Internet-Draft draft-ietf-tls-md5-sha1-deprecate September 2021
Velvindron, et al. Expires 24 March 2022 [Page]
Internet Engineering Task Force
5246 (if approved)
Intended Status:
Standards Track
L.V. Velvindron
K.M. Moriarty
A.G. Ghedini
Cloudflare Inc.

Deprecating MD5 and SHA-1 signature hashes in (D)TLS 1.2


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.

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

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This Internet-Draft will expire on 24 March 2022.

Table of Contents

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] 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. Note that the 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

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:

Table 1
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.

IANA is also requested to update the Reference for the TLS SignatureAlgorithm and TLS HashAlgorithm registries to refer to this RFC:







7. 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.

8. 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, Sean Turner, Christopher Wood and David Cooper for their feedback.

9. References

9.1. Normative References

Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <>.
Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS) Protocol Version 1.2", RFC 5246, DOI 10.17487/RFC5246, , <>.
Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, , <>.
Rescorla, E., "The Transport Layer Security (TLS) Protocol Version 1.3", RFC 8446, DOI 10.17487/RFC8446, , <>.
Salowey, J. and S. Turner, "IANA Registry Updates for TLS and DTLS", RFC 8447, DOI 10.17487/RFC8447, , <>.

9.2. Informative References

CA/Browser Forum, "Ballot 118 -- SHA-1 Sunset (passed)", , <>.
Barker, E.B. and A.R. Roginsky, "Transitioning the Use of Cryptographic Algorithms and Key Lengths", , <>.
Turner, S. and L. Chen, "Updated Security Considerations for the MD5 Message-Digest and the HMAC-MD5 Algorithms", RFC 6151, DOI 10.17487/RFC6151, , <>.
Stevens, M.S., Bursztein, E.B., Karpman, P.K., Albertini, A.A., and Y.M. Markov, "The first collision for full SHA-1", , <>.
Bhargavan, K.B. and G.L. Leurent, "Transcript Collision Attacks: Breaking Authentication in TLS, IKE, and SSH", , <>.
Wang, X.W., Yin, Y.Y., and H.Y. Yu, "Finding Collisions in the Full SHA-1", , <>.

Authors' Addresses

Loganaden Velvindron
Rose Hill
Phone: +230 59762817
Kathleen Moriarty
Center for Internet Security
East Greenbush, NY
United States of America
Alessandro Ghedini
Cloudflare Inc.