untraceable.
- If CA issues an anonymous certificate using a blind signature
(see below), the CA cannot verify the contents of the certificate,
making the certificate untraceable and essentially forgeable.
(If a CA signs a certificate without examining its content, even
after verifying a user's identity, certificates issued by the CA
are essentially forgeable.)
To address the issues described above, we extend the simple
separation-of-authority concept already defined in the RA/CA PKI
model. First we restate the requirements in a more precise and
concise fashion, and introduce a basic model for achieving the
goals from a more general perspective.
3. Requirements
This document describes a new separation-of-authority model and
protocols for certificate issuance in a way that enables issuing
Park Expires November 22, 2009 [Page 5]
Internet-Draft Traceable Anonymous Certificate May 2009
traceable anonymous certificates, while maintaining compatibility
with the standards used in existing PKIs. To do this, the
following requirements must be satisfied.
- The traceable anonymous certificate MUST be a syntactically
valid X.509 certificate in which the Subject field contains a
pseudonym.
- There must be technical means to counter a claim by a
malicious user who later denies having participated in the
activities that resulted in issuing a TAC. Specifically, when a
user is identified and requests issuance of a TAC, the mechanisms
employed MUST ensure that the user to whom the TAC is issued is the
one who requested the TAC (unless that user transfers the private
key to another party, unknown to the RA/CA).
- The traceability and revocation functions MUST support the
linkage between a user's true identity and the pseudonym in a
certificate issued to the user. Thus the solution MUST enable
determining a true identity from the anonymous certificate, upon
agreement among the authorities who collaborated to issue the
certificate.
4. Traceable Anonymous Certificate Model
A TAC is an EE certificate issued by a pair of entities that
operate in a split responsibility mode: a Blind Issuer (BI) and
an Anonymity Issuer (AI). The pair appear as a single CA to the
outside world, e.g., they are represented by a single CA
certificate. The public key in the CA certificate is used to verify
certificates issued by this CA in the normal fashion,
i.e., a relying party processes a TAC just like any other EE
certificates.
In this model the BI acts as a RA. It interacts with a user to
verify the user's "real" identity, just like a normal RA.
The BI maintains a database that can be used to map a TAC to the
user to whom it was issued, but only with the cooperation of the
AI.
This mapping will be initiated only if there is evidence that the
user to whom the TAC was issued has abused the anonymity provided
by the TAC.
Park Expires November 22, 2009 [Page 6]
Internet-Draft Traceable Anonymous Certificate May 2009
The AI acts as a CA. It validates a certificate request submitted
by the user, using a standard certificate request format such as
PKCS10. The AI performs the functions common to a CA, including a
private key proof of possession (PoP) check, a name uniqueness
check among all certificates issued by it, assignment of a serial
number, etc. To effect issuance of the TAC, the AI interacts with
the BI, over a secure channel, to jointly create the signature on
the TAC, and sends the signed TAC to the user.
The AI does this without learning the user's real identity
(either from the user or from the BI).
The result of this split functionality between the BI and the AI
is that neither can unilaterally act to reveal the real user
identity. The AI has knowledge of the certificate issued to the
user, but no knowledge of the user's real identity. The BI knows
the user's real identity, but has no knowledge of the certificate
issued to that user. Only if the AI and BI collaborate can they
map the TAC issued to a user to the real identity of that user.
This system is not perfect. For example, it assumes that the AI
and BI collaborate to reveal a user's real identity only under
appropriate circumstances. The details of the procedural security
means by which this assurance is achieved are outside the scope of
this document. Nonetheless, there are security benefits to adopting
this model described in this document, based on the technical
approach used to enable separation of the BI and AI functions.
For example, the BI and AI can be operated by different
organizations in geographically separate facilities, and managed
by different staff. As a result, one can have higher confidence
in the anonymity offered to a user by the system, as opposed to a
monolithic CA operating model that relies only on procedural
security controls to ensure anonymity.
5. Issuing a TAC
The follow subsections describe the procedures and the protocols
employed to issue a TAC. To begin, BI and AI collaborate to
generate a public key pair (that represents the CA as seen by
relying parties) using a threshold signature scheme. Such schemes
have been defined for RSA. The details of how this is accomplished
depend on the algorithm in question, and thus are not described here.
The reader is referred to [17] where procedures for implementing RSA
Park Expires November 22, 2009 [Page 7]
Internet-Draft Traceable Anonymous Certificate May 2009
threshold signatures are described. A DSA-based threshold signature
scheme will be incorporated into a future version of TAC [16].
Note that this split signing model for certificate issuance is an
especially simple case of a threshold signature; the private key
used to sign a TAC is divided into exactly two shares, one held by
the BI and one held by the AI. Both shares must be used, serially,
to create a signature on a TAC. After the key pair for the (nominal)
CA has been generated and the private key split between the BI and
the AI, the public key is published, e.g., in a self-signed
certificate that represents the TAC CA.
Another public key cryptographic function that is an essential
part of this system is called "blind signing". To create a blind
signature one party encrypts a value to be signed, e.g., a hash
value of a certificate, and passes it to the signer. The signer
digitally signs the encrypted value, and returns it to the first
party. The first party inverts the encryption it applied with the
random value in the first place, to yield a signature on the
underlying data, e.g., a hash value.
This technique enables the signer to digitally sign a message,
without seeing the content of the message. This is the simplest
approach to blind signing; it requires that the public key needed
to invert the encryption not be available to the blind signer.
Other blind signing techniques avoid the need for this restriction,
but are more complex.
The tricky part of a cryptographic blinding function is that is must
be associative and commutative, with regard to a public key signature
function. Let B be a blinding function, B-INV is its inverse, and S
is a public key signature. The following relationship must hold:
B-INV( S (B (X) ) ) = B-INV( B( S (X) ) ) = S (X). RSA can be use
to blind a value with random value and to sign a blinded value,
because the modular exponentiation operation used by RSA for both
signature and for encryption is associative and commutative.
