Deprecating MD5 and SHA-1 signature hashes in TLS 1.2
draft-ietf-tls-md5-sha1-deprecate-06

Document Type Active Internet-Draft (tls WG)
Authors Loganaden Velvindron  , Kathleen Moriarty  , Alessandro Ghedini 
Last updated 2021-03-29
Replaces draft-lvelvindron-tls-md5-sha1-deprecate
Stream Internet Engineering Task Force (IETF)
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Stream WG state Submitted to IESG for Publication (wg milestone: Jul 2020 - Submit "Deprecating ... )
Document shepherd Joseph Salowey
Shepherd write-up Show (last changed 2021-01-21)
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Kathleen Moriarty  for 75 days
Loganaden Velvindron  for 75 days
Alessandro Ghedini  for 75 days
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Send notices to joe@salowey.net, loganaden@gmail.com
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Internet Engineering Task Force                            L. Velvindron
Internet-Draft                                             cyberstorm.mu
Updates: 5246 7525 (if approved)                             K. Moriarty
Intended status: Standards Track                       Dell Technologies
Expires: September 30, 2021                                   A. Ghedini
                                                         Cloudflare Inc.
                                                          March 29, 2021

         Deprecating MD5 and SHA-1 signature hashes in TLS 1.2
                  draft-ietf-tls-md5-sha1-deprecate-06

Abstract

   The MD5 and SHA-1 hashing algorithms are increasingly vulnerable to
   attack and this document deprecates their use in TLS 1.2 digital
   signatures.  However, this document does not deprecate SHA-1 in HMAC
   for record protection.  This document updates RFC 5246 and RFC 7525.

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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   Internet-Drafts are draft documents valid for a maximum of six months
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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on September 30, 2021.

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
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   publication of this document.  Please review these documents
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   include Simplified BSD License text as described in Section 4.e of

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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   2
   2.  Signature Algorithms  . . . . . . . . . . . . . . . . . . . .   3
   3.  Certificate Request . . . . . . . . . . . . . . . . . . . . .   3
   4.  Server Key Exchange . . . . . . . . . . . . . . . . . . . . .   3
   5.  Certificate Verify  . . . . . . . . . . . . . . . . . . . . .   3
   6.  Updates to RFC5246  . . . . . . . . . . . . . . . . . . . . .   3
   7.  Updates to RFC7525  . . . . . . . . . . . . . . . . . . . . .   4
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   4
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   10. Acknowledgement . . . . . . . . . . . . . . . . . . . . . . .   5
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .   5
     11.1.  Normative References . . . . . . . . . . . . . . . . . .   5
     11.2.  Informative References . . . . . . . . . . . . . . . . .   6
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   6

1.  Introduction

   The usage of MD5 and SHA-1 for signature hashing in 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 Wang, et. al, attack and the
   potential for brute-force attack.  In 2016, researchers from INRIA
   identified a new class of transcript collision attacks on TLS (and
   other protocols) that rely on efficient collision-finding algorithms
   on the underlying hash constructions [Transcript-Collision].
   Further, in 2017, researchers from Google and CWI Amsterdam
   [SHA-1-Collision] proved SHA-1 collision attacks were practical.
   This document updates [RFC5246] and [RFC7525] in such a way that MD5
   and SHA-1 MUST NOT be used for digital signatures.  However, this
   document does not deprecate SHA-1 in HMAC for record protection.

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.

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2.  Signature Algorithms

   Clients MUST NOT include MD5 and SHA-1 in the signature_algorithms
   extension.  If a client does not send a signature_algorithms
   extension, then the server MUST abort the handshake and send a
   handshake_failure alert, except when digital signatures are not used
   (for example, when using PSK ciphers).

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 a client receives a MD5 or SHA-1 signature in a ServerKeyExchange
   message it MUST abort the connection with the 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 handshake_failure or
   insufficient_security alert.

