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Versions: (draft-housley-lamps-cms-update-alg-id-protect) 00

Network Working Group                                         R. Housley
Internet-Draft                                            Vigil Security
Updates: 5652 (if approved)                             January 21, 2020
Intended status: Standards Track
Expires: July 24, 2020


     Update to the Cryptographic Message Syntax (CMS) for Algorithm
                         Identifier Protection
             draft-ietf-lamps-cms-update-alg-id-protect-00

Abstract

   This document updates the Cryptographic Message Syntax (CMS)
   specified in RFC 5652 to ensure that algorithm identifiers are
   adequately protected.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on July 24, 2020.

Copyright Notice

   Copyright (c) 2020 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   described in the Simplified BSD License.



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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Require use the same hash algorithm . . . . . . . . . . . . .   3
     3.1.  RFC 5652, Section 5.3 . . . . . . . . . . . . . . . . . .   3
     3.2.  RFC 5652, Section 5.4 . . . . . . . . . . . . . . . . . .   4
     3.3.  RFC 5652, Section 5.6 . . . . . . . . . . . . . . . . . .   4
     3.4.  Backward Compatibility Considerations . . . . . . . . . .   5
     3.5.  Timestamp Compatibility Considerations  . . . . . . . . .   5
   4.  Recommend inclusion of the CMSAlgorithmProtection attribute .   5
     4.1.  RFC 5652, Section 14  . . . . . . . . . . . . . . . . . .   6
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   6
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   6
     8.2.  Informative References  . . . . . . . . . . . . . . . . .   7
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   8

1.  Introduction

   This document updates the Cryptographic Message Syntax (CMS)
   [RFC5652] to ensure that algorithm identifiers are adequately
   protected.

   The CMS Signed-data Content Type [RFC5652], unlike X.509 certificates
   [RFC5280], can be vulnerable to algorithm substitution attacks.  In
   an algorithm substitution attack, the attacker changes either the
   algorithm identifier or the parameters associated with the algorithm
   identifier to change the verification process used by the recipient.
   The X.509 certificate structure protects the algorithm identifier and
   the associate parameters by signing them.

   In an algorithm substitution attack, the attacker looks for a
   different algorithm that produces the same result as the algorithm
   used by the originator.  As an example, if the signer of a message
   used SHA-256 [SHS] as the digest algorithm to hash the message
   content, then the attacker looks for a weaker hash algorithm that
   produces a result that is of the same length.  The attacker's goal is
   to find a different message that results in the same hash value,
   which is commonly called a collision.  Today, there are many hash
   functions that produce 256-bit results.  One of them may be found to
   be weak in the future.

   Further, when a digest algorithm produces a larger result than is
   needed by a digital signature algorithm, the digest value is reduced
   to the size needed by the signature algorithm.  This can be done both



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   by truncation and modulo operations, with the simplest being
   straightforward truncation.  In this situation, the attacker needs to
   find a collision with the reduced digest value.  As an example, if
   the message signer uses SHA-512 [SHS] as the digest algorithm and
   ECDSA with the P-256 curve [DSS] as the signature algorithm, then the
   attacker needs to find a collision with the first half of the digest.

   Similar attacks can be mounted against parameterized algorithm
   identifiers.  When looking at randomized hash functions, such as the
   example in [RFC6210], the algorithm identifier parameter includes a
   random value that can be manipulated by an attacker looking for
   collisions.  Some other algorithm identifiers include complex
   parameter structures, and each value provides another opportunity for
   manipulation by an attacker.

   This document makes two updates to CMS to provide similar protection
   for the algorithm identifier.  First, it mandates a convention
   followed by many implementations by requiring the originator to use
   the same hash algorithm to compute the digest of the message content
   and the digest of signed attributes.  Second, it recommends that the
   originator include the CMSAlgorithmProtection attribute [RFC6211].

