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Versions: 00 01 02 03 04 05 RFC 5752

S/MIME WG                                             Sean Turner, IECA
Internet Draft                                 Jim Schaad, Soaring Hawk
Intended Status: Standard Track                        January 22, 2008
Expires: July 22, 2008



                       Multiple Signatures in S/MIME
                     draft-ietf-smime-multisig-04.txt


Status of this Memo

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   This Internet-Draft will expire on July 22, 2008.

Copyright Notice

   Copyright (C) The IETF Trust (2008).

Abstract

   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



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   the multiple-signatures attribute, its generation rules, and its
   processing rules to allow signers to convey multiple SignerInfo while
   protecting against downgrade attacks. Additionally, this attribute
   may assist during periods of algorithm migration.

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].

Discussion

   This draft is being discussed on the 'ietf-smime' mailing list. To
   subscribe, send a message to ietf-smime-request@imc.org with the
   single word subscribe in the body of the message. There is a Web site
   for the mailing list at <http://www.imc.org/ietf-smime/>.

Table of Contents

   1. Introduction...................................................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...........................................7
      4.2. EncapsulatedContentInfo Type..............................7
      4.3. SignerInfo Type...........................................7
      4.4. Message Digest Calculation Process........................8
         4.4.1. multiple-signatures Signed Attribute Generation......8
         4.4.2. Message Digest calculation Process...................8
      4.5. Signature Generation Process..............................8
      4.6. Signature Verification Process............................8
   5. Signature Evaluation Processing................................9
      5.1. Evaluation of a SignerInfo object.........................9
      5.2. Evaluation of a SignerInfo Set...........................10
      5.3. Evaluation of a SignedData Set...........................11
   6. Security Considerations.......................................12
   7. IANA Considerations...........................................12
   8. References....................................................12
      8.1. Normative References.....................................12
      8.2. Informative References...................................13
   Appendix A. ASN.1 Module.........................................14
   Appendix B. Background...........................................16
      B.1. Attacks..................................................16
      B.2. Hashes in CMS............................................16



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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 include 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 an
   existing one(s) removed without perturbing the remaining
   signature(s).

   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.

   Note this attribute ought not be confused with the countersignature
   attribute, see 11.4 of [CMS], as this is not intended to sign over an
   existing signature rather it is to provide a pointer to additional
   signer's signatures that are all at the same level. That is
   countersignature provides a serial signature while the attribute
   defined herein provides pointers to parallel signature by the same
   signer.

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
   object has multiple SignerInfos, then the attacker, whether it be
   Alice, Bob, or Mallory, can remove SignerInfos without the relying
   party being aware that more than one was generated.










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   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.

   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 premiage 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
   points to the 2nd and 3rd SignerInfos.  The 2nd SignerInfo points to
   the 1st and 3rd SignerInfos. The 3rd SignerInfo points to the 1st and
   2nd SignerInfos.

   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 }
















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   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:

      -- aldId 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).

     - 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 section 5.1-5.6 in [CMS].







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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 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.

     - 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(s).

       -- Include their signerInfo.

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 a 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.








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4.4. Message Digest Calculation Process

4.4.1. multiple-signatures Signed Attribute Generation

   The procedure for generating the multiple-signatures signed attribute
   are 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].

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 SignerInfos 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.



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   The hash corresponding to the 1st SignerInfo must match the value in
   the multiple-signatures attribute that points to the 1st SignerInfo
   present in the 2nd and 3rd SignerInfos.  The hash corresponding to
   the 2nd SignerInfo must match the value in the multiple-signatures
   attribute that points to the 2nd SignerInfo present in the 1st and
   3rd SignerInfos. The hash corresponding to the 3rd SignerInfo must
   match the value in the multiple-signatures attribute that points to
   the 3rd SignerInfo present in the 1st and 2nd SignerInfos.

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 SignerInfos being present in a singled
   SignedData object, or there are 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
   which 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.

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.




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   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 addition processing is
   described in [SMIME-CERT].  One piece of information that is part of
   this additional 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 SignerInfos into a result for
   a SignedData object requires knowledge of the results for the
   individual SignerInfo objects, the require 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.

