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Update to the Cryptographic Message Syntax (CMS) for Algorithm Identifier Protection
draft-ietf-lamps-cms-update-alg-id-protect-04

The information below is for an old version of the document.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 8933.
Author Russ Housley
Last updated 2020-08-27
Replaces draft-housley-lamps-cms-update-alg-id-protect
RFC stream Internet Engineering Task Force (IETF)
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Stream WG state Submitted to IESG for Publication
Document shepherd Tim Hollebeek
Shepherd write-up Show Last changed 2020-06-30
IESG IESG state Became RFC 8933 (Proposed Standard)
Consensus boilerplate Yes
Telechat date (None)
Responsible AD Roman Danyliw
Send notices to Tim Hollebeek <tim.hollebeek@digicert.com>
IANA IANA review state IANA OK - No Actions Needed
draft-ietf-lamps-cms-update-alg-id-protect-04
Network Working Group                                         R. Housley
Internet-Draft                                            Vigil Security
Updates: 5652 (if approved)                              August 27, 2020
Intended status: Standards Track
Expires: February 28, 2021

     Update to the Cryptographic Message Syntax (CMS) for Algorithm
                         Identifier Protection
             draft-ietf-lamps-cms-update-alg-id-protect-04

Abstract

   This document updates the Cryptographic Message Syntax (CMS)
   specified in RFC 5652 to ensure that algorithm identifiers in signed-
   data and authenticated-data content types 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
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on February 28, 2021.

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
   Provisions Relating to IETF Documents
   (https://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 must
   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.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Required 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.  Recommended inclusion of the CMSAlgorithmProtection attribute   6
     4.1.  RFC 5652, Section 14  . . . . . . . . . . . . . . . . . .   6
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   7
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   7
     8.2.  Informative References  . . . . . . . . . . . . . . . . .   8
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   9

1.  Introduction

   This document updates the Cryptographic Message Syntax (CMS)
   [RFC5652] to ensure that algorithm identifiers in signed-data and
   authenticated-data content types 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 associated 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 called a cross-algorithm 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 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.  Required 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 signedAttrs 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 the signedAttrs field is present in the SignerInfo, then the
      same digest algorithm MUST be used to compute both the digest of
      the SignedData encapContentInfo eContent, which is carried in the
      message-digest attribute, and the digest of the DER-encoded
      signedAttrs, 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 encapContentInfo
      eContent OCTET STRING, which is carried in the message-digest
      attribute, and the digest of the collection of attributes that are
      signed.

   nit: there may be a grammar nit here, relating to the parallelism of
   "compute the digest of" - I think "the collection of attributes that
   are signed" should also have an "of" or "digest of" prefix.

3.3.  RFC 5652, Section 5.6

   Change the paragraph discussing the signed attributes 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

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      be the same as the value of the messageDigest attribute included
      in the signedAttributes of the SignedData signerInfo.

   NEW:

      The recipient MUST NOT rely on any message digest values computed
      by the originator.  If the SignedData signerInfo includes the
      signedAttrs field, 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 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 signedAttrs field of the SignedData signerInfo.

3.4.  Backward Compatibility Considerations

   The new requirement introduced above might lead to incompatibility
   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.

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
   originator-provided digest in the MessageImprint.

   When producing the TimeStampToken, the TSA MUST use the same digest
   algorithm to compute the digest of the encapContentInfo eContent,
   which is an OCTET STRING that contains the TSTInfo, and the message-
   digest attribute within the SignerInfo.

   To ensure that TimeStampToken values that were generated before this
   update remain valid, no requirement is placed on a TSA to ensure that
   the digest algorithm for the TimeStampToken matches the digest
   algorithm for the MessageImprint embedded within the TSTInfo.

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4.  Recommended 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.

4.1.  RFC 5652, Section 14

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

   ADD:

      While there are no known algorithm substitution attacks today, 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 properties of the CMS [RFC5652] signed-data and
   authenticated-data content types are updated to offer protection for
   algorithm identifiers, which makes algorithm substitution attacks
   significantly more difficult.

   For the signed-data content type, the improvements specified in this
   document force an attacker to mount a hash algorithm substitution
   attack on the overall signature, not just on the message digest of
   the encapContentInfo eContent.

   Some digital signature algorithms have prevented hash function
   substitutions by including a digest algorithm identifier as an input
   to the signature algorithm.  As discussed in [HASHID], such a
   "firewall" may not be effective or even possible with newer signature
   algorithms.  For example, RSASSA-PKCS1-v1_5 [RFC8017] protects the
   digest algorithm identifier, but RSASSA-PSS [RFC8017] does not.
   Therefore, it remains important that a signer have a way to signal to
   a recipient which digest algorithms are allowed to be used in
   conjunction with the verification of an overall signature.  This

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   signaling can be done as part of the specification of the signature
   algorithm, in an X.509v3 certificate extension [RFC5280], or some
   other means.  The Digital Signature Standard (DSS) [DSS] takes the
   first approach by requiring the use of an "approved" one-way hash
   algorithm.

   For the authenticated-data content type, the improvements specified
   in this document force an attacker to mount a MAC algorithm
   substitution attack, which is difficult because the attacker does not
   know the authentication key.

   The CMSAlgorithmProtection attribute [RFC6211] offers protection for
   the algorithm identifiers used in the signed-data and authenticated-
   data content types.  However, no protection is provided for the
   algorithm identifiers in the enveloped-data, digested-data, or
   encrypted-data content types.  Likewise, The CMSAlgorithmProtection
   attribute provides no protection for the algorithm identifiers used
   in the authenticated-enveloped-data content type defined in
   [RFC5083].  A mechanism for algorithm identifier protection for these
   content types is work for the future.

7.  Acknowledgements

   Many thanks to Jim Schaad and Peter Gutmann; without knowing it, they
   motivated me to write this document.  Thanks to Roman Danyliw, Ben
   Kaduk, and Peter Yee for their careful review and editorial
   suggestions.

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>.

   [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>.

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

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   [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 186-4, July 2013.

   [HASHID]   Kaliski, B., "On Hash Function Firewalls in Signature
              Schemes", Lecture Notes in Computer Science, Volume 2271,
              DOI 10.1007/3-540-45760-7_1, February 2002.

   [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>.

   [RFC8017]  Moriarty, K., Ed., Kaliski, B., Jonsson, J., and A. Rusch,
              "PKCS #1: RSA Cryptography Specifications Version 2.2",
              RFC 8017, DOI 10.17487/RFC8017, November 2016,
              <https://www.rfc-editor.org/info/rfc8017>.

   [SHS]      National Institute of Standards and Technology (NIST),
              "Secure Hash Standard", FIPS Publication 180-4, August
              2015.

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Author's Address

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

   Email: housley@vigilsec.com

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