Re: [TLS] Consistency for Signature Algorithms?

[Ilari Liusvaara <[e-mail]>]

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