   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 RFC 822 name exists in
     either the subject name or the subject alt name they are the same


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     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

   For simple applications, the requirement can be made that the result
   of evaluating a set of SignedData objects is the worst outcome of the
   items.  (I.e. one failure means the entire item fails).  However not
   all applications will want to have this behavior.

   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 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


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   signatures which 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 performing the signature must ensure 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 affects 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. IANA Considerations

   None: All identifiers are already registered.  Please remove this
   section prior to publication as an RFC.

8. References

8.1. Normative References

   [RFC2119]     Bradner, S., "Key words for use in RFCs to Indicate
                 Requirement Levels", RFC 2119, BCP 14, March 1997.

   [CMS]         Housley, R., "Cryptographic Message Syntax (CMS)", RFC
                 3852, July 2004.

   [PROFILE]     Housley, R., Polk, W., Ford, W., and D. Solo,
                 "Internet X.509 Public Key Infrastructure Certificate
                 and Certificate Revocation List (CRL) Profile", RFC
                 3280, April 2002.





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   [SMIME-CERT]  Ramsdell, B., and S. Turner, "Secure Multipurpose
                 Internet Mail Extensions (S/MIME) Version 3.2
                 Certificate Handling", work in progress.

   [SMIME-MSG]   Ramsdell, B., and S. Turner, "Secure Multipurpose
                 Internet Mail Extensions (S/MIME) Version 3.2 Message
                 Specification", work in progress.

   [ESS]         Hoffman, P., "Enhanced Security Services for S/MIME",
                 RFC 2634, June 1999.

   [ESSCertID]   Schaad, J., "ESS Update: Adding CertID Algorithm
                 Agility", RFC 5035, August 2007.

8.2. Informative References

   [ATTACK]      Hoffman, P., Schneier, B., "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) pkcs-9(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 3852 [CMS], 12.1

        DigestAlgorithmIdentifier, SignatureAlgorithmIdentifier
        FROM CryptographicMessageSyntax2004
          { iso(1) member-body(2) us(840) rsadsi(113549)
            pkcs(1) pkcs-9(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) pkcs-9(9) smime(16) modules(0) id-mod-ess-2006(30) }

   ;

   -- Section 3.0

   id-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 informative appendix that looks at the locations of hashes
   CMS and possible attacks against them.

B.1. Attacks

   The following types of resistance against known attacks, see
   [ATTACK], is needed:

     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 simplier than a preimage attack.

B.2. Hashes in CMS

   Within a SignedInfo there are two places where hashes are applied and
   hence can be attacked: the Body and the SignedAttributes.  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) Alice creates the Body to be hashed:

        i) Alice attacks the hash: This would require a successful
        Collision Resistance attack.

        ii) Mallory attacks the hash: This would require a successful
        Second Preimage Reistance attack.

      b) Alice hashes a body provided by Bob:

        i) Alice attacks the hash:  This would require a successful
        Second Preimage Attack.

        ii) Bob attacks the hash:  This would require a successful
        Collision Resistance attack.  This can be upgraded to requiring
        a successful Second Preimage Attack if Alice hash the ability


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        to "change" the content of the body in some fashion.  (One
        example would be to use a keyed hash function.)

        iii) Mallory attacks the hash: This would require a successful
        Second Preimage Attack.

      c) Alice signs using a hash value provided by Bob.  (In this case
      Alice is presumed to never see the body in question.)

        i) Alice attacks hash: This would require a successful Preimage
        Attack.

        ii) Bob attacks hash: This would require a successful Collision
        Resistance attack.  Unlike case (b), there is nothing that
        Alice can do to upgrade the attack required.

        iii) Mallory attacks the hash:  This would require a success
        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 SignedAttributes (i.e., the complete 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 be doing 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
   attribute list.)

     a) Alice attacks the hash: This requires a successful Collision
     Resistance Attack.

     b) Mallory attacks the hash: This requires a successful Second
     Preimage Resistance attack.

     c) Bob attacks the hash and provides the body hash used:  This case
     is analogous to the current attacks [ATTACK].  Based on prediction
     of the signed attributes Alice will be using and the provided hash


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     value and function.  (It is expected that if Alice uses a keyed
     hashing function as part of the signature this attack will be more
     difficult.)

   It should be noted that both of these attacks are considered to be
   more difficult that 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 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
   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.































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Author's 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

































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Full Copyright Statement

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