Re: [TLS] Renumbering the new SignatureSchemes

David Benjamin <davidben@chromium.org> Tue, 20 September 2016 15:56 UTC

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From: David Benjamin <davidben@chromium.org>
Date: Tue, 20 Sep 2016 15:56:01 +0000
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To: Ilari Liusvaara <ilariliusvaara@welho.com>
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Subject: Re: [TLS] Renumbering the new SignatureSchemes
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On Tue, Sep 20, 2016 at 11:33 AM Ilari Liusvaara <ilariliusvaara@welho.com>;
wrote:

> 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).
>

The ecdsa_p256_sha256 business means that the old scheme isn't quite
accurate. And if we are to drop the old scheme, it was intentional on my
part that RSA-PSS did not look like it, even though it still fit. I think
that paid off. No one's going to implement Ed25519 for a while, so RSA-PSS
is our smoke test that this SignatureScheme idea is sane. (Both for interop
and for making sure removing the hash/sig decomposition in implementations
internally is sound.)

David