Re: [TLS] Should TLS 1.3 servers send "signature_algorithms" extensions

Eric Rescorla <ekr@rtfm.com> Mon, 19 September 2016 23:13 UTC

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From: Eric Rescorla <ekr@rtfm.com>
Date: Mon, 19 Sep 2016 16:12:43 -0700
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To: Xiaoyin Liu <xiaoyin.l@outlook.com>
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Subject: Re: [TLS] Should TLS 1.3 servers send "signature_algorithms" extensions
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On Mon, Sep 19, 2016 at 3:56 PM, Xiaoyin Liu <xiaoyin.l@outlook.com>; wrote:

> Hello,
>
>
>
> There seems to be a conflict in the TLS 1.3 spec on whether servers should
> send “signature_algorithms” extension or not. In section 4.2.2 Signature
> Algorithms <https://tlswg.github.io/tls13-spec/#signature-algorithms>;, it
> says:
>
> Servers which are authenticating via a certificate MUST indicate so by
> sending the client an empty “signature_algorithms” extension.
>
>
>
> But in section 8.2 MTI Extensions
> <https://tlswg.github.io/tls13-spec/#mti-extensions>;, it says:
>
> Servers MUST NOT send the “signature_algorithms” extension
>
>
>
> So should a server send am empty “signature_algorithms” extension or not
> in ServerHello?
>

Section 8.2 is a bug in the spec. Servers need to send sig_algs if they are
signing.

David Benjamin has suggested an alternative encoding which I may put in a
future draft, but for -15, you need to send it.

-Ekr


>
> Thank you!
>
> Xiaoyin
>
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