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Re: [TLS] signature_algorithms_cert extension

Eric Rescorla <ekr@rtfm.com> Thu, 18 January 2018 13:17 UTC

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From: Eric Rescorla <ekr@rtfm.com>
Date: Thu, 18 Jan 2018 05:16:19 -0800
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To: Matt Caswell <matt@openssl.org>
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Subject: Re: [TLS] signature_algorithms_cert extension
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On Thu, Jan 18, 2018 at 3:29 AM, Matt Caswell <matt@openssl.org> wrote:

> The specification of the new signature_algorithms_cert seems somewhat
> lacking to me. There is very little description about how it should be
> interpreted. About the best I can get from the spec is this:
>
>    The "signature_algorithms_cert" extension applies to signatures in
>    certificates and the "signature_algorithms" extension, which
>    originally appeared in TLS 1.2, applies to signatures in
>    CertificateVerify messages.
>
> But in section 4.4.2.2 we see this:
>
>    All certificates provided by the server MUST be signed by a signature
>    algorithm that appears in the "signature_algorithms" extension
>    provided by the client, if they are able to provide such a chain (see
>    Section 4.2.3).  Certificates that are self-signed or certificates
>    that are expected to be trust anchors are not validated as part of
>    the chain and therefore MAY be signed with any algorithm.
>
>
> Is this an oversight? Should this reference "signature_algorithms_cert"
> as well/instead?
>

Yes, it's an oversight that it didn't get added here.


Some questions:
>
> - Is "signature_algorithms_cert" mandatory to implement for servers? It
> does not appear in 9.2 so I am assuming not. There is some text in 4.2.3
>  which says what to do if "signature_algorithms_cert" is not present -
> which seems to confirm that it is not mandatory for clients at least.
>

Both sides need to implement it for the purposes of filtering the
certificates
they send. Neither side need send it if it has a consistent policy for
CertVerify
and chain validation.


- Are we allowed to ignore "signature_algorithms_cert" if we can't build
> a chain and honour its contents?
>

Same rules as signature_algorithms.

I have filed https://github.com/tlswg/tls13-spec/pull/1142 to clarify these
points.

-Ekr



>
> Matt
>
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