Re: [TLS] Consistency for Signature Algorithms?

[Hubert Kario <[e-mail]>]

On Friday, 21 July 2017 15:38:32 CEST Benjamin Kaduk wrote:
> On 07/21/2017 08:23 AM, 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.
> 
> I assume you saw
> https://www.ietf.org/mail-archive/web/tls/current/msg23714.html which
> raised a different question in this same general area.
> 
> I do not see how the response here cannot be the same as it was there:
> namely, that the current formulation is assumed to have WG consensus,
> having been through two WGLCs; there would need to be rather strong
> reasons to make changes at this stage.

MTI (section 9.1) says only that ecdsa_secp256r1_sha256 is mandatory (MUST) 
and no word about any of the other two.

given that we already have CAs that use P-384 for signatures. I'd say this is 
a big conflict between theory and practice.

> > 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)
> > 
> > With the implementers hat on, I'd prefer to drop the curves from signature
> > algorithm names/specifications and return to TLS 1.2 behaviour.
> > With my security hat on, I'd say that we should set the minimal key sizes
> > for RSA-PSS signatures too, as we did with ECDSA.
> > 
> > Any other ideas?
> > 
> >  1 - Nick Sullivan from Cloudflare provided me with some stats from random
> > 
> > 50000 client hellos from early 2017:
> > 
> > Sigalgs:
> > ECDSA + SHA-256 = 39104 (78.2%)
> > ECDSA + (SHA-256 + SHA-384 + SHA-512) = 28678 (57.4%)
> > ECDSA + (SHA-256 + SHA-384 + !SHA-512) = 8934 (17.9%)
> > ECDSA + (SHA-256 + !SHA-384 + !SHA-512) = 1492 (2.98%)
> > 
> > Note: many of the 1492 seem to be on iOS and only support:
> > [RSA-SHA-384, RSA-SHA-256, RSA-SHA1, ECDSA-SHA256, ECDSA-SHA1]
> > 
> > Curves:
> > none = 757 (1.51%)
> > P-256 = 49243 (98.5%)
> > P-384 = 49233 (98.5%)
> > P-256 + P-384 = 49233 (98.5%)
> > P-256 + !P-384 = 10 (0.02%)
> > !P-256 + P-384 = 0 (0%)
> > 
> > 
> > _______________________________________________
> > TLS mailing list
> > [e-mail]
> > https://www.ietf.org/mailman/listinfo/tls


-- 
Regards,
Hubert Kario
Senior Quality Engineer, QE BaseOS Security team
Web: www.cz.redhat.com
Red Hat Czech s.r.o., Purkyňova 115, 612 00  Brno, Czech Republic