]> Deprecating MD5 and SHA-1 Signature Hashes in TLS 1.2 and DTLS 1.2 cyberstorm.mu
Rose Hill Mauritius +230 59762817 logan@cyberstorm.mu
Center for Internet Security
East Greenbush NY United States of America Kathleen.Moriarty.ietf@gmail.com
Cloudflare Inc.
alessandro@cloudflare.com
Security TLS tls 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.
Introduction The usage of MD5 and SHA-1 for signature hashing in (D)TLS 1.2 is specified in . MD5 and SHA-1 have been proven to be insecure, subject to collision attacks . In 2011, detailed the security considerations, including collision attacks for MD5. NIST formally deprecated use of SHA-1 in 2011 and disallowed its use for digital signatures at the end of 2013, based on both the attack described in 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 . Further, in 2017, researchers from Google and Centrum Wiskunde & Informatica (CWI) Amsterdam proved SHA-1 collision attacks were practical. This document updates 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 .
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 when, and only when, they appear in all capitals, as shown here.
Signature Algorithms Clients MUST include the signature_algorithms extension. Clients MUST NOT include MD5 and SHA-1 in this extension.
Certificate Request Servers SHOULD NOT include MD5 and SHA-1 in CertificateRequest messages.
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.
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.
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 . The following entries have been updated; other entries in the registry remain the same.
Value Description Recommended Reference
0x0201 rsa_pkcs1_sha1 N [RFC9155]
0x0203 ecdsa_sha1 N [RFC9155]
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.
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 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.
References Normative References Informative References Transitioning the Use of Cryptographic Algorithms and Key Lengths Ballot 118 -- SHA-1 Sunset (passed) CA/Browser Forum Transcript Collision Attacks: Breaking Authentication in TLS, IKE, and SSH The First Collision for Full SHA-1 Finding Collisions in the Full SHA-1
Acknowledgements The authors would like to thank for his help in writing the initial draft version of this document. We are also grateful to , , , , and for their feedback.