Re: [TLS] Consistency for Signature Algorithms?

[Benjamin Kaduk <[e-mail]>]

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.

-Ben


> 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%)
>
>
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