Re: [TLS] draft-ietf-tls-tls13-21 posted

Martin Thomson <martin.thomson@gmail.com> Thu, 06 July 2017 02:27 UTC

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From: Martin Thomson <martin.thomson@gmail.com>
Date: Thu, 06 Jul 2017 12:27:18 +1000
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To: Kazu Yamamoto <kazu@iij.ad.jp>
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Subject: Re: [TLS] draft-ietf-tls-tls13-21 posted
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On 6 July 2017 at 12:19, Kazu Yamamoto <kazu@iij.ad.jp> wrote:
> The definitions would be:
>
>        HKDF-Expand-Label(Secret, Label, *Value*, Length) =
>             HKDF-Expand(Secret, HkdfLabel, Length)
>
>        struct {
>            uint16 length = *Value.length*;
>            opaque label<7..255> = "tls13 " + Label;
>            opaque hash_value<0..255> = *Value*;
>        } HkdfLabel;

Length is the size of the output, so you don't want to assign
Value.length to that field in the struct.  Also, you forgot to rename
hash_value in the struct.

The name "context" strikes me as a good choice here.  It shares a neat
parallel with the name used in exporters.  But I trust that ekr will
find a reasonable way of solving this.