On Fri, Jul 21, 2017 at 02:41:50PM +0100, Dr Stephen Henson wrote: > On 21/07/2017 14:23, Hubert Kario wrote: > > Signature Algorithms for ECDSA now define both the curve and the hash > > algorithm: > > > > ecdsa_secp256r1_sha256(0x0403), ecdsa_secp384r1_sha384(0x0503), > > ecdsa_secp521r1_sha512(0x0603), > > > > This is in contrast to the TLS 1.2 protocol, where any hash can be used > > with any curve. > > > > There are good reasons for that change: - less combinations to test - > > establishes the low water mart for security > > > > I see few problems with that though: 1). there are not insignificant number > > of clients that advertise support for all (at least P-256 and P-384) > > curves, but don't advertise support for hashes stronger than SHA-256 with > > ECDSA[1] 2). This is inconsistent with RSA-PSS behaviour, where key size is > > completely detached from the used hash algorithm. 3). This is not how ECDSA > > signatures in X.509 work, so it doesn't actually limit the signatures on > > certificates (in other words, as an implementer you need to support all > > hashes with all curves either way) > > > > There is another and significant problem with the change. In TLS 1.2 support > for a curve was indicated in the supported curves extension and it implied > support for ECDH and ECDSA. This has changed in TLS 1.3: curves in the > supported groups extension are for ECDH only. Support for a curve for ECDSA is > indicated in the signature algorithms extension.
I suppose some new dual-version TLS 1.2/1.3 libraries might have the same issue as mine: supported groups is just plain ignored for ECDSA, and siganture algorithms have the TLS 1.3 meanings, even in TLS 1.2. > I agree though that there is an anomaly here. For example AFAICS in > certificates for TLS1.3 you're allowed (with some caveats) to use a > P-256+SHA-1 signature (which has security concerns) but P-256 + SHA512 is not > allowed at all. Is that likely to be a problem in practice? Are there many > such certificates about in the wild? Actually, P-256+SHA512 is allowed with a caveat, even if the certificate is not self-signed. And with the same caveat, server can send a certificate signed with P-256+SHA3-512, despite TLS codepoint for it having never existed (not many clients can validate it through). -Ilari _______________________________________________ TLS mailing list TLS@ietf.org https://www.ietf.org/mailman/listinfo/tls
