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 > > TLS@ietf.org > > 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
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