The TAC issuance process described below requires an ability for the
BI, the AI, and the user to employ secure communication
channels between one another.
Use of TLS [19] is one suitable means to establish such channels,
although other options also are acceptable. To this end, this
document assumes TLS as the default secure communication channel,
and thus requires that the BI and the AI have X.509 certificates
that represent them.
Park Expires November 22, 2009 [Page 8]
Internet-Draft Traceable Anonymous Certificate May 2009
These certificates are independent of the certificate that
represents the CA (formed by the BI and the AI) and may be either
self-signed or issued by other CA(s).
Appendix B provides a top level description of the application of
TLS to these message exchanges.
5.1. Steps in issuing a TAC
Figure 1. depicts the procedures for issuing a TAC. The lines
represent steps in the issuance process, and the numbers refer to
these steps.
1 +---------------+
+<-------->| Blind |
| 2 | Issuer (BI)|
| +---------------+
+-------+ | ^
| user |<------------>| 4 | 5
+-------+ | v
| 3 +----------------+
+--------->| |
| | Anonymity |
| | Issuer (AI) |
+<-------- | |
6 +----------------+
Figure 1. TAC issuance Procedures
Step 1 : A user authenticates himself to BI. This may be
effected via an in-person meeting or electronically. The same
sorts of procedures that RAs use for normal certificate issuance
are used here. Such procedures are not standardized, and thus
they are not described here in detail. For purposes of the TAC
architecture, we require the BI to establish a record in a
Park Expires November 22, 2009 [Page 9]
Internet-Draft Traceable Anonymous Certificate May 2009
database for the user, and to generate a (locally) unique
identifier, called the UserKey, that will serve as a (database)
key for the record. The UserKey value MUST NOT be generated in an
fashion that permits any external entity (including the AI) to
infer a user's real identity from its value. (For example, is the
user's name is used as an input to a one-way hash algorithm to
generate the UserKey value, then additional random data must be
used as an input to prevent simple guessing attacks.) Associated
with the UserKey in this database is an expiration time. The
expiration time is used by BI and the AI to reject session-level
replay attacks in some exchanges, and to enable BI and AI to
garbage collect database records if a user initiates but does not
complete the certificate request process.
It is RECOMMENDED that the UserKey be a random or pseudo-random
value. Whenever the BI passes a UserKey to an external party, or
accepts the UserKey from an external party (e.g., the AI), the
value is embedded in digitally signed CMS object called a Token,
accompanied by the time stamp noted above. The signature on a
Token is generated by the BI. (Note that the certificate used is just
a certificate suitable for use with CMS, and is NOT the split-key
certificate used to verify TAC.)
The following ASN.1 syntax represents the UserKey and an expiration
time:
UserKey ::= OCTET STRING
Timeout ::= GeneralizedTime
In the context of this specification, GeneralizedTime value MUST
be expressed in Greenwich Mean Time (Zulu) and MUST include
seconds(YYYYMMDDHHMMSSZ).
Step 2 : BI presents to the user a data structure called a Token.
The Token must be conveyed to the user via a secure channel, e.g.,
in person or via a secure communication channel. The secure channel
is required here to prevent a wiretapper from being able to acquire
the Token. For example, if the user establishes a one-way
authenticated TLS session to the BI in Step 1, this session could
be used to pass the Token back to the user.
The Token serves two purposes. During TAC issuance, the Token
is used to verify that a request to the AI has been submitted by a
user who is registered with the BI (and thus there is a record in
BI's database with the real identity of the user). This is necessary
Park Expires November 22, 2009 [Page 10]
Internet-Draft Traceable Anonymous Certificate May 2009
to ensure that the TAC can later be traced to the user. If there is
a request to reveal the real identity of a user, the AI will release
the Token to the entity requesting that a TAC be traced, and that
entity will pass the Token to the BI, to enable tracing the TAC.
If the BI does not perform its part of the certificate issuance
procedure (in Step 6) before the Token expires, the BI can delete
the Token from the database as a means of garbage collection. The
timeout value in a Token is selected by the BI.
The Token is a ContentInfo with a contentType of id-kisa-tac-token
and a content that hold a SignedData of CMS SignedData object [6],
signed by the BI, where the eContent (EncapsulatedContentInfo) is a
SEQUENCE consisting of the UserKey and Timeout and eContentType MUST
be id-data.
EncapsulatedContentInfo ::= SEQUENCE {
eContentType ContentType, -- OBJECT IDENTIFIER : id-data
eContent [0] EXPLICIT OCTET STRING OPTIONAL }
-- DER encoded with the input of 'SEQUENCE of the UserKey and
-- Timeout'
id-data OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840)
rsadsi(113549) pkcs(1) pkcs7(7) 1 }
The signature (SignatureValue of SignerInfo) is generated using
the BI's private signature key, corresponding to the public key
present in the BI's certificate. (Note that this certificate is
just a certificate suitable for use with TLS, and is NOT the split-
key certificate used to verify a TAC.) The certificate(certificates)
MUST be present. Appendix A provides the ASN.1 syntax for the Token,
as a profiled CMS ContentInfo object. Appendix C provides the CMS
SignedData object profile for wrapping the Token.
Token ::= ContentInfo
Upon receipt of the Token, the user SHOULD verify the signature
using the BI public key and note the Timeout value to ensure that
the certificate request process is completed prior to that time.
Step 3 : The user prepares a certificate request in a standard
format, e.g., PKCS10 [3] or CMC[4]. The Subject field of the
certificate contains a pseudonym generated by the user. It is
anticipated that the CA (BI + AI) may provide software for users
to employ in constructing certificate requests.
