Internet Engineering Task Force (IETF) S. Turner
Request for Comments: 5752 IECA
Category: Standards Track J. Schaad
ISSN: 2070-1721 Soaring Hawk
January 2010
Multiple Signatures in Cryptographic Message Syntax (CMS)
Abstract
Cryptographic Message Syntax (CMS) SignedData includes the SignerInfo
structure to convey per-signer information. SignedData supports
multiple signers and multiple signature algorithms per signer with
multiple SignerInfo structures. If a signer attaches more than one
SignerInfo, there are concerns that an attacker could perform a
downgrade attack by removing the SignerInfo(s) with the 'strong'
algorithm(s). This document defines the multiple-signatures
attribute, its generation rules, and its processing rules to allow
signers to convey multiple SignerInfo objects while protecting
against downgrade attacks. Additionally, this attribute may assist
during periods of algorithm migration.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc5752.
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Turner & Schaad Standards Track [Page 1]RFC 5752 Multiple Signatures in S/MIME January 2010
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Table of Contents
1. Introduction ....................................................3
1.1. Conventions Used in This Document ..........................3
2. Rationale .......................................................3
2.1. Attribute Design Requirements ..............................4
3. Multiple Signature Indication ...................................5
4. Message Generation and Processing ...............................6
4.1. SignedData Type ............................................6
4.2. EncapsulatedContentInfo Type ...............................7
4.3. SignerInfo Type ............................................7
4.4. Message Digest Calculation Process .........................7
4.4.1. multiple-signatures Signed Attribute Generation .....7
4.4.2. Message Digest Calculation Process ..................7
4.5. Signature Generation Process ...............................8
4.6. Signature Verification Process .............................8
5. Signature Evaluation Processing .................................8
5.1. Evaluation of a SignerInfo Object ..........................9
5.2. Evaluation of a SignerInfo Set .............................9
5.3. Evaluation of a SignedData Set ............................10
6. Security Considerations ........................................11
7. References .....................................................11
7.1. Normative References ......................................11
7.2. Informative References ....................................12
Appendix A. ASN.1 Module...........................................13
Appendix B. Background.............................................15
B.1. Attacks....................................................15
B.2. Hashes in CMS..............................................15
Turner & Schaad Standards Track [Page 2]RFC 5752 Multiple Signatures in S/MIME January 2010
1. Introduction
The Cryptographic Message Syntax (CMS; see [CMS]) defined SignerInfo
to provide data necessary for relying parties to verify the signer's
digital signature, which is also included in the SignerInfo
structure. Signers include more than one SignerInfo in a SignedData
if they use different digest or signature algorithms. Each
SignerInfo exists independently and new SignerInfo structures can be
added or existing ones removed without perturbing the remaining
signatures.
The concern is that if an attacker successfully attacked a hash or
signature algorithm, the attacker could remove all SignerInfo
structures except the SignerInfo with the successfully attacked hash
or signature algorithm. The relying party is then left with the
attacked SignerInfo even though the relying party supported more than
just the attacked hash or signature algorithm.
A solution is to have signers include a pointer to all the signer's
SignerInfo structures. If an attacker removes any SignerInfo, then
relying parties will be aware that an attacker has removed one or
more SignerInfo objects.
Note that this attribute ought not be confused with the
countersignature attribute (see Section 11.4 of [CMS]) as this is not
intended to sign over an existing signature. Rather, it is to
provide a pointer to additional signatures by the signer that are all
at the same level. That is, countersignature provides a serial
signature while the attribute defined herein provides pointers to
parallel signatures by the same signer.
1.1. Conventions Used in This Document
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. Rationale
The rationale for this specification is to protect against downgrade
attacks that remove the 'strong' signature to leave the 'weak'
signature, which has presumably been successfully attacked. If a CMS
SignedData object has multiple SignerInfo objects, then the attacker,
whether it be Alice, Bob, or Mallory, can remove a SignerInfo object
without the relying party being aware that more than one was
generated.