6.  Updates to RFC5246

   [RFC5246], The Transport Layer Security (TLS) Protocol Version 1.2,
   suggests that implementations can assume support for MD5 and SHA-1 by
   their peer.  This update changes the suggestion to assume support for
   SHA-256 instead, due to MD5 and SHA-1 being deprecated.

   In Section 7.4.1.4.1: the text should be revised from:

   OLD:

   "Note: this is a change from TLS 1.1 where there are no explicit
   rules, but as a practical matter one can assume that the peer
   supports MD5 and SHA- 1."

   NEW:

   "Note: This is a change from TLS 1.1 where there are no explicit
   rules, but as a practical matter one can assume that the peer
   supports SHA-256."

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7.  Updates to RFC7525

   [RFC7525], Recommendations for Secure Use of Transport Layer Security
   (TLS) and Datagram Transport Layer Security (DTLS) recommends use of
   SHA-256 as a minimum requirement.  This update moves the minimum
   recommendation to use stronger language deprecating use of both SHA-1
   and MD5.  The prior text did not explicitly include MD5 or SHA-1; and
   this text adds guidance to ensure that these algorithms have been
   deprecated.

   Section 4.3:

   OLD:

   When using RSA, servers SHOULD authenticate using certificates with
   at least a 2048-bit modulus for the public key.  In addition, SHA-256
   hash algorithm MUST be used (see [CAB-Baseline] for more details).
   Clients SHOULD indicate to servers that they request SHA-256, by
   using the "Signature Algorithms" extension defined in TLS 1.2.

   NEW:

   Servers SHOULD authenticate using certificates with at least a
   2048-bit modulus for the public key.

   In addition, the SHA-256 hash algorithm MUST be used; and SHA-1 or
   MD5 MUST NOT be used (see [CAB-Baseline] for more details).  Clients
   MUST indicate to servers that they request SHA-256, by using the
   "Signature Algorithms" extension defined in TLS 1.2.

8.  IANA Considerations

   The document updates the "TLS SignatureScheme" registry to change the
   recommended status of SHA-1 based signature schemes to N (not
   recommended) as defined by [RFC8447].  The following entries are to
   be updated:

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

   Other entries of the registry remain the same.

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9.  Security Considerations

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

10.  Acknowledgement

   The authors would like to thank Hubert Kario for his help in writing
   the initial draft.  We are also grateful to Daniel Migault, Martin
   Thomson and David Cooper for their feedback.

11.  References

11.1.  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>.

   [RFC7525]  Sheffer, Y., Holz, R., and P. Saint-Andre,
              "Recommendations for Secure Use of Transport Layer
              Security (TLS) and Datagram Transport Layer Security
              (DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC7525, May
              2015, <https://www.rfc-editor.org/info/rfc7525>.

   [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>.

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11.2.  Informative References

   [CAB-Baseline]
              CA/Browser Forum, "Baseline Requirements for the Issuance
              and Management of Publicly-Trusted Certificates Version
              1.1.6", 2013, <https://www.cabforum.org/documents.html>.

   [NISTSP800-131A-R2]
              Barker, E. and A. Roginsky, "Transitioning the Use of
              Cryptographic Algorithms and Key Lengths", 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", March
              2019, <http://shattered.io/static/shattered.pdf>.

   [Transcript-Collision]
              Bhargavan, K. and G. Leurent, "Transcript Collision
              Attacks: Breaking Authentication in TLS, IKE, and SSH",
              February 2016, <https://www.mitls.org/downloads/
              transcript-collisions.pdf>.

   [Wang]     Wang, X., Yin, Y., and H. Yu, "Finding Collisions in the
              Full SHA-1", 2005.

Authors' Addresses

   Loganaden Velvindron
   cyberstorm.mu
   Rose Hill
   MU

   Phone: +230 59762817
   Email: logan@cyberstorm.mu

   Kathleen Moriarty
   Dell Technologies

   Email: Kathleen.Moriarty.ietf@gmail.com

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   Alessandro Ghedini
   Cloudflare Inc.

   Email: alessandro@cloudflare.com

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