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Re: [TLS] Empty CertificateRequest.supported_signature_algorithms in TLS 1.2

Viktor Dukhovni <ietf-dane@dukhovni.org> Wed, 21 November 2018 21:56 UTC

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From: Viktor Dukhovni <ietf-dane@dukhovni.org>
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Date: Wed, 21 Nov 2018 16:56:43 -0500
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Subject: Re: [TLS] Empty CertificateRequest.supported_signature_algorithms in TLS 1.2
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> On Nov 21, 2018, at 4:50 PM, Martin Thomson <martin.thomson@gmail.com> wrote:
> 
> In attempting to fix a bug related to this, a question came up about
> what the semantics of an empty value is here.  Some implementations
> seem to infer that empty means {*,SHA1}, which effectively assumes
> that an empty value is equivalent to an absent signature_algorithms
> extension (Section 7.4.1.4.1)
> 
> [...]
> 
> We think that treating an empty supported_signature_algorithms field
> as an error is the best response and plan to implement that change.
> We'll send a fatal alert if we receive one.

Sounds reasonable, provided the application did or would have actually
provided a client certificate to use.  If the application would have
continued with an empty certificate list, and no CertificateVerify,
then hanging up is arguably counter-productive.  Of course the server
should still not send a certificate request that could never be satisfied,
so perhaps snatching victory from the jaws of defeat is not worth the
trouble even when the client has no certificate to send?

-- 
	Viktor.

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