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RFC 5752 Multiple Signatures in S/MIME January 2010
Removal of a SignerInfo does not render the signature invalid nor
does it constitute an error. In the following scenario, a signer
generates a SignedData with two SignerInfo objects, one with a 'weak'
algorithm and one with a 'strong' algorithm; there are three types of
relying parties:
1) Those that support only a 'weak' algorithm. If both SignerInfo
objects are present, the relying party processes the algorithm it
supports. If both SignerInfo objects are not present, the relying
party can easily determine that another SignerInfo has been
removed, but not changed. In both cases, if the 'weak' signature
verifies, the relying party MAY consider the signature valid.
2) Those that support only a 'strong' algorithm. If both SignerInfo
objects are present, the relying party processes the algorithm it
supports. If both SignerInfo objects are not present, the relying
party can easily determine that another SignerInfo has been
removed, but the relying party doesn't care. In both cases, if
the 'strong' signature verifies, the relying party MAY consider
the signature valid.
3) Those that support both a 'weak' algorithm and a 'strong'
algorithm. If both SignerInfo objects are present, the relying
party processes both algorithms. If both SignerInfo objects are
not present, the relying party can easily determine that another
SignerInfo has been removed. In both cases, if the 'strong'
and/or 'weak' signatures verify, the relying party MAY consider
the signature valid. (Policy may dictate that both signatures are
required to validate if present.)
Local policy MAY dictate that the removal of the 'strong' algorithm
results in an invalid signature. See Section 5 for further
processing.
2.1. Attribute Design Requirements
The attribute will have the following characteristics:
1) Use CMS attribute structure;
2) Be computable before any signatures are applied;
3) Contain enough information to identify individual signatures
(i.e., a particular SignerInfo); and
4) Contain enough information to resist collision, preimage, and
second preimage attacks.
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3. Multiple Signature Indication
The multiple-signatures attribute type specifies a pointer to a
signer's other multiple-signatures attribute(s). For example, if a
signer applies three signatures, there must be two attribute values
for multiple-signatures in each SignerInfo. The 1st SignerInfo
object points to the 2nd and 3rd SignerInfo objects. The 2nd
SignerInfo object points to the 1st and 3rd SignerInfo objects. The
3rd SignerInfo object points to the 1st and 2nd SignerInfo objects.
The multiple-signatures attribute MUST be a signed attribute. The
number of attribute values included in a SignerInfo is the number of
signatures applied by a signer less one. This attribute is multi-
valued, and there MAY be more than one AttributeValue present. The
following object identifier identifies the multiple-signatures
attribute:
id-aa-multipleSignatures OBJECT IDENTIFIER ::= {
iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs9(9)
id-aa(16) 51 }
multiple-signatures attribute values have the ASN.1 type
MultipleSignatures:
MultipleSignatures ::= SEQUENCE {
bodyHashAlg DigestAlgorithmIdentifier,
signAlg SignatureAlgorithmIdentifier,
signAttrsHash SignAttrsHash,
cert ESSCertIDv2 OPTIONAL}
SignAttrsHash ::= SEQUENCE {
algID DigestAlgorithmIdentifier,
hash OCTET STRING }
The fields in MultipleSignatures have the following meaning:
- bodyHashAlg includes the digest algorithmIdentifier for the
referenced multiple-signatures attribute.
- signAlg includes the signature algorithmIdentifier for the
referenced multiple-signatures attribute.
- signAttrsHash has two fields:
-- algId MUST match the digest algorithm for the SignerInfo in
which this multiple-signatures attribute value is placed.
-- hash is the hash value of the signedAttrs (see Section 4.3).
Turner & Schaad Standards Track [Page 5]RFC 5752 Multiple Signatures in S/MIME January 2010
- cert is optional. It identities the certificate used to sign the
SignerInfo that contains the other multiple-signatures
attribute(s). It MUST be present if the fields in the other
multiple-signatures attribute(s) are the same.
