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Re: [TLS] PRF Negotiation - Finished "gotcha"

Martin Thomson <martin.thomson@gmail.com> Fri, 18 April 2014 16:46 UTC

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Date: Fri, 18 Apr 2014 09:46:20 -0700
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From: Martin Thomson <martin.thomson@gmail.com>
To: Michael StJohns <msj@nthpermutation.com>
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Subject: Re: [TLS] PRF Negotiation - Finished "gotcha"
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On 18 April 2014 08:05, Michael StJohns <msj@nthpermutation.com> wrote:
> As I was working through the "finished" message email I just sent, I
> realized something about PRF negotiation.   The client, when it sends a
> ClientHello, doesn't necessarily know which PRF it will be using (assuming
> there are multiple PRFs defined in the offered cipher suites) until it gets
> the ServerHello back.  I think that means that the client either needs to
> keep a copy of the ClientHello around (or the data to rebuild it), or needs
> to keep multiple HASH states - one for each PRF algorithm.
>
> It may be useful to add a paragraph on PRF negotiation implications that
> provides this guidance.

Indeed.

There is another way to avoid this problem: don't negotiate the hash
function used for the PRF.  SHA-256 might be good enough until we next
revise the protocol.

https://github.com/tlswg/tls13-spec/issues/26

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