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RFC 9155 PROPOSED STANDARD TLS · HASHES

No MD5/SHA-1 in TLS 1.2: Deprecating MD5 and SHA-1 Signature Hashes in TLS 1.2 and DTLS 1.2

TLS 1.2 and DTLS 1.2 may no longer use MD5 or SHA-1 in digital signatures.

On rfc-editor.org ↗
PUBLISHED
December 2021
AUTHORS
L. Velvindron, K. Moriarty, A. Ghedini
STREAM
IETF
BUILDS ON
RFC 5246 — TLS 1.2
RSA_PKCS1_SHA1 CODE
0x0201
ECDSA_SHA1 CODE
0x0203
ALERT ON VIOLATION
illegal_parameter
DOI
10.17487/RFC9155
LINEAGE · FROM THE RFC INDEXOpen full graph →
'08RFC 5246TLS 1.2'21RFC 9155No MD5/SHA-1 in TLS 1.2

RFC text

Contents
Original text · RFC Editor View on rfc-editor.org ↗ 
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: logan@cyberstorm.mu


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

   Email: Kathleen.Moriarty.ietf@gmail.com


   Alessandro Ghedini
   Cloudflare Inc.

   Email: alessandro@cloudflare.com

AI ELI5

On this page
PROBLEMTLS 1.2 allowed MD5 and SHA-1 for signature hashing, but both are subject to collision attacks.
MECHANISMRFC 9155 updates RFC 5246 so clients and servers MUST NOT use MD5 or SHA-1 in signature algorithm lists, key exchange and certificate verification messages, and IANA marks SHA-1 schemes as not recommended.
WHY IT MATTERSMD5 and SHA-1 MUST NOT be used for digital signatures in (D)TLS 1.2, while SHA-1 with HMAC for record protection is not deprecated.

§1Why MD5 and SHA-1 Were Retired

RFC 5246 specifies MD5 and SHA-1 for signature hashing in (D)TLS 1.2. Both have been shown to be insecure and subject to collision attacks. The RFC cites NIST deprecating SHA-1 in 2011 and disallowing it for digital signatures at the end of 2013, and a 2017 result showing that SHA-1 collision attacks are practical.

The CA/Browser Forum has also deprecated SHA-1 for certificate signatures. RFC 9155 updates RFC 5246 so MD5 and SHA-1 MUST NOT be used for digital signatures. SHA-1 with HMAC, as used in record protection, is not deprecated.

Two old secret-stamp designs can now be faked, so they should no longer be used to prove who signed something.

§2The Rules for Each Message

Each handshake message gets its own rule.

  • signature_algorithms extension: clients MUST include it and MUST NOT include MD5 and SHA-1 in it.
  • CertificateRequest: servers SHOULD NOT include MD5 and SHA-1.
  • ServerKeyExchange: servers MUST NOT include them. A client that sees one MUST abort the connection with an illegal_parameter alert.
  • CertificateVerify: clients MUST NOT include them. A server that receives one MUST abort with an illegal_parameter alert.

At every step of the greeting, neither side may offer the old stamps, and if one shows up anyway, the other side hangs up.

§3IANA Registry Updates

IANA changed the recommended status of SHA-1-based schemes in the "TLS SignatureScheme" registry to "N" (not recommended), as defined by RFC 8447. The entries are 0x0201 (rsa_pkcs1_sha1) and 0x0203 (ecdsa_sha1).

IANA also updated the reference for the "TLS SignatureAlgorithm" and "TLS HashAlgorithm" registries to include this document.

The official list now marks the two SHA-1 entries with a warning label.

§4Security Considerations

The concern is that (D)TLS 1.2 implementations might fall back to SHA-1. The document deprecates MD5 and SHA-1 for digital signatures to address that, while leaving SHA-1 with HMAC for record protection alone.

The worry is that computers quietly slip back to the old stamp, so the rule says not to.

§5References

[RFC5246]RFC 5246Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS) Protocol Version 1.2", RFC 5246, August 2008.August 2008[RFC8446]RFC 8446Rescorla, E., "The Transport Layer Security (TLS) Protocol Version 1.3", RFC 8446, August 2018.August 2018[RFC8447]RFC 8447Salowey, J. and S. Turner, "IANA Registry Updates for TLS and DTLS", RFC 8447, August 2018.August 2018[RFC6151]RFC 6151Turner, S. and L. Chen, "Updated Security Considerations for the MD5 Message-Digest and the HMAC-MD5 Algorithms", RFC 6151, March 2011.March 2011