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

3.  Require use the same hash algorithm

   This section updates [RFC5652] to require the originator to use the
   same hash algorithm to compute the digest of the message content and
   the digest of signed attributes.

3.1.  RFC 5652, Section 5.3

   Change the paragraph describing the digestAlgorithm as follows:

   OLD:

      digestAlgorithm identifies the message digest algorithm, and any
      associated parameters, used by the signer.  The message digest is
      computed on either the content being signed or the content
      together with the signed attributes using the process described in
      Section 5.4.  The message digest algorithm SHOULD be among those
      listed in the digestAlgorithms field of the associated SignerData.



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      Implementations MAY fail to validate signatures that use a digest
      algorithm that is not included in the SignedData digestAlgorithms
      set.

   NEW:

      digestAlgorithm identifies the message digest algorithm, and any
      associated parameters, used by the signer.  The message digest is
      computed on either the content being signed or the content
      together with the signed attributes using the process described in
      Section 5.4.  The message digest algorithm SHOULD be among those
      listed in the digestAlgorithms field of the associated SignerData.
      If signedAttrs are present in the SignerInfo, then the same digest
      algorithm MUST be used to compute the digest of the SignedData
      encapContentInfo eContent, which is carried in the message-digest
      attribute, and to compute the digest of the DER-encoded SET OF
      signed attributes, which is passed to the signature algorithm.
      Implementations MAY fail to validate signatures that use a digest
      algorithm that is not included in the SignedData digestAlgorithms
      set.

3.2.  RFC 5652, Section 5.4

   Add the following paragraph as the second paragraph in Section 5.4:

   ADD:

      When the signedAttrs field is present, the same digest algorithm
      MUST be used to compute the digest of the the encapContentInfo
      eContent OCTET STRING, which is carried in the message-digest
      attribute, and the collection of attributes that are signed.

3.3.  RFC 5652, Section 5.6

   Change the paragraph discussing the signedAttributes as follows:

   OLD:

      The recipient MUST NOT rely on any message digest values computed
      by the originator.  If the SignedData signerInfo includes
      signedAttributes, then the content message digest MUST be
      calculated as described in Section 5.4.  For the signature to be
      valid, the message digest value calculated by the recipient MUST
      be the same as the value of the messageDigest attribute included
      in the signedAttributes of the SignedData signerInfo.

   NEW:




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      The recipient MUST NOT rely on any message digest values computed
      by the originator.  If the SignedData signerInfo includes
      signedAttributes, then the content message digest MUST be
      calculated as described in Section 5.4, using the same digest
      algorithm to compute the digest of the the encapContentInfo
      eContent OCTET STRING and the message-digest attribute.  For the
      signature to be valid, the message digest value calculated by the
      recipient MUST be the same as the value of the messageDigest
      attribute included in the signedAttributes of the SignedData
      signerInfo.

3.4.  Backward Compatibility Considerations

   The new requirement introduced above might lead to compatibility with
   an implementation that allowed different digest algorithms to be used
   to compute the digest of the message content and the digest of signed
   attributes.  The signatures produced by such an implementation when
   two different digest algorithms are used will be considered invalid
   by an implementation that follows this specification.  However, most,
   if not all, implementations already require the originator to use the
   same digest algorithm for both operations.

   READER:

      If you have an implementation that allows different digest
      algorithms to be used to compute the digest of the message content
      and the digest of signed attributes, please tell us on the
      spasm@ietf.org mail list.

3.5.  Timestamp Compatibility Considerations

   The new requirement introduced above might lead to compatibility
   issues for timestamping systems when the originator does not wish to
   share the message content with the Time Stamp Authority (TSA)
   [RFC3161].  In this situation, the originator sends a TimeStampReq to
   the TSA that includes a MessageImprint, which consists of a digest
   algorithm identifier and a digest value, then the TSA uses the digest
   in the MessageImprint.  As a result, the signature algorithm used by
   the TSA needs to be compatible with the digest algorithm selected by
   the originator for the MessageImprint.

