Deprecating MD5 and SHA-1 signature hashes in TLS 1.2
draft-ietf-tls-md5-sha1-deprecate-03

Internet Engineering Task Force                            L. Velvindron
Internet-Draft                                             cyberstorm.mu
Updates: 5246 7525 (if approved)                             K. Moriarty
Intended status: Standards Track                                Dell EMC
Expires: November 15, 2020                                    A. Ghedini
                                                         Cloudflare Inc.
                                                            May 14, 2020

         Deprecating MD5 and SHA-1 signature hashes in TLS 1.2
                  draft-ietf-tls-md5-sha1-deprecate-03

Abstract

   The MD5 and SHA-1 hashing algorithms are steadily weakening in
   strength and their deprecation process should begin for their use in
   TLS 1.2 digital signatures.  However, this document does not
   deprecate SHA-1 in HMAC for record protection.

Status of This Memo

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   This Internet-Draft will expire on November 15, 2020.

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   the Trust Legal Provisions and are provided without warranty as
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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.  Security Considerations . . . . . . . . . . . . . . . . . . .   4
   9.  Acknowledgement . . . . . . . . . . . . . . . . . . . . . . .   4
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .   5
     10.1.  Normative References . . . . . . . . . . . . . . . . . .   5
     10.2.  Informative References . . . . . . . . . . . . . . . . .   5
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   6

1.  Introduction

   The usage of MD5 and SHA-1 for signature hashing in TLS 1.2 is
   specified in RFC 5246 [RFC5246].  MD5 and SHA-1 have been proven to
   be insecure, subject to collision attacks.  RFC 6151 [RFC6151]
   details the security considerations, including collision attacks for
   MD5, published in 2011.  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 RFC 5246 [RFC5246] and RFC7525
   [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", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

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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 [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.

   OLD:

   In Section 7.4.1.4.1: the text should be revised from " 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 [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 and this text adds it to ensure it is understood as having 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, the use
   of the SHA-256 hash algorithm is RECOMMENDED (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 use of the SHA-256 hash algorithm is RECOMMENDED,
   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.  Security Considerations

   Concerns with TLS 1.2 implementations falling back to SHA-1 is an
   issue.  This draft 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.

9.  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.

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10.  References

10.1.  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>.

   [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>.

10.2.  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>.

   [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>.

   [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>.

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Authors' Addresses

   Loganaden Velvindron
   cyberstorm.mu
   Rose Hill
   MU

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

   Kathleen Moriarty
   Dell EMC

   Email: Kathleen.Moriarty.ietf@gmail.com

   Alessandro Ghedini
   Cloudflare Inc.

   Email: alessandro@cloudflare.com

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