The following is an example:
SignedData
DigestAlg=sha1,sha256
SignerInfo1 SignerInfo2
digestAlg=sha1 digestAlg=sha256
signatureAlg=dsawithsha1 signatureAlg=ecdsawithsha256
signedAttrs= signedAttrs=
signingTime1 signingTime1
messageDigest1 messageDigest2
multiSig1= multiSig2=
bodyHash=sha256 bodyHash=sha1
signAlg=ecdsawithsha256 signAlg=dsawithsha1
signAttrsHash= signAttrsHash=
algID=sha1 algID=sha256
hash=value1 hash=value2
4. Message Generation and Processing
The following are the additional procedures for message generation
when using the multiple-signatures attribute. These paragraphs track
with Sections 5.1-5.6 in [CMS].
4.1. SignedData Type
The following steps MUST be followed by a signer when generating
SignedData:
- The signer MUST indicate the CMS version.
- The signer SHOULD include the digest algorithm used in
SignedData.digestAlgorithms, if the digest algorithm's identifier
is not already present.
- The signer MUST include the encapContentInfo. Note that the
encapContentInfo is the same for all signers in this SignedData.
- The signer SHOULD add certificates sufficient to contain
certificate paths from a recognized "root" or "top-level
certification authority" to the signer, if the signer's
certificates are not already present.
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- The signer MAY include the Certificate Revocation Lists (CRLs)
necessary to validate the digital signature, if the CRLs are not
already present.
- The signer MUST:
-- Retain the existing signerInfo objects.
-- Include their signerInfo object(s).
4.2. EncapsulatedContentInfo Type
The procedures for generating EncapsulatedContentInfo are as
specified in Section 5.2 of [CMS].
4.3. SignerInfo Type
The procedures for generating SignerInfo are as specified in Section
4.4.1 of [CMS] with the following addition:
The signer MUST include the multiple-signatures attribute in
signedAttrs.
4.4. Message Digest Calculation Process
4.4.1. multiple-signatures Signed Attribute Generation
The procedure for generating the multiple-signatures signed attribute
is as follows:
1) All other signed attributes are placed in the respective
SignerInfo structures, but the signatures are not yet computed for
the SignerInfo.
2) The multiple-signatures attributes are added to each of the
SignerInfo structures with the SignAttrsHash.hash field containing
a zero-length octet string.
3) The correct SignAttrsHash.hash value is computed for each of the
SignerInfo structures.
4) After all hash values have been computed, the correct hash values
are placed into their respective SignAttrsHash.hash fields.
4.4.2. Message Digest Calculation Process
The remaining procedures for generating the message-digest attribute
are as specified in Section 5.4 of [CMS].
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4.5. Signature Generation Process
The procedures for signature generation are as specified in Section
5.5 of [CMS].
4.6. Signature Verification Process
The procedures for signature verification are as specified in Section
5.6 of [CMS] with the following addition:
If the SignedData signerInfo includes the multiple-signatures
attribute, the attribute's values must be calculated as described in
Section 4.4.1.
For every SignerInfo to be considered present for a given signer, the
number of MultipleSignatures AttributeValue(s) present in a given
SignerInfo MUST equal the number of SignerInfo objects for that
signer less one and the hash value present in each of the
MultipleSignatures AttributeValue(s) MUST match the output of the
message digest calculation from Section 4.4.1 for each SignerInfo.
The hash corresponding to the n-th SignerInfo must match the value in
the multiple-signatures attribute that points to the n-th SignerInfo
present in all other SignerInfo objects.
5. Signature Evaluation Processing
This section describes recommended processing of signatures when
there are more than one SignerInfo present in a message. This may be
due to either multiple SignerInfo objects being present in a single
SignedData object or multiple SignerData objects embedded in each
other.
The text in this section is non-normative. The processing described
is highly recommended, but is not forced. Changes in the processing
that have the same results with somewhat different orders of
processing is sufficient.