4.  Recommend inclusion of the CMSAlgorithmProtection attribute

   This section updates [RFC5652] to recommend that the originator
   include the CMSAlgorithmProtection attribute [RFC6211] whenever
   signed attributes or authenticated attributes are present.





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4.1.  RFC 5652, Section 14

   Add the following paragraph as the eighth paragraph in Section 14:

   ADD:

      While no known algorithm substitution attacks are known at this
      time, the inclusion of the algorithm identifiers used by the
      originator as a signed attribute or an authenticated attribute
      makes such an attack significantly more difficult.  Therefore, the
      originator of a Signed-data content type that includes signed
      attributes SHOULD include the CMSAlgorithmProtection attribute
      [RFC6211] as one of the signed attributes.  Likewise, the
      originator of an Authenticated-data content type that includes
      authenticated attributes SHOULD include the CMSAlgorithmProtection
      attribute [RFC6211] as one of the authenticated attributes.

5.  IANA Considerations

   This document makes no requests of the IANA.

6.  Security Considerations

   The security considerations of [RFC5652] are updated ensure that
   algorithm identifiers are adequately protected, which makes algorithm
   substitution attacks significantly more difficult.

   The CMSAlgorithmProtection attribute [RFC6211] offers protection the
   algorithm identifiers used in the signed-data and authenticated-data
   content types.  There is not currently protection mechanism for the
   algorithm identifiers used in the enveloped-data, digested-data, or
   encrypted-data content types.  Likewise there us not currently
   protection mechanism for the algorithm identifiers used in the
   authenticated-enveloped-data content type defined in [RFC5083].

7.  Acknowledgements

   Many thanks to Jim Schaad and Peter Gutmann; without knowing it, they
   motivated me to write this document.

8.  References

8.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.



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   [RFC5652]  Housley, R., "Cryptographic Message Syntax (CMS)", STD 70,
              RFC 5652, DOI 10.17487/RFC5652, September 2009,
              <https://www.rfc-editor.org/info/rfc5652>.

   [RFC6211]  Schaad, J., "Cryptographic Message Syntax (CMS) Algorithm
              Identifier Protection Attribute", RFC 6211,
              DOI 10.17487/RFC6211, April 2011,
              <https://www.rfc-editor.org/info/rfc6211>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

8.2.  Informative References

   [DSS]      National Institute of Standards and Technology (NIST),
              "Digital Signature Standard (DSS)", FIPS
              Publication 186-3, June 2009.

   [RFC3161]  Adams, C., Cain, P., Pinkas, D., and R. Zuccherato,
              "Internet X.509 Public Key Infrastructure Time-Stamp
              Protocol (TSP)", RFC 3161, DOI 10.17487/RFC3161, August
              2001, <https://www.rfc-editor.org/info/rfc3161>.

   [RFC5083]  Housley, R., "Cryptographic Message Syntax (CMS)
              Authenticated-Enveloped-Data Content Type", RFC 5083,
              DOI 10.17487/RFC5083, November 2007,
              <https://www.rfc-editor.org/info/rfc5083>.

   [RFC5280]  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, DOI 10.17487/RFC5280, May 2008,
              <https://www.rfc-editor.org/info/rfc5280>.

   [RFC6210]  Schaad, J., "Experiment: Hash Functions with Parameters in
              the Cryptographic Message Syntax (CMS) and S/MIME",
              RFC 6210, DOI 10.17487/RFC6210, April 2011,
              <https://www.rfc-editor.org/info/rfc6210>.

   [SHS]      National Institute of Standards and Technology (NIST),
              "Secure Hash Standard", FIPS Publication 180-3, October
              2008.








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Author's Address

   Russ Housley
   Vigil Security
   516 Dranesville Road
   Herndon, VA  20170
   US

   Email: housley@vigilsec.com










































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