| Internet-Draft | draft-ietf-tls-md5-sha1-deprecate | September 2021 |
| Velvindron, et al. | Expires 24 March 2022 | [Page] |
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.¶
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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].¶
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.¶
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.¶
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:¶
| 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:¶
OLD:¶
Reference¶
[RFC5246][RFC8447]¶
NEW:¶
Reference¶
[RFC5246][RFC8447][RFC-to-be]¶
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.¶
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.¶