Order of operations:
1) Evaluate each SignerInfo object independently.
2) Combine the results of all SignerInfo objects at the same level
(i.e., attached to the same SignerData object).
3) Combine the results of the nested SignerData objects. Note that
this should ignore the presence of other CMS objects between the
SignedData objects.
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5.1. Evaluation of a SignerInfo Object
When evaluating a SignerInfo object, there are three different pieces
that need to be examined.
The first piece is the mathematics of the signature itself (i.e., can
one actually successfully do the computations and get the correct
answer?). This result is one of three results. The mathematics
succeeds, the mathematics fails, or the mathematics cannot be
evaluated. The type of things that lead to the last state are non-
implementation of an algorithm or required inputs, such as the public
key, are missing.
The second piece is the validation of the source of the public key.
For CMS, this is generally determined by extracting the public key
from a certificate. The certificate needs to be evaluated. This is
done by the procedures outlined in [PROFILE]. In addition to the
processing described in that document, there may be additional
requirements on certification path processing that are required by
the application in question. One such set of additional processing
is described in [SMIME-CERT]. One piece of information that is part
of this additional certificate path processing is local and
application policy. The output of this processing can actually be
one of four different states: Success, Failure, Indeterminate, and
Warning. The first three states are described in [PROFILE]; Warning
would be generated when it is possible that some information is
currently acceptable, but may not be acceptable either in the near
future or under some circumstances.
The third piece of the validation is local and application policy as
applied to the contents of the SignerInfo object. This would cover
such issues as the requirements on mandatory signed attributes or
requirements on signature algorithms.
5.2. Evaluation of a SignerInfo Set
Combining the results of the individual SignerInfo objects into a
result for a SignedData object requires knowledge of the results for
the individual SignerInfo objects, the required application policy,
and any local policies. The default processing if no other rules are
applied should be:
1) Group the SignerInfo objects by the signer.
2) Take the best result from each signer.
3) Take the worst result from all of the different signers; this is
the result for the SignedData object.
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Application and local policy can affect each of the steps outlined
above.
In Step 1:
- If the subject name or subject alternative name(s) cannot be used
to determine if two SignerInfo objects were created by the same
identity, then applications need to specify how such matching is to
be done. As an example, the S/MIME message specification [SMIME-
MSG] could say that as long as the same rfc822Name exists in either
the subject name or the subject alt name they are the same
identity. This would be true even if other information that did
not match existed in these fields.
- Some applications may specify that this step should be skipped;
this has the effect of making each SignerInfo object independent of
all other SignerInfo objects even if the signing identity is the
same. Applications that specify this need to be aware that
algorithm rollover will not work correctly in this case.
In Step 2:
- The major policy implication at this step is the treatment of and
order for the indeterminate states. In most cases, this state
would be placed between the failure and warning states. Part of
the issue is the question of having a multi-state or a binary
answer as to success or failure of an evaluation. Not every
application can deal with the statement "try again later". It may
also be dependent on what the reason for the indeterminate state
is. It makes more sense to try again later if the problem is that
a CRL cannot be found than if you are not able to evaluate the
algorithm for the signature.
In Step 3:
- The same policy implications from Step 2 apply here.
5.3. Evaluation of a SignedData Set
Simple applications will generally use the worst single outcome
(success, unknown, failure) as the outcome for a set of SignedData
objects (i.e., one failure means the entire item fails). However,
not all applications will want to have this behavior.
Turner & Schaad Standards Track [Page 10]RFC 5752 Multiple Signatures in S/MIME January 2010
A work flow application could work as follows:
The second signer will modify the original content, keep the original
signature, and then sign the message. This means that only the
outermost signature is of significance during evaluation. The second
signer is asserting that they successfully validated the inner
signature as part of its processing.
A Signed Mail application could work as follows:
If signatures are added for the support of [ESS] features, then the
fact that an outer layer signature cannot be validated can be treated
as a non-significant failure. The only thing that matters is that
the originator signature is valid. This means that all outer layer
signatures that fail can be stripped from the message prior to
display leaving only the inner-most valid signature to be displayed.