Park Expires November 22, 2009 [Page 11]
Internet-Draft Traceable Anonymous Certificate May 2009
If so, then this software can generate a candidate Subject name
to minimize the likelihood of a collision. If the user selects a
candidate pseudonym without such support, the likelihood of a
subject name collision probably will be greater, increasing the
likelihood that the certificate request will be rejected or that
the AI will have to generate a pseudonym for the user.
After constructing the certificate request, the user sends it,
along with the Token from Step 2, to the AI, via a secure channel.
This channel MUST be encrypted and one-way authenticated, i.e.,
the user MUST be able to verify that it is communicating with the
AI, but the AI MUST NOT be able to verify the real identity of the
user. Typical use of TLS for secure web site access satisfies this
requirement. The certificate request of PKCS10 [3] or CMC[4] carries
the Token from Step 2.
The Token is carried as an attribute in a certificate request
(CertificationRequestInfo.attributes) where the attrType MUST be
id-kisa-tac below in PKCS10 format. The Token is set to
attrValues(Certificate Request Controls) where the attrType MUST be
id-kisa-tac in CMC[4] format. The TAC request message profile is
described in the section 5.3.
Step 4 : The AI, upon receipt of the certificate request
containing a Token, verifies that the request is consistent with
the processing defined for the request format (PKCS10).
If a Subject name is present, it verifies that the proposed
pseudonym is unique. The AI also verifies the signature on the
Token and, if it is valid, checks the Timeout value to reject a
replay attack based on an "timed-out" Token.
A Token with an old Timeout value is rejected out-of-hand by the
AI. (After a Token's Timeout time is reached, the AI deletes Token
from its cache.) Next, the AI compares the received Token against
a cache of recent (i.e., not "timed out"), validated Tokens. The AI
matches the resubmitted request to the original request, and responds
accordingly. For example, if a duplicate is detected, the certificate
request can be rejected as a replay.
If the Subject field contains a Subject name already issued by the
AI, the AI MUST either reject the certificate request, or
substitute a pseudonym it generates, depending on the policy of
the TAC CA. If the certificate request is acceptable, the AI
assigns a serial number and constructs a tbsCertificate (i.e.,
the final form of the certificate payload, ready to be signed).
Park Expires November 22, 2009 [Page 12]
Internet-Draft Traceable Anonymous Certificate May 2009
The AI then computes a hash over this data structure and blinds
the hash value. (The AI blinds the hash value using a key from a
public-key encryption pair where neither key is ever made public.
The other key from this pair is used by the AI in Step 6 to "un-
blind" the signed hash value.)
The AI sends the CMS ContentInfo object of TokenandBlindHash to the
BI, via a two-way authenticated and encrypted channel.
The two-way authentication and encryption is required to ensure
that the AI is sending these values to the BI, to allow the BI to
verify that the values were transmitted by the AI, and to prevent
a wiretapper from acquiring the Token. A TLS session in which
both parties employ certificates to authenticate one another is
the RECOMMENDED way to achieve this communication.
The TokenandBlindHash is a CMS ContentInfo with a contentType of id-
kisa-tac-tokenandblindhash and a content that hold a SignedData of
CMS SignedData object [6], signed by the AI, where the eContent
(EncapsulatedContentInfo) is a SEQUENCE consisting of the Token and
BlindedCertificateHash, and eContentType MUST be id-data.
EncapsulatedContentInfo ::= SEQUENCE {
eContentType ContentType, -- OBJECT IDENTIFIER : id-data
eContent [0] EXPLICIT OCTET STRING OPTIONAL }
-- DER encoded with the input of 'SEQUENCE of the Token and
-- BlindedCertificateHash'
The signature (SignatureValue of SignerInfo) is generated using
the AI's private signature key, corresponding to the public key
present in the AI's certificate. (Note that this certificate is
just a certificate suitable for use with TLS, and is NOT the split-
key certificate used to issue a TAC.) The certificate(certificates)
MUST be present.
The following ASN.1 syntax represents the Token and
BlindedCertificateHash :
Token ::= ContentInfo
BlinedCertificateHash ::= OCTET STRING
Token is the value of ContentInfo in the certificate request message
(CertificationRequestInfo.attributes) from the Step 3.
BlindedCertificateHash is the blinded hash value for the
tbsCertificate.
Park Expires November 22, 2009 [Page 13]
Internet-Draft Traceable Anonymous Certificate May 2009
Appendix A provides the ASN.1 syntax for the Token, as a profiled CMS
ContentInfo object. Appendix C provides the CMS SignedData object
profile for wrapping the Token.
TokenandBlindHash ::= ContentInfo
Step 5 : The BI receives the Token and blinded certificate
hash via the secure channel described above. First the BI verifies
the signature on the TokenandBlindHash generated by AI and then
verifies the signature on the Token to ensure that it is a
legitimate Token generated by the BI. Next, the BI checks its
database to ensure that the UserKey value from the Token is present
and that the Token has not been used to authorized issuance of a
certificate previously.
This check is performed to ensure that the BI has authenticated the
user and entered the user's real identity into the BI's database.
Each Token authorizes issuance of only one certificate, so the
check also ensures that the same Token is not used to authorized
issuance of more than one certificate. These checks ensure that
the certificate issued by the AI to this user will be traceable,
if needed.
The BI uses its share of the threshold private
signature key to sign the blinded certificate hash and returns
the CMS SignedData back to AI, where it has the 'SEQUENCE of Token
and PartiallySignedCertificateHash' in SignedData.eContent field.
The following ASN.1 syntax represents the Token and
PartiallySignedCertificateHash :
Token ::= ContentInfo
PartiallySignedCertificateHash ::= OCTET STRING
Token is the token value of the TokenandBlindHash(eContent in which
'SEQUENCE consisting of the both Token and
PartiallySignedCertificateHash') from the Step 4.
