Re: [TLS] Delegated Credentials Question about PSS

"Martin Thomson" <mt@lowentropy.net> Mon, 21 October 2019 22:34 UTC

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Date: Tue, 22 Oct 2019 09:33:56 +1100
From: "Martin Thomson" <mt@lowentropy.net>
To: "Watson Ladd" <watsonbladd@gmail.com>
Cc: "Nick Sullivan" <nick@cloudflare.com>, "TLS List" <tls@ietf.org>
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Subject: Re: [TLS] Delegated Credentials Question about PSS
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On Thu, Oct 17, 2019, at 14:32, Watson Ladd wrote:
> In TLS 1.3 it seems to have been assumed this wouldn't happen and we 
> could split signature algorithms from signature algorithms cert.
> 
> If that's not actually the case it affects more than just DCs. DCs are 
> a good way to restore extensibility if there is a problem here, 
> provided we can come up with a solution.

Yeah, I think that this is the clincher.

FWIW, in Firefox we have a separation between TLS and the certificate validation logic.  The latter cares about the SPKI of all certificates in the certification path because it applies policy related to choice of keys.  As a result, that code cares about DC also.  So we don't really get to advertise an algorithm until it is supported in both places.  For that reason signature_algorithms_cert, as much as it is intended to address this sort of split, doesn't really help us.

Logically, there is a split between the certification path construction and the policy pieces, and the structure of the code recognizes this, but in practice they are somewhat coupled.