6. Security Considerations
Security considerations from the hash and signature algorithms used
to produce the SignerInfo apply.
If the hashing and signing operations are performed by different
entities, the entity creating the signature must ensure that the hash
comes from a "trustworthy" source. This can be partially mitigated
by requiring that multiple hashes using different algorithms are
provided.
This attribute cannot be relied upon in the event that all of the
algorithms used in the signer attribute are 'cracked'. It is not
possible for a verifier to determine that a collision could not be
found that satisfies all of the algorithms.
Local policy and applications greatly affect signature processing.
The application of local policy and the requirements specific to an
application can both affect signature processing. This means that a
signature valid in one context or location can fail validation in a
different context or location.
7. References
7.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[CMS] Housley, R., "Cryptographic Message Syntax (CMS)", RFC
5652, September 2009.
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[PROFILE] 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.
[SMIME-CERT] Ramsdell, B. and S. Turner, "Secure/Multipurpose
Internet Mail Extensions (S/MIME) Version 3.2
Certificate Handling", RFC 5750, January 2010.
[SMIME-MSG] Ramsdell, B. and S. Turner, "Secure/Multipurpose
Internet Mail Extensions (S/MIME) Version 3.2 Message
Specification", RFC 5751, January 2010.
[ESS] Hoffman, P., Ed., "Enhanced Security Services for
S/MIME", RFC 2634, June 1999.
[ESSCertID] Schaad, J., "Enhanced Security Services (ESS) Update:
Adding CertID Algorithm Agility", RFC 5035, August
2007.
7.2. Informative References
[ATTACK] Hoffman, P. and B. Schneier, "Attacks on Cryptographic
Hashes in Internet Protocols", RFC 4270, November 2005.
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Appendix A. ASN.1 Module
MultipleSignatures-2008
{ iso(1) member-body(2) us(840) rsadsi(113549)
pkcs(1) pkcs9(9) smime(16) modules(0)
id-mod-multipleSig-2008(34) }
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 5652 [CMS], 12.1
DigestAlgorithmIdentifier, SignatureAlgorithmIdentifier
FROM CryptographicMessageSyntax2004
{ iso(1) member-body(2) us(840) rsadsi(113549)
pkcs(1) pkcs9(9) smime(16) modules(0) cms-2004(24) }
-- Imports from RFC 5035 [ESSCertID], Appendix A
ESSCertIDv2
FROM ExtendedSecurityServices-2006
{ iso(1) member-body(2) us(840) rsadsi(113549)
pkcs(1) pkcs9(9) smime(16) modules(0) id-mod-ess-2006(30) }
;
-- Section 3.0
id-aa-multipleSignatures OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs9(9) id-aa(2) 51 }
MultipleSignatures ::= SEQUENCE {
bodyHashAlg DigestAlgorithmIdentifier,
signAlg SignatureAlgorithmIdentifier,
signAttrsHash SignAttrsHash,
cert ESSCertIDv2 OPTIONAL }
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SignAttrsHash ::= SEQUENCE {
algID DigestAlgorithmIdentifier,
hash OCTET STRING }
END -- of MultipleSignatures-2008
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Appendix B. Background
This is an informational appendix. This appendix enumerates all
locations in CMS where hashes are used and the possible attacks on
those hash locations.
B.1. Attacks
As noted in [ATTACK], the following types of resistance are needed
against known attacks:
1) Collision Resistance: Find x and y where x != y and H(x) = H(y)
2) Preimage Resistance: Given y, find x where H(x) = y
3) Second Preimage Resistance: Given y, find x where H(x) = H(y)
Note: It is known that a collision resistance attack is simpler than
a second preimage resistance attack, and it is presumed that a second
preimage resistance attack is simpler than a preimage attack.