PartiallySignedCertificateHash is the signature value generated by
BI's share of the threshold private signature key on
BlindedCertificateHash from the Step 4.
The TokenandPartiallySignedCertificateHash is a CMS ContentInfo with
a contentType of id-kisa-tac-tokenandpartially and a content that
hold a SignedData of CMS SignedData object [6], signed by the BI,
Park Expires November 22, 2009 [Page 14]
Internet-Draft Traceable Anonymous Certificate May 2009
where the eContent(EncapsulatedContentInfo) is a SEQUENCE consisting
of the Token and PartiallySignedCertificateHash, and eContentType
MUST be id-data.
EncapsulatedContentInfo ::= SEQUENCE {
eContentType ContentType, -- OBJECT IDENTIFIER : id-data
eContent [0] EXPLICIT OCTET STRING OPTIONAL }
-- DER encoded with the input of 'SEQUENCE of the Token and
-- PartiallySignedCertificateHash'
The signature (SignatureValue of SignerInfo) is generated using
the BI's private signature key, corresponding to the public key
present in the BI's certificate. (Note that this certificate is
just a certificate suitable for use with TLS, and is NOT the split-
key certificate used to issue a TAC.) The certificate(certificates)
MUST be present. Appendix A provides the ASN.1 syntax for the Token,
as a profiled CMS SignedData object. Appendix C provides the CMS
SignedData object profile for wrapping the Token.
TokenandPartiallySignedCertificateHash ::= ContentInfo
Step 6 : Upon receipt of the TokenandPartiallySignedCertificateHash,
the AI verifies the signature on the PartiallySignedCertificateHash,
generated by BI and then matches the Token against its list of
outstanding requests to the BI. The AI then "un-blinds" the
blindHashValue, using the other key from the key pair employed Step 4.
This reveals the partially-signed certificate hash. The AI then
applies its part of the split private key to complete the signature
of the certificate for the user.
It records the certificate and the Token value in its database, to
enable later tracing of the certificate to the real user identity,
if needed. The AI transmits the completed certificate to the user,
via the response message from the request protocol employed by the
user in Step 3, PKCS10.
The user may now employ the certificate with any PKI-enabled
application or protocol that makes use of X.509 certificates
(consistent with the key usage, and EKU values in the certificate).
Note that the user should be prepared to accommodate delays in the
certificate issuance process. For example, a connection between the
user and the AI might fail sometime after the user submits a
certificate request at the end of Step 3 and before the AI returns
the certificate at the end of Step 6. If this happens, the user
should resubmit the request. The AI and BI retain sufficient state
Park Expires November 22, 2009 [Page 15]
Internet-Draft Traceable Anonymous Certificate May 2009
to be able to match the resubmitted request to the original request,
and respond accordingly. If the process failed in steps 5 or 6, the
AI returns an error indication to the user.
5.2. Mapping a TAC to a User's Real Identity
If a user to whom a TAC has been issued abuses the anonymity
provided by the TAC, the TAC can be traced to the identity of that
user. Mapping a TAC to a user's real identity is a four step
process, described below and illustrated in Figure 2.
C +---------------+
+<-------->| Blind |
| D | Issuer (BI)|
| +---------------+
+---------+ |
| Relying |<---------->|
| Party | |
+---------+ |
| A +----------------+
+<-------->| Anonymity |
B | Issuer (AI) |
+----------------+
Figure 2. Revealing a TAC User's Real Identity
Step A: The AI verifies the assertion by an aggrieved party
that a TAC user has abused the anonymity provided by his TAC.
The procedures used by AI to verify that such abuse has occurred
are outside the scope of this document. No protocol is defined
here for the interaction between the aggrieved party and AI.
The only technical requirement is that the TAC of the offending
Park Expires November 22, 2009 [Page 16]
Internet-Draft Traceable Anonymous Certificate May 2009
user be provided to the AI. If AI determines that there is
sufficient evidence of abuse to trace the TAC to the user, the AI
revokes the TAC, by listing its serial number on the next CRL
issued by the AI.
An AI unilaterally manages the CRL for a TAC. Because RFC 5280
implementations are not required to process indirect CRLs,
we create a second certificate for the CA, under the TAC CA. Revoked
EE certificates issued by the TAC CA are recorded on this CRL and
validated using this second CA certificate.
This CA certificate will have the cRLSign bit set in KeyUsage
extension, but not the keyCertSign bit. The private key for this
certificate will be held by the AI, so that it can issue CRLs
unilaterally.
The Subject DN will be the same in both CA certificates, which
reinforces the notion that the CRL issuer is the same entity as the
TAC issuer, and that this CRL is not an indirect CRL. Because the
CRL issuer does not issue any certificates itself, there is no
possible serial number conflict. This will be the only CA
certificate issued under the TAC CA certificate (and thus it will
be signed jointly by the BI and AI). We recommend that the CRL for
this CA certificate be similarly long-lived, as it too needs to be
signed by the BI and AI. Each EE TAC certificate MUST contain a
CRLDP that points to the CRL issued by this CA, to ensure that RPs
know to check this CRL vs. the CRL that covers only the CRL CA.
(If the AI uses OCSP [15] to convey the revocation status of TACs, an
equivalent procedure is employed.) If it is later determined that the
revocation was not warranted, a new TAC can be issued, to preserve
the anonymity of the user in future transactions.
Step B: The AI searches its database, e.g., based on the serial
number in the TAC, to locate the Token that was passed between
the AI and BI during the issuance process (Steps 5 and 6 above).
The AI passes this Token to the aggrieved party via an encrypted
and two-way authenticated channel. Encryption is required to
prevent disclosure of the Token, and two-way authentication is
required to ensure that the aggrieved party and the AI know that
they are communicating with each other. Two-way authenticated TLS
is the RECOMMENDED means of implementing this channel, though
other approaches are allowed.
