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.

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.

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The usage of MD5 and SHA-1 for signature hashing in (D)TLS 1.2 is specified in [RFC 5246]. MD5 and SHA-1 have been proven to be insecure, subject to collision attacks [Wang]. In 2011, [RFC 6151] 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 [RFC 5246] 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].

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 [RFC 2119] [RFC 8174] when, and only when, they appear in all capitals, as shown here.

Clients MUST include the signature_algorithms extension. Clients MUST NOT include MD5 and SHA-1 in this extension.

Servers SHOULD NOT include MD5 and SHA-1 in CertificateRequest messages.

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.

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.

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 [RFC 8447]. The following entries have been updated; other entries in the registry remain the same. 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.

Concerns with (D)TLS 1.2 implementations falling back to SHA-1 is an issue. This document updates the TLS 1.2 specification [RFC 5246] 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.

[RFC2119]

S. Bradner, "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]

T. Dierks, 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]

B. Leiba, "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]

E. Rescorla, "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]

J. Salowey, 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>.

[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]

E. Barker, 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]

S. Turner, 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]

M. Stevens, E. Bursztein, P. Karpman, A. Albertini, and Y. Markov, "The First Collision for Full SHA-1", 2017,
<https://eprint.iacr.org/2017/190>.

[Transcript-Collision]

K. Bhargavan, 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]

X. Wang, Y. Yin, 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>.

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.

Loganaden Velvindron

Kathleen Moriarty

Alessandro Ghedini