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 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? Steve. -- Dr Stephen N. Henson. Founder member of the OpenSSL project: http://www.openssl.org/ _______________________________________________ TLS mailing list TLS@ietf.org https://www.ietf.org/mailman/listinfo/tls
