On Fri, Jul 21, 2017 at 8:00 AM, Ilari Liusvaara <ilariliusva...@welho.com> wrote:
> 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). > But this doesn't affect security if i understand the model correctly. > > > > > -Ilari > > > > > > _______________________________________________ > TLS mailing list > TLS@ietf.org > https://www.ietf.org/mailman/listinfo/tls >
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