Park Expires November 22, 2009 [Page 17]
Internet-Draft Traceable Anonymous Certificate May 2009
Step C and D : The aggrieved party transits the Token to the BI,
via an encrypted and two-way authenticated channel. The channel
MUST be encrypted to prevent disclosure of the Token, and two-way
authentication is required to ensure that the aggrieved party and
the BI know that they are communicating with each other.
If specified by the CP for the TAC CA, the BI will independently
determine that there is sufficient evidence of abuse to trace the
TAC to the user, before proceeding. The BI verifies its signature
on the Token, to verify that this is a Token generated by it and
presumably released to the aggrieved party by the AI. Next the BI
searches its database using the UserKey value extracted from the
Token. The BI retrieves the user's real identity and provides it
to the aggrieved party. (By requiring the aggrieved party to
interact with both the AI and the BI, the BI can verify that it is
dealing with an aggrieved party, not with the AI acting
unilaterally.)
5.3. TAC Request Message Format Profile
TAC request MAY use either of PKCS10 or CMC. An AI MUST support
PKCS10 and MAY support CMC.
5.3.1. PKCS10 Profile
This profile refines the specification in PKCS10 [3], as it relates
to TAC. A Certificate Request Message object, formatted according to
PKCS10, is passed to the AI.
This profile applies the following additional constraints to fields
that may appear in a CertificationRequestInfo:
Version
This field is mandatory and MUST have the value 0.
Subject
This field MUST be present. If the value of this field is empty,
AI will generate a subject name that is unique in the context of
certificates issued by this issuer. If the Subject
field contains a Subject name already issued by the AI, the AI
MUST either reject the certificate request, or substitute a
pseudonym it generates, depending on the policy of the TAC CA.
Park Expires November 22, 2009 [Page 18]
Internet-Draft Traceable Anonymous Certificate May 2009
SubjectPublicKeyInfo
This field specifies the subject's public key and the
algorithm with which the key is used.
Attributes
PKCS10[3] defines the attributes field as key-value pairs
where the key is an OID and the value's structure depends on the
key. The attribute field MUST include id-kisa-tac attribute, which
holds the Token and is defined below. The Attributes field MAY also
contain X509v3 Certificate Extensions and any PKCS9 [7]
extensionRequest attributes that the subscriber would like to have
included in the certificate. The profile for extensions in
certificate requests is specified in the RFC5280 [2].
5.3.2. CMC Profile
This profile refines the Certificate Request messages in Certificate
Management over CMS in CMC [4], as it relates to TACs.
A Certificate Request messages, formatted according to the
CMC[4], is passed to the AI .
With the exception of the public key related fields, the CA is
permitted to alter any requested field when issuing a corresponding
certificate.
This profile recommends the full PKI Request of the two types of PKI
requests (Simple or Full PKI Request) and the PKI Request SHOULD be
encapsulated in SignedData with an encapsulated content type of id-
cct-PKIData.
This profile applies the following additional constraints to fields
that may appear in a Certificate Request Template of CRMF[5]:
Version
This field MAY be absent, or MAY specify the request of a
Version 3 Certificate. It SHOULD be omitted.
SerialNumber
As per CRMF [5], this field is assigned by the CA and MUST
be omitted in this profile.
SigningAlgorithm
As per CRMF [5], this field is assigned by the CA and MUST
be omitted in this profile.
Park Expires November 22, 2009 [Page 19]
Internet-Draft Traceable Anonymous Certificate May 2009
Issuer
This field is assigned by the CA and MUST be omitted in
this profile.
Validity
This field MAY be omitted. If omitted, the AI will issue
a Certificate with Validity dates as determined by the TAC CA
policy. If specified, then the CA MAY override the requested
values with dates as determined by the TAC CA policy.
Subject
This field MUST be present. If the value of this
field is empty, in which case the AI MUST generate a
subject name that is unique in the context of certificates issued
by this issuer. If the Subject field contains a Subject name
already issued by the AI, the AI MUST either reject the certificate
request, or substitute a pseudonym it generates, depending on the
policy of the TAC CA.
PublicKey
This field MUST be present.
This profile also refines constraints that may appear in a
Certificate Request controls:
The Token is set to attrValues (CertRequest.controls) where the
attrType MUST be id-kisa-tac.
This profile also refines constraints that may appear in a
Certificate Request controls : The Token is set to attrValues where
the attrType MUST be id-kisa-tac.
The Certification Request Formats based on PKCS10 can be referred to
5.3.1 PKCS10 Profile.
6. Security Considerations
The anonymity provided by the architecture and protocols defined
in this document is conditional. Moreover, if the user employs
the same TAC for multiple transactions (with the same or different
parties), the transactions can be linked through the use of the
same TAC. Thus the anonymity guarantee is "weak" even though the
user's real identity is still hidden.
Park Expires November 22, 2009 [Page 20]
Internet-Draft Traceable Anonymous Certificate May 2009
To achieve stronger anonymity, a user may acquire multiple TACs,
through distinct iterations of the protocol. Since each TAC is
generated independently, it should not be possible for a relying
party to discover a link between pseudonyms unless the tracing
feature of this scheme is invoked. If the TAC has a long validity
interval, this increases the probability that the identity of a
TAC user will be discovered, e.g., as a result of linking user
transactions across multiple servers. Thus we recommend that each
TAC CA consider carefully how long the validity for a TAC
certificate should be. In the course of issuing a TAC, the AI and the
user interact directly. Thus the AI may have access to lower layer
information (e.g., an IP address) that might reveal the user's
identity. A user concerned about this sort of possible identity
compromise should use appropriate measures to conceal such
information, e.g., a network anonymity service such as Tor [insert an
informative reference to Tor].