B.2. Hashes in CMS
Within a SignerInfo there are two places where hashes are applied and
hence can be attacked: the body and the signed attributes. The
following outlines the entity that creates the hash, the entity that
attacks the hash, and the type of resistance required:
1) Hash of the Body (i.e., the octets comprising the value of the
encapContentInfo.eContent OCTET STRING omitting the tag and length
octets, as per 5.4 of [CMS]).
a) If Alice creates the body to be hashed, then:
i) Alice can attack the hash. This attack requires a
successful collision resistance attack.
ii) Mallory can attack the hash. This attack requires a
successful second preimage resistance attack.
b) If Alice hashes a body provided by Bob, then:
i) Alice can attack the hash. This attack requires a
successful second preimage attack.
Turner & Schaad Standards Track [Page 15]RFC 5752 Multiple Signatures in S/MIME January 2010
ii) Bob can attack the hash. This attack requires a successful
Collision Resistance attack. If Alice has the ability to
"change" the content of the body in some fashion, then this
attack requires a successful second preimage attack. (One
example would be to use a keyed hash function.)
iii) Mallory can attack the hash. This attack requires a
successful second preimage attack.
c) If Alice signs using a hash value provided by Bob (in this
case, Alice is presumed to never see the body in question),
then:
i) Alice can attack the hash. This attack requires a
successful preimage attack.
ii) Bob can attack the hash. This attack requires a successful
collision resistance attack. Unlike case (b), there is
nothing that Alice can do to upgrade the attack.
iii) Mallory can attack the hash. This requires a successful
preimage attack if the content is unavailable to Mallory and
a successful second preimage attack if the content is
available to Mallory.
2) Hash of signed attributes (i.e., the complete Distinguished
Encoding Rules (DER) encoding of the SignedAttrs value contained
in the signedAttrs field, as per 5.4 of [CMS]).
There is a difference between hashing the body and hashing the
SignedAttrs value in that one should not accept a sequence of
attributes to be signed from a third party. In fact, one should
not accept attributes to be included in the signed attributes list
from a third party. The attributes are about the signature you
are applying and not about the body. If there is meta-information
that needs to be attached to the body by a third party, then they
need to provide their own signature and you need to add a
countersignature. (Note: The fact that the signature is to be
used as a countersignature is a piece of information that should
be accepted, but it does not directly provide an attribute that is
inserted in the signed attribute list.)
a) Alice can attack the hash. This requires a successful
collision resistance attack.
b) Mallory can attack the hash. This requires a successful second
preimage resistance attack.
Turner & Schaad Standards Track [Page 16]RFC 5752 Multiple Signatures in S/MIME January 2010
c) Bob can attack the hash and Bob controls the value of the
message digest attribute used. This case is analogous to the
current attacks [ATTACK]. Bob can attack the hash value
generated by Alice based on a prediction of the signed
attributes and the hash algorithm Alice will be using to create
the signature. If Bob successfully predicts these items, the
attack requires a successful collision resistance attack. (It
is expected that if Alice uses a keyed hashing function as part
of the signature, this attack will be more difficult as Bob
would have a harder time prediction the key value.)
It should be noted that both of these attacks are considered to be
more difficult than the attack on the body since more structure is
designed into the data to be hashed than is frequently found in the
body and the data is shorter in length than that of the body.
The successful prediction of the signing-time attribute is expected
to be more difficult than with certificates as the time would not
generally be rounded. Time stamp services can make this more
unpredictable by using a random delay before issuing the signature.
Allowing a third party to provide a hash value could potentially make
an attack simpler when keyed hash functions are used since there is
more data than can be modified without changing the overall structure
of the signed attribute structure.
Authors' Addresses
Sean Turner
IECA, Inc.
3057 Nutley Street, Suite 106
Fairfax, VA 22031
USA
EMail: turners@ieca.com
Jim Schaad
Soaring Hawk Consulting
EMail: jimsch@exmsft.com
Turner & Schaad Standards Track [Page 17]
