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

Eric Rescorla <ekr@rtfm.com> Wed, 21 November 2018 22:01 UTC

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From: Eric Rescorla <ekr@rtfm.com>
Date: Wed, 21 Nov 2018 14:00:55 -0800
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To: Martin Thomson <martin.thomson@gmail.com>
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Subject: Re: [TLS] Empty CertificateRequest.supported_signature_algorithms in TLS 1.2
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On Wed, Nov 21, 2018 at 1:50 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)
>
> The text on CertificateRequest is less clear about what to do.  That's
> understandable because it doesn't have to deal with the value being
> absent because it's not optional.  All we have to go on is this from
> Section 7.4.8:
>
>    The hash and signature algorithms used in the signature MUST be
>    one of those present in the supported_signature_algorithms field
>    of the CertificateRequest message.
>
> 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.
>

Yes, I believe this is the right approach.

-Ekr


> This is consistent with our handling of the signature_algorithms
> extension, where we treat an empty list as a failure.
>
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