This document makes no provisions for certificate renewal or rekey;
we recommend TAC users acquire new TACs periodically, to further
reduce the likelihood of linkage. It also may be possible to
determine the identity of a user via information carried by lower
level protocols, or by other, application-specific means.
For example, the IP address of the user might be used to identify
him/her. For this reason, we recommend that a TAC be used primarily
to access web services with anonymity. Note that the TAC
architecture described in this document is not capable of using
certificates for use with SMIME, because there is no provision to
issue two certificates (one for encryption and one for
signatures) that contain the same (anonymous) Subject name.
An analogous problem might arise if a user visits a site (and
does not conceal his/her identity), the site deposits a "cookie"
into the user's browser cache, and the user later visits a site and
employs a TAC with the presumption of anonymity.
The use of a TAC is a tool to help a user preserve anonymity,
but it is not, per se, a guarantee of anonymity. We recommend that
each TAC CA issue certificates with only one lifetime, in order to
avoid the complexity that might arise otherwise. If a TAC CA
offered certificates with different lifetimes, then it would need
to communicate this information from the BI to AI in a way that
does not unduly compromise the anonymity of the user.
This architecture uses the UserKey to link a TAC to the
corresponding real user identity. The UserKey is generated in a
fashion to ensure that it cannot be examined to determine a
user's real identity. UserKey values are maintained in two
distinct databases: the BI database maps a UserKey to a real user
Park Expires November 22, 2009 [Page 21]
Internet-Draft Traceable Anonymous Certificate May 2009
identity, and the AI database maps a TAC to a UserKey. The UserKey
is always carried in a signed data object, a Token. The Token is
signed to allow the BI to verify its authenticity, to prevent
attacks based on guessing UserKey values. The Token also carries a
Timeout value to allow the AI and BI to reject session-level
replay attacks, and to facilitate garbage collection of Ai and BI
databases.
Threshold cryptography is employed to enable strong separation of
the BI and AI functions, and to ensure that both must cooperate to
issue certificates under the aegis of a TAC CA. (The AI and BI must
ensure that the threshold cryptographic scheme they employ does not
provide an advantage to either party based on the way the key
splitting is effected.) Blind signatures are used with threshold
cryptography to preserve the separation of functions, i.e.,
to prevent the BI from learning the hash value of the TAC issued
by the AI.
Message exchanges between a user and the BI or the AI, between the
AI and BI, and between an aggrieved party and the AI and BI all make
use of secure channels. These channels are encrypted to prevent
disclosure of the Token value and of the pseudonym in the TAC
request and response and in a tracing request. The channels are
two-way authenticated to allow the AI and BI to verify their
respective identities when communication with one another, and
one-way authenticated to allow the user to verify their
identities when he communicates with them. Two-way authentication
is employed for communication between an aggrieved party and the
AI and BI, to allow all parties to verify the identity of one
another.
There is an opportunity for the AI to return the wrong UserKey to
an aggrieved party, which will result in tracing a certificate to
the wrong real user identity. This appears to be unavoidable in
any scheme of this sort, since the database maintained by the BI
is intentionally ignorant of any info relating a UserKey to a TAC.
A TAC CA MUST describe in its CP how long it will retain the data
about certificates it issued, beyond the lifetime of these
certificates. This will help a prospective TAC subject gauge the
likelihood of unauthorized of his/her identity as a result of a
compromise of this retained data. It also alerts relying parties
of the timeframe (after expiration of a certificate) in which an
alleged abuse must be brought to the attention of the AI and BI,
before the data linking a cert to the real user identity is
destroyed.
Park Expires November 22, 2009 [Page 22]
Internet-Draft Traceable Anonymous Certificate May 2009
7. IANA Considerations
This document does not require any IANA registration.
8. Acknowledgments
Tim Polk (NIST), Stefan Santesson(ACC-sec.com), Jim Schaad (Soaring
Hawk), David A. Cooper (NIST), SeokLae Lee, JongHyun Baek, SoonTae
Park (KISA), Taekyoung Kwon (Sejong Univ.), JungHee Cheon (Seoul
National Univ.), and YongDae Kim Minnesota Univ.) have significantly
contributed to work on the concept of TAC and identified security
issues. Their comments enhanced the maturity of the document.
9. References
9.1. Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[2] D. Cooper, S. Santesson, S. Farrell, S. Boeyen, R. Housley,
W. Polk, "Internet X.509 Public Key Infrastructure
Certificate and Certificate Revocation List (CRL) Profile,"
RFC 5280, May 2008.
[3] M. Nystrom, B Kaliski "PKCS10 : Certificate Request Syntax
Specification version 1.7", RFC 2986, November 2000.
[4] J. Schaad, M. Myers, M. Nystrom, B Kaliski "Certificate
Management over CMS (CMC)", RFC 5272, June 2008.
[5] J. Schadd, "Internet X.509 Public Key Infrastructure
Certificate Request Message Format(CRMF)", RFC 4211,September 2005.
[6] R. Hously, "Cryptographic Message Syntax(CMS)", RFC 3852, July
2004.
[7] M. Nystrom, B. Kaliski, "PKCS #9: Selected Object Classes and
Attribute Types Version 2.0", RFC 2985, November 2000
9.2. Informative References
[8] S. Brands, Rethinking public key infrastructures and digital
certificates - Building in Privacy, PHD thesis, Eindhoven
Institute of Technology, Eindhoven, The Netherlands, 1999.
Park Expires November 22, 2009 [Page 23]
Internet-Draft Traceable Anonymous Certificate May 2009
[9] D. Chaum, "Blind signature system," CRYPTO '83, Plenum
Press, page 153, 1984.
[10] J. Graaf and O. Carvalho, "Reflecting on X.509 and LDAP, or
How separating identity and attributes could simplify a PKI"
WSEG 2004, pp. 37-48.
