Reviewer: Daniel Migault Review result: Ready with Nits Hi,
I reviewed this document as part of the IoT Directorate's ongoing effort to review all IETF documents being processed by the IESG. These comments were written primarily for the benefit of the Security Area Directors. Document authors, document editors, and WG chairs should treat these comments just like any other IETF Last Call comments. Review Results: Ready with Nits Please find my comments below. Yours, Daniel Deprecating MD5 and SHA-1 signature hashes in TLS 1.2 draft-ietf-tls-md5-sha1-deprecate-04 [...] 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] and [RFC7525] 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. <mglt> RFC6194 may be mentioned as a reference for not deprecating HMAC-SHA-1 as well as an additional reference to [NISTSP800-131A-R2]. Reading the text the situation of HMAC with MD5 is unclear. Since we specify that SHA-1 is not deprecated for HMAC we may specify the status for HMAC with MD5. Given RFC6151 I hope the reason is that MD5 and HMAC-MD5 has already been deprecated but I have not found this. Maybe that would worth mentioning it is deprecated already. </mglt> [...] 2. Signature Algorithms Clients MUST NOT include MD5 and SHA-1 in the signature_algorithms extension. If a client does not send a signature_algorithms extension, then the server MUST abort the handshake and send a handshake_failure alert, except when digital signatures are not used (for example, when using PSK ciphers). <mglt> It seems to me that the server behavior might be defined as well. In our case this could be something around the lines the server MUST ignore MD5 and SHA1 values in the signature algorithm extension. </mglt> 3. Certificate Request Servers SHOULD NOT include MD5 and SHA-1 in CertificateRequest messages. <mglt> It seems to me that the same level of authentication should be provided for both peers and that server MUST NOT include MD5 or SHA-1. A SHOULD NOT status might be welcome for a smooth transition. At that time, collision for MD5 and SHA1 are known for years. It is likely that software that still need MD5 or SHA1 are likely to never upgrade, so I doubt a smooth path worth being taken. </mglt> 4. Server Key Exchange Servers MUST NOT include MD5 and SHA-1 in ServerKeyExchange messages. If a client receives a MD5 or SHA-1 signature in a ServerKeyExchange message it MUST abort the connection with the illegal_parameter alert. <mglt> As per section 2, the client has clearly indicated it does not support signature with MD5/SHA1, so Server Key Exchange should not end up with signature with SHA1/MD5. """ If the client has offered the "signature_algorithms" extension, the signature algorithm and hash algorithm MUST be a pair listed in that extension. """ It also seems to me that the constraint of including a MD5 and SHA-1 signature is related to the Certificate. I suspect that some clarification are needed here. Since the case where the extension becomes mandatory, the quoted text above of RFC 5246 might be updated as well, though this does not appear that necessary. </mglt> 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 handshake_failure or insufficient_security alert. <mglt> 6. Certificate Unless I am missing something, it seems to me that signature may also be found in the Certificate messages for the chain as well in the restriction of the signature algorithm. The end certificate is associated to the peer while other certificate are related to a CA. It seems that client and server behavior may be specified. The quoted text below may be helpful to clarify. """ If the client provided a "signature_algorithms" extension, then all certificates provided by the server MUST be signed by a hash/signature algorithm pair that appears in that extension. """ </mglt> 6. Updates to RFC5246 [RFC5246], The Transport Layer Security (TLS) Protocol Version 1.2, suggests that implementations can assume support for MD5 and SHA-1 by their peer. This update changes the suggestion to assume support for SHA-256 instead, due to MD5 and SHA-1 being deprecated. In Section 7.4.1.4.1: the text should be revised from: OLD: "Note: this is a change from TLS 1.1 where there are no explicit rules, but as a practical matter one can assume that the peer supports MD5 and SHA- 1." NEW: "Note: This is a change from TLS 1.1 where there are no explicit rules, but as a practical matter one can assume that the peer supports SHA-256." <mglt> I am reading the Note as an explanation on why sha was taken as the default hash function with the following rules. """ If the client does not send the signature_algorithms extension, the server MUST do the following: - If