On Tue, Sep 20, 2016 at 11:33 AM Ilari Liusvaara <ilariliusva...@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
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