[11] R. Grimm and P. Aichroth, "Privacy Protection for Signed
Media Files: A Separation-of-Duty Approach to the Lightweight
DRM (LWDRM) System," ACM MM&Sec'04, pp. 93-99, 2004
[12] R. Rivest, A. Shamir, and L. Adleman, "A method for
obtaining digital signature and public-key cryptosystems,"
Communications of the ACM, vol. 21, no. 2, pp.120-126, 1978.
[13] X.509, "Information technology - Open Systems
Interconnection - The Directory: Public-key and attribute
certificate frameworks," ITU-T Recommendation X.509, March
2000. Also avaiable at ISO/IEC 9594-8, 2001.
[14] S. Rafaeli, M. Rennhard, L. Mathy, B. Plattner, and D.
Hutchison, "An Architecture for Pseudonymous e-Commerce,"
AISB'01 Symposium on Information Agents for Electronic
Commerce, pp. 33-41, 2001.
[15] M. Myers, R. Ankney, A. Malpani, S. Galperin, C. Adams,
"X.509 Internet Public Key Infrastructure Online Certificate
Status Protocol - OCSP", June 1999.
[16] Philip MacKenzie and Michael K. Reiter, "Two-Party Generation
of DSA Signature", Crypto 2001.
[17] Shaohua Tang, "Simple Threshold RSA Signature Scheme Based on
Simple Secret Sharing", 2005
[18] Taekyoung Kwon, Jung Hee Cheon, Yongdae Kim, Jae-Il Lee,
"Privacy Protection in PKIs : A Separation of Authority
Approach", 2007
[19] T. Dierks, "The Transport Layer Security (TLS) Protocol Version
1.2", 2008
[20] B.Ramsdell, "S/MIME verson 3 Message Specification", 1999
Park Expires November 22, 2009 [Page 24]
Internet-Draft Traceable Anonymous Certificate May 2009
APPENDIX A: Traceable Anonymous Certificate ASN.1 Modules
DEFINITIONS IMPLICIT TAGS ::=
BEGIN
-- EXPORTS All
-- The types and values defined in this module are exported for
-- use in the other ASN.1 modules. Other applications may use
-- them for their own purposes.
IMPORTS
-- Imports from RFC 3280 [PROFILE], Appendix A.1
AlgorithmIdentifier, Certificate, CertificateList,
CertificateSerialNumber, Name FROM PKIX1Explicit88
{ iso(1) identified-organization(3) dod(6)
internet(1) security(5) mechanisms(5) pkix(7)
mod(0) pkix1-explicit(18) }
-- Imports from CMS
ContentInfo, SignedData FROM
CryptographicMessageSyntax2004{ iso(1)
member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-9(9)
smime(16) modules(0) cms-2004(24)}
UserKey ::= OCTET STRING
Timeout ::= GeneralizedTime
BlinedCertificateHash ::= OCTET STRING
PartiallySignedCertificateHash ::= OCTET STRING
EncapsulatedContentInfo ::= SEQUENCE {
eContentType ContentType, -- OBJECT IDENTIFIER : id-data
eContent [0] EXPLICIT OCTET STRING OPTIONAL }
id-data OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840)
rsadsi(113549) pkcs(1) pkcs7(7) 1 }
Token ::= ContentInfo
Park Expires November 22, 2009 [Page 25]
Internet-Draft Traceable Anonymous Certificate May 2009
TokenandBlindHash ::= ContentInfo
TokenandPartiallySignedCertificateHash ::= ContentInfo
id-KISA OBJECT IDENTIFIER ::= {iso(1) member-body(2) korea(410)
kisa(200004)}
id-npki OBJECT IDENTIFIER ::= {id-KISA 10}
id-attribute OBJECT IDENTIFIER ::= {id-npki 1}
id-kisa-tac OBJECT IDENTIFIER ::= {id-attribute 1}
id-kisa-tac-token OBJECT IDENTIFIER ::= { id-kisa-tac 1}
id-kisa-tac-tokenandblindbash OBJECT IDENTIFIER ::= { id-kisa-tac 2}
id-kisa-tac-tokenandpartially OBJECT IDENTIFIER ::= { id-kisa-tac 3}
END
Park Expires November 22, 2009 [Page 26]
Internet-Draft Traceable Anonymous Certificate May 2009
APPENDIX B: TAC message exchanges over Transport Layer Security
TAC message exchanges between a user and the BI or the AI, between
the AI and BI, and between an aggrieved party and the AI and BI
all make use of secure channels to prevent disclosure of the Token
value and of the pseudonym in the TAC request and response and in a
tracing request. The Transport Layer Security Protocol v1.2 [19]
(TLS) is a suitable security protocol to protect these message
exchanges and this document recommends use of TLS to protect these
exchanges. The following text describes how the handshake part of TLS
should be employed to protect each type of exchange. Note that no
specific cipher suites are specified for use here; the choice of
suites is up to the client and servers, as is commonly the case.
B.1. Message exchanges between a User and the BI or the AI
The channels between a User and the BI or the AI are one-way
authenticated to allow the user to verify their identities when he
communicates with them.
User BI or AI
ClientHello -------->
ServerHello
Certificate
<-------- ServerHelloDone
ClientKeyExchange
[ChangeCipherSpec]
Finished -------->
[ChangeCipherSpec]
<--------- Finished
TAC Message <---------> TAC Message
Figure 3. TAC Message exchanges between a User and the BI or the AI
B.2. Message exchanges between the BI and the AI
The channels between the BI and the AI are two-way authenticated to
allow the AI and BI to verify their respective identities when
communication with one another.
