Re: [TLS] Renumbering the new SignatureSchemes

Ilari Liusvaara <ilariliusvaara@welho.com> Tue, 20 September 2016 15:38 UTC

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Date: Tue, 20 Sep 2016 18:33:27 +0300
From: Ilari Liusvaara <ilariliusvaara@welho.com>
To: David Benjamin <davidben@chromium.org>
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Subject: Re: [TLS] Renumbering the new SignatureSchemes
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On Tue, Sep 20, 2016 at 03:07:51PM +0000, David Benjamin wrote:
> Hi folks,
> 
> I've just uploaded this PR to slightly tweak SignatureScheme numbering:
> https://github.com/tlswg/tls13-spec/pull/641
> 
> In principle, we should only have needed to burn values starting with known
> HashAlgorithms, but TLS 1.2 said:
> 
>    signature
>       This field indicates the signature algorithm that may be used.
>       The values indicate anonymous signatures, RSASSA-PKCS1-v1_5
>       [PKCS1] and DSA [DSS], and ECDSA [ECDSA], respectively.  The
>       "anonymous" value is meaningless in this context but used in
>       Section 7.4.3.  It MUST NOT appear in this extension.
> 
> We'd started RSA-PSS along the train to get shipped in Chrome to get early
> warning on any interoperability issues. We ran into an implementation which
> enforced this MUST NOT. It's a MUST NOT in 1.2, so it seems prudent to
> allocate around it and avoid ending in known SignatureAlgorithms. Thus,
> rather than only burning {0x00-0x06, *}, we also burn {*, 0x00-0x03}. This
> has the added benefit that TLS 1.2 dissector tools don't get confused.

Heck, I think one could put the RSA-PSS ones as 0404, 0504 and 0604,
as those do have the indicated "prehashes".

And one could probably also stick Ed25519/Ed448 in 00xx, as those have
no prehash, which is exactly what "hash #0" is about.

(Of course, this all is pretty pointless bikeshedding).


-Ilari