the negotiated key exchange algorithm is one of (RSA, DHE_RSA, DH_RSA, RSA_PSK, ECDH_RSA, ECDHE_RSA), behave as if client had sent the value {sha1,rsa}. - If the negotiated key exchange algorithm is one of (DHE_DSS, DH_DSS), behave as if the client had sent the value {sha1,dsa}. - If the negotiated key exchange algorithm is one of (ECDH_ECDSA, ECDHE_ECDSA), behave as if the client had sent value {sha1,ecdsa}. """ The current document does not update the default hash function from sha to sha256 to avoid interoperability issue where one peer takes sha while the other one takes sha-256. As a results, these rules and the "Note" may eventually all together be replaced by your text of section 2. The following text may also be removed: """ If the client supports only the default hash and signature algorithms (listed in this section), it MAY omit the signature_algorithms extension. """ Regarding the Note, it seems to be that the removal of support for MD5/SHA1 will result in interoperability issues. At this point, the issue is due to the obsolescence of the implementation as deprecation of SHA1/Md5 has started a long time ago. It is unclear to me how normative is interpreted "can assume". Was the support of MD5/SHA1 a SHOULD or a MUST? In both case, if we were willing to maintain interoperability between software that only implemented MD5/SHA1, we should take a slower path and introducing SHA-256 and having were MD5/SHA1 kept for interoperability purpose before being deprecated. I do not think we should take that path as implementations that currently do not support SHA-256 are unlikely to be updated and that deprecation of SHA1/MD5 has started a long time ago. I would however mention the issue of interoperability in the section but not in the text to update. In the text to update I would maybe suggest that the support of SHA-256 comes with a normative MUST statement. </mglt> Velvindron, et al. Expires April 12, 2021 [Page 3] Internet-Draft draft-ietf-tls-md5-sha1-deprecate October 2020 7. Updates to RFC7525 [RFC7525], Recommendations for Secure Use of Transport Layer Security (TLS) and Datagram Transport Layer Security (DTLS) recommends use of SHA-256 as a minimum requirement. This update moves the minimum recommendation to use stronger language deprecating use of both SHA-1 and MD5. The prior text did not explicitly include MD5 or SHA-1; and this text adds guidance to ensure that these algorithms have been deprecated.. Section 4.3: OLD: When using RSA, servers SHOULD authenticate using certificates with at least a 2048-bit modulus for the public key. In addition, the use of the SHA-256 hash algorithm is RECOMMENDED (see [CAB-Baseline] for more details). Clients SHOULD indicate to servers that they request SHA-256, by using the "Signature Algorithms" extension defined in TLS 1.2. NEW: Servers SHOULD authenticate using certificates with at least a 2048-bit modulus for the public key. In addition, the use of the SHA-256 hash algorithm is RECOMMENDED; and SHA-1 or MD5 MUST NOT be used (see [CAB-Baseline] for more details). Clients MUST indicate to servers that they request SHA- 256, by using the "Signature Algorithms" extension defined in TLS 1.2. <mglt> I understand the reason we do specify that hash algorithms that MUST NOT been used. This is fine in the context of this document, but it seems to me that if we were writing the updated specification we may have rather mentioned a minimum level of security hash function needs to be met - in our case SHA-256. I leave the co-authors make the appropriated choice. </mglt> 8. 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: +--------+----------------+-------------+-------------------+ | Value | Description | Recommended | Reference | +--------+----------------+-------------+-------------------+ | 0x0201 | rsa_pkcs1_sha1 | N | [RFC8446][RFCTBD] | | 0x0203 | ecdsa_sha1 | N | [RFC8446][RFCTBD] | +--------+----------------+-------------+-------------------+ Other entries of the resgistry remain the same. <mglt> It seems to me that TLS 1.2 is using the TLS hash and TLS signature registry TLS signature registry and TLS 1.3 is using Signature Scheme. I suspect that TLS hash values for sha1 and md5 should be deprecated. And RFCTBD should be added for sha1 and md5. Note that the SHOULD NOT status for CertificateRequest may have prevented such deprecation. A side effect is these code points for signature scheme that were assigned for compatibility with legacy (TLS 1.2) signatures must not be used anymore - if there are no more valid with TLS 1.2. </mglt> _______________________________________________ TLS mailing list TLS@ietf.org https://www.ietf.org/mailman/listinfo/tls