Park Expires November 22, 2009 [Page 27]
Internet-Draft Traceable Anonymous Certificate May 2009
BI AI
ClientHello -------->
ServerHello
Certificate
CertificateRequest
<-------- ServerHelloDone
Certificate
ClientKeyExchange
CertificateVerify
[ChangeCipherSpec]
Finished -------->
[ChangeCipherSpec]
<--------- Finished
TAC Message <---------> TAC Message
Figure 4. TAC Message exchanges between BI and AI
B.3. Message exchanges between the aggrieved party and the AI and or
the BI
The channels between a User and the BI or the AI are two-way
authenticated, to allow both parties to verify the identity of one
another.
User BI or AI
ClientHello -------->
ServerHello
Certificate
CertificateRequest
<-------- ServerHelloDone
Certificate
ClientKeyExchange
CertificateVerify
[ChangeCipherSpec]
Finished -------->
[ChangeCipherSpec]
<--------- Finished
TAC Message <---------> TAC Message
Figure 5. TAC Message exchanges between a aggrieved party and the
BI or the AI
Park Expires November 22, 2009 [Page 28]
Internet-Draft Traceable Anonymous Certificate May 2009
APPENDIX C: Cryptographic Message Syntax Profile for TAC Token.
Using the Cryptographic Message Syntax(CMS)[6], TAC Token is a type
of signed-data object. The general format of a CMS object is :
ContentInfo ::= SEQUENCE {
contentType ContentType,
content [0] EXPLICIT ANY DEFINED BY contentType }
ContentType ::= OBJECT IDENTIFIER
As a TAC is a signed-data object, it uses the corresponding OID,
1.2.840.113549.1.1.2.
C.1. Signed-Data Content Type
According to the CMS specification, the signed-data content type
shall have ASN.1 type SignedData:
SignedData ::= SEQUENCE {
version CMSVersion,
digestAlgorithms DigestAlgorithmIdentifiers,
encapContentInfo EncapsulatedContentInfo,
certificates [0] IMPLICIT CertificateSet OPTIONAL,
crls [1] IMPLICIT RevocationInfoChoices OPTIONAL,
signerInfos SignerInfos }
DigestAlgorithmIdentifiers ::= SET OF DigestAlgorithmIdentifier
SignerInfos ::= SET OF SignerInfo
The elements of the signed-data content type are as follows:
Version
The version is the syntax version number. It MUST be 3,
corresponding to the signerInfo structure having version number 3.
digestAlgorithms
This field specifies digest Algorithms.
encapContentInfo
This element is defined in Appendix C.1.1.
Park Expires November 22, 2009 [Page 29]
Internet-Draft Traceable Anonymous Certificate May 2009
certificates
The certificates element MUST be included and MUST
contain only the single PKI EE certificate needed to validate this
CMS Object. The CertificateSet type is defined in section 10 of
RFC3852 [6].
crls
The crls element MUST be omitted.
signerInfos
This element is defined in Appendix C.1.2.
C.1.1. encapContentInfo
encapContentInfo is the signed content, consisting of a content type
identifier and the content itself.
EncapsulatedContentInfo ::= SEQUENCE{
eContentType ContentType,
eContent [0] EXPLICIT OCTET STRING OPTIONAL }
ContentType ::= OBJECT IDENTIFIER
The elements of this signed content type are as follows:
eContentType
The ContentType for an TAC Token is id-data and has the
numerical value of 1.2.840.113549.1.7.1.
eContent
The content of an TAC Token is the DER encoded with the
input of 'SEQUENCE of UserKey and Timeout'.
C.1.2. signerInfos
SignerInfo is defined under CMS as:
SignerInfo ::= SEQUENCE {
version CMSVersion,
sid SignerIdentifier,
digestAlgorithm DigestAlgorithmIdentifier,
signedAttrs [0] IMPLICIT SignedAttributes OPTIONAL,
signatureAlgorithm SignatureAlgorithmIdentifier,
Park Expires November 22, 2009 [Page 30]
Internet-Draft Traceable Anonymous Certificate May 2009
signature SignatureValue,
unsignedAttrs [1] IMPLICIT UnsignedAttributes OPTIONAL }
The content of the SignerInfo element are as follows:
Version
The version number MUST be 3, corresponding with the
choice of SubjectKeyIdentifier for the sid.
Sid
The sid is defined as:
SignerIdentifier ::= CHOICE {
issuerAndSerialNumber IssuerAndSerialNumber,
subjectKeyIdentifier [0] SubjectKeyIdentifier }
For a TAC Token, the sid MUST be a SubjectKeyIdentifier.
digestAlgorithm
This field specifies digest Algorithms.
signedAttrs
The signedAttr element MUST be omitted.
SignatureAlgorithm
This field specifies the signature Algorithm.
Signature
The signature value is defined as:
SignatureValue ::= OCTET STRING
The signature characteristics are defined by the digest
and signature algorithms.
UnsignedAttrs
unsignedAttrs MUST be omitted.
Park Expires November 22, 2009 [Page 31]
Internet-Draft Traceable Anonymous Certificate May 2009
Author's Addresses
SangHwan Park
Korea Information Security Agency
78, Garak-Dong, Songpa-Gu, Seoul, Korea
Email: shpark@kisa.or.kr
Haeryong Park
Korea Information Security Agency
78, Garak-Dong, Songpa-Gu, Seoul, Korea
Email: hrpark@kisa.or.kr
YooJae Won
Korea Information Security Agency
78, Garak-Dong, Songpa-Gu, Seoul, Korea
Email: yjwon@kisa.or.kr
JaeIl Lee
Korea Information Security Agency
78, Garak-Dong, Songpa-Gu, Seoul, Korea
Email: jilee@kisa.or.kr
Stephen Kent
BBN Technologies
10 Moulton Street Cambridge, MA 02138
Email: kent@bbn.com
Park Expires November 22, 2009 [Page 32]