RFC 9155

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

📅 published December 2021, from ietf

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

Okay bestie, this is RFC 9155 💖 It's a Standards Track document from the IETF, published December 2021 (ISSN 2070-1721). The authors are Loganaden Velvindron (cyberstorm.mu), Kathleen Moriarty (CIS), and Alessandro Ghedini (Cloudflare Inc.). It updates RFC 5246 🎀

Abstract

MD5 and SHA-1 are getting easier to break, so this doc says goodbye to them as digital signature hashes in TLS 1.2 and DTLS 1.2 🥀 But it's not a full breakup! SHA-1 with HMAC, the one used in record protection, is still fine and is NOT deprecated here 🫶 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, got public review, and was approved for publication by the Internet Engineering Steering Group (IESG). If you want more info on Internet Standards, check out Section 2 of RFC 7841 📚

Current status, errata, and feedback info live at https://www.rfc-editor.org/info/rfc9155 🦋

Copyright Notice

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) as in effect on the date of publication. Please read these carefully, because they spell out your rights and restrictions. Code Components pulled out of this document must include Revised BSD License text, as described in Section 4.e of the Trust Legal Provisions, and come without warranty as described in that license 🙈

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 ✨

So MD5 and SHA-1 for signature hashing in (D)TLS 1.2 are specified in [RFC5246]. Both have been proven insecure to collision attacks [Wang] 😭 Back in 2011, [RFC6151] went over the security considerations, including MD5 collision attacks. Also in 2011, NIST formally deprecated SHA-1 [NISTSP800-131A-R2] and banned it for digital signatures by the end of 2013, based on the [Wang] attack and the potential for brute-force attacks 💀 In 2016, researchers at the National Institute for Research in Digital Science and Technology (INRIA) found a new class of transcript collision attacks on TLS (and other protocols). These rely on efficient collision-finding algorithms on the underlying hash constructions [Transcript-Collision] 🔥 Then in 2017, researchers from Google and Centrum Wiskunde & Informatica (CWI) Amsterdam [SHA-1-Collision] proved SHA-1 collision attacks are practical. Rip 🥀

This document updates [RFC5246] so that MD5 and SHA-1 MUST NOT be used for digital signatures 🚫 However, it does not deprecate SHA-1 with HMAC, as used in record protection 🫶 Also, the CA/Browser Forum (CABF) has deprecated SHA-1 for certificate signatures too [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 gets a ServerKeyExchange message indicating MD5 or SHA-1, 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 gets a CertificateVerify message with MD5 or SHA-1, it MUST abort the connection with an illegal_parameter alert 💀

6. IANA Considerations 📚

IANA updated the "TLS SignatureScheme" registry by changing the recommended status of SHA-1-based signature schemes to "N" (not recommended), as defined by [RFC8447] ✨ Only the entries below changed. Everything else in the registry stays the same 🍓

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

                                  Table 1

IANA also updated the "TLS SignatureAlgorithm" and "TLS HashAlgorithm" registries so they point to this document as well as RFCs 5246 and 8447 🔥

7. Security Considerations 🥀

Okay this section is important, so listen up 👀 Falling back to SHA-1 in (D)TLS 1.2 implementations is a real problem. This document updates the TLS 1.2 specification [RFC5246] to deprecate support for MD5 and SHA-1 for digital signatures 🚨 But again, SHA-1 with HMAC, as used in record protection, is NOT deprecated 🫶

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 thank Hubert Kario for helping write the initial draft. They're 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

📦 where this page comes from

ietf: rfc9155 2021-12
💖 girlified by claude-haiku-5-5 on 2026-10-18: a language model's rewrite of the original, under its terms