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

Xiaoyin Liu <xiaoyin.l@outlook.com> Mon, 19 September 2016 23:18 UTC

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From: Xiaoyin Liu <xiaoyin.l@outlook.com>
To: Eric Rescorla <ekr@rtfm.com>
Thread-Topic: [TLS] Should TLS 1.3 servers send "signature_algorithms" extensions
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Subject: Re: [TLS] Should TLS 1.3 servers send "signature_algorithms" extensions
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Thank you for your explanation, Eric!

Xiaoyin

From: Eric Rescorla [mailto:ekr@rtfm.com]
Sent: Monday, September 19, 2016 7:13 PM
To: Xiaoyin Liu <xiaoyin.l@outlook.com>;
Cc: tls@ietf.org
Subject: Re: [TLS] Should TLS 1.3 servers send "signature_algorithms" extensions



On Mon, Sep 19, 2016 at 3:56 PM, Xiaoyin Liu <xiaoyin.l@outlook.com<mailto: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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