Internet Engineering Task Force (IETF)                     L. Velvindron
Request for Comments: 9155                                 cyberstorm.mu
Updates: 5246                                                K. Moriarty
Category: Standards Track                                            CIS
ISSN: 2070-1721                                               A. Ghedini
                                                        Cloudflare Inc.
                                                          December 2021


  Deprecating MD5 and SHA-1 Signature Hashes in TLS 1.2 and DTLS 1.2

Abstract

  The MD5 and SHA-1 hashing algorithms are increasingly vulnerable to
  attack, and this document deprecates their use in TLS 1.2 and DTLS
  1.2 digital signatures.  However, this document does not deprecate
  SHA-1 with Hashed Message Authentication Code (HMAC), as used in
  record protection.  This document updates RFC 5246.

Status of This Memo

  This is an Internet Standards Track document.

  This document is a product of the Internet Engineering Task Force
  (IETF).  It represents the consensus of the IETF community.  It has
  received public review and has been approved for publication by the
  Internet Engineering Steering Group (IESG).  Further information on
  Internet Standards is available in Section 2 of RFC 7841.

  Information about the current status of this document, any errata,
  and how to provide feedback on it may be obtained at
  https://www.rfc-editor.org/info/rfc9155.

Copyright Notice

  Copyright (c) 2021 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 Revised BSD License text as described in Section 4.e of the
  Trust Legal Provisions and are provided without warranty as described
  in the Revised BSD License.

Table of Contents

  1.  Introduction
    1.1.  Requirements Language
  2.  Signature Algorithms
  3.  Certificate Request
  4.  Server Key Exchange
  5.  Certificate Verify
  6.  IANA Considerations
  7.  Security Considerations
  8.  References
    8.1.  Normative References
    8.2.  Informative References
  Acknowledgements
  Authors' Addresses

1.  Introduction

  The usage of MD5 and SHA-1 for signature hashing in (D)TLS 1.2 is
  specified in [RFC5246].  MD5 and SHA-1 have been proven to be
  insecure, subject to collision attacks [Wang].  In 2011, [RFC6151]
  detailed the security considerations, including collision attacks for
  MD5.  NIST formally deprecated use of SHA-1 in 2011
  [NISTSP800-131A-R2] and disallowed its use for digital signatures at
  the end of 2013, based on both the attack described in [Wang] and the
  potential for brute-force attack.  In 2016, researchers from the
  National Institute for Research in Digital Science and Technology
  (INRIA) identified a new class of transcript collision attacks on TLS
  (and other protocols) that relies on efficient collision-finding
  algorithms on the underlying hash constructions
  [Transcript-Collision].  Further, in 2017, researchers from Google
  and Centrum Wiskunde & Informatica (CWI) Amsterdam [SHA-1-Collision]
  proved SHA-1 collision attacks were practical.  This document updates
  [RFC5246] in such a way that MD5 and SHA-1 MUST NOT be used for
  digital signatures.  However, this document does not deprecate SHA-1
  with HMAC, as used in record protection.  Note that the CA/Browser
  Forum (CABF) has also deprecated use of SHA-1 for use in certificate
  signatures [CABF].

1.1.  Requirements Language

  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.

2.  Signature Algorithms

  Clients MUST include the signature_algorithms extension.  Clients
  MUST NOT include MD5 and SHA-1 in this extension.

3.  Certificate Request

  Servers SHOULD NOT include MD5 and SHA-1 in CertificateRequest
  messages.

4.  Server Key Exchange

  Servers MUST NOT include MD5 and SHA-1 in ServerKeyExchange messages.
  If the client receives a ServerKeyExchange message indicating MD5 or
  SHA-1, then it MUST abort the connection with an illegal_parameter
  alert.

5.  Certificate Verify

  Clients MUST NOT include MD5 and SHA-1 in CertificateVerify messages.
  If a server receives a CertificateVerify message with MD5 or SHA-1,
  it MUST abort the connection with an illegal_parameter alert.

