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Internet Engineering Task Force (IETF)                         S. TurnerRequest for Comments: 5752                                          IECACategory: Standards Track                                      J. SchaadISSN: 2070-1721                                             Soaring Hawk                                                            January 2010Multiple 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 inSection 2 of RFC 5741.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://www.rfc-editor.org/info/rfc5752.Copyright Notice   Copyright (c) 2010 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject toBCP 78 and the IETF Trust's Legal   Provisions Relating to IETF Documents   (http://trustee.ietf.org/license-info) in effect on the date of   publication of this document.  Please review these documents   carefully, as they describe your rights and restrictions with respect   to this document.  Code Components extracted from this document mustTurner & Schaad              Standards Track                    [Page 1]

RFC 5752              Multiple Signatures in S/MIME         January 2010   include Simplified BSD License text as described in Section 4.e of   the Trust Legal Provisions and are provided without warranty as   described in the Simplified BSD License.   This document may contain material from IETF Documents or IETF   Contributions published or made publicly available before November   10, 2008.  The person(s) controlling the copyright in some of this   material may not have granted the IETF Trust the right to allow   modifications of such material outside the IETF Standards Process.   Without obtaining an adequate license from the person(s) controlling   the copyright in such materials, this document may not be modified   outside the IETF Standards Process, and derivative works of it may   not be created outside the IETF Standards Process, except to format   it for publication as an RFC or to translate it into languages other   than English.Table of Contents1. Introduction ....................................................31.1. Conventions Used in This Document ..........................32. Rationale .......................................................32.1. Attribute Design Requirements ..............................43. Multiple Signature Indication ...................................54. Message Generation and Processing ...............................64.1. SignedData Type ............................................64.2. EncapsulatedContentInfo Type ...............................74.3. SignerInfo Type ............................................74.4. Message Digest Calculation Process .........................74.4.1. multiple-signatures Signed Attribute Generation .....74.4.2. Message Digest Calculation Process ..................74.5. Signature Generation Process ...............................84.6. Signature Verification Process .............................85. Signature Evaluation Processing .................................85.1. Evaluation of a SignerInfo Object ..........................95.2. Evaluation of a SignerInfo Set .............................95.3. Evaluation of a SignedData Set ............................106. Security Considerations ........................................117. References .....................................................117.1. Normative References ......................................117.2. Informative References ....................................12Appendix A. ASN.1 Module...........................................13Appendix B. Background.............................................15B.1. Attacks....................................................15B.2. Hashes in CMS..............................................15Turner & Schaad              Standards Track                    [Page 2]

RFC 5752              Multiple Signatures in S/MIME         January 20101.  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.Turner & Schaad              Standards Track                    [Page 3]

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.  SeeSection 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.Turner & Schaad              Standards Track                    [Page 4]

RFC 5752              Multiple Signatures in S/MIME         January 20103.  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 (seeSection 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=value24.  Message Generation and Processing   The following are the additional procedures for message generation   when using the multiple-signatures attribute.  These paragraphs track   with Sections5.1-5.6 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.Turner & Schaad              Standards Track                    [Page 6]

RFC 5752              Multiple Signatures in S/MIME         January 2010   - 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 inSection4.4.1 of [CMS] with the following addition:   The signer MUST include the multiple-signatures attribute in   signedAttrs.4.4.  Message Digest Calculation Process4.4.1.  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].Turner & Schaad              Standards Track                    [Page 7]

RFC 5752              Multiple Signatures in S/MIME         January 20104.5.  Signature Generation Process   The procedures for signature generation are as specified inSection5.5 of [CMS].4.6.  Signature Verification Process   The procedures for signature verification are as specified inSection5.6 of [CMS] with the following addition:   If the SignedData signerInfo includes the multiple-signatures   attribute, the attribute's values must be calculated as described inSection 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 fromSection 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.Turner & Schaad              Standards Track                    [Page 8]

RFC 5752              Multiple Signatures in S/MIME         January 20105.1.  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.Turner & Schaad              Standards Track                    [Page 9]

RFC 5752              Multiple Signatures in S/MIME         January 2010   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.  References7.1.  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)",RFC5652, September 2009.Turner & Schaad              Standards Track                   [Page 11]

RFC 5752              Multiple Signatures in S/MIME         January 2010   [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.Turner & Schaad              Standards Track                   [Page 12]

RFC 5752              Multiple Signatures in S/MIME         January 2010Appendix A.  ASN.1 ModuleMultipleSignatures-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 fromRFC 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 fromRFC 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 3.0id-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 }Turner & Schaad              Standards Track                   [Page 13]

RFC 5752              Multiple Signatures in S/MIME         January 2010SignAttrsHash ::= SEQUENCE {  algID            DigestAlgorithmIdentifier,  hash             OCTET STRING }END -- of MultipleSignatures-2008Turner & Schaad              Standards Track                   [Page 14]

RFC 5752              Multiple Signatures in S/MIME         January 2010Appendix 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.comTurner & Schaad              Standards Track                   [Page 17]