6.  IANA Considerations

  IANA has updated the "TLS SignatureScheme" registry by changing the
  recommended status of SHA-1-based signature schemes to "N" (not
  recommended), as defined by [RFC8447].  The following entries have
  been updated; other entries in the registry remain the same.

     +========+================+=============+=====================+
     | Value  |  Description   | Recommended |      Reference      |
     +========+================+=============+=====================+
     | 0x0201 | rsa_pkcs1_sha1 |      N      | [RFC8446] [RFC9155] |
     +--------+----------------+-------------+---------------------+
     | 0x0203 |   ecdsa_sha1   |      N      | [RFC8446] [RFC9155] |
     +--------+----------------+-------------+---------------------+

                                 Table 1

  IANA has also updated the reference for the "TLS SignatureAlgorithm"
  and "TLS HashAlgorithm" registries to refer to this document in
  addition to RFCs 5246 and 8447.

7.  Security Considerations

  Concerns with (D)TLS 1.2 implementations falling back to SHA-1 is an
  issue.  This document updates the TLS 1.2 specification [RFC5246] to
  deprecate support for MD5 and SHA-1 for digital signatures.  However,
  this document does not deprecate SHA-1 with HMAC, as used in record
  protection.

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>.

  [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
             (TLS) Protocol Version 1.2", RFC 5246,
             DOI 10.17487/RFC5246, August 2008,
             <https://www.rfc-editor.org/info/rfc5246>.

  [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>.

  [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
             Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
             <https://www.rfc-editor.org/info/rfc8446>.

  [RFC8447]  Salowey, J. and S. Turner, "IANA Registry Updates for TLS
             and DTLS", RFC 8447, DOI 10.17487/RFC8447, August 2018,
             <https://www.rfc-editor.org/info/rfc8447>.

8.2.  Informative References

  [CABF]     CA/Browser Forum, "Ballot 118 -- SHA-1 Sunset (passed)",
             October 2014, <https://cabforum.org/2014/10/16/ballot-118-
             sha-1-sunset/>.

  [NISTSP800-131A-R2]
             Barker, E. and A. Roginsky, "Transitioning the Use of
             Cryptographic Algorithms and Key Lengths", NIST Special
             Publication 800-131A, Revision 2,
             DOI 10.6028/NIST.SP.800-131Ar2, March 2019,
             <https://nvlpubs.nist.gov/nistpubs/SpecialPublications/
             NIST.SP.800-131Ar2.pdf>.

  [RFC6151]  Turner, S. and L. Chen, "Updated Security Considerations
             for the MD5 Message-Digest and the HMAC-MD5 Algorithms",
             RFC 6151, DOI 10.17487/RFC6151, March 2011,
             <https://www.rfc-editor.org/info/rfc6151>.

  [SHA-1-Collision]
             Stevens, M., Bursztein, E., Karpman, P., Albertini, A.,
             and Y. Markov, "The First Collision for Full SHA-1", 2017,
             <https://eprint.iacr.org/2017/190>.

  [Transcript-Collision]
             Bhargavan, K. and G. Leurent, "Transcript Collision
             Attacks: Breaking Authentication in TLS, IKE, and SSH",
             DOI 10.14722/ndss.2016.23418, February 2016,
             <https://hal.inria.fr/hal-01244855/document>.

  [Wang]     Wang, X., Yin, Y., and H. Yu, "Finding Collisions in the
             Full SHA-1", DOI 10.1007/11535218_2, 2005,
             <https://www.iacr.org/archive/
             crypto2005/36210017/36210017.pdf>.

Acknowledgements

  The authors would like to thank Hubert Kario for his help in writing
  the initial draft version of this document.  We are also grateful to
  Daniel Migault, Martin Thomson, Sean Turner, Christopher Wood, and
  David Cooper for their feedback.

Authors' Addresses

  Loganaden Velvindron
  cyberstorm.mu
  Rose Hill
  Mauritius

  Phone: +230 59762817
  Email: [email protected]


  Kathleen Moriarty
  Center for Internet Security
  East Greenbush, NY
  United States of America

  Email: [email protected]


  Alessandro Ghedini
  Cloudflare Inc.

  Email: [email protected]

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