Re: [Cfrg] draft-irtf-cfrg-eddsa -- one final proposal for domain separation (context labels) for ed25519

Daniel Kahn Gillmor <dkg@fifthhorseman.net> Wed, 20 April 2016 12:51 UTC

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From: Daniel Kahn Gillmor <dkg@fifthhorseman.net>
To: "Salz, Rich" <rsalz@akamai.com>, "Paterson, Kenny" <Kenny.Paterson@rhul.ac.uk>, "cfrg@ietf.org" <cfrg@ietf.org>, "draft-irtf-cfrg-eddsa.all@ietf.org" <draft-irtf-cfrg-eddsa.all@ietf.org>
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Cc: Ondřej Surý <ondrej@sury.org>, Robert Edmonds <edmonds@debian.org>, "Kaduk, Ben" <bkaduk@akamai.com>, Martin Thomson <martin.thomson@gmail.com>
Subject: Re: [Cfrg] draft-irtf-cfrg-eddsa -- one final proposal for domain separation (context labels) for ed25519
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On Wed 2016-04-20 07:27:00 -0400, Salz, Rich wrote:
> This is okay with me, except for one pedantic clarification.  "Empty
> string" has a specific meaning in C, it's a single NUL byte.  Since
> our other uses of context including the NUL terminator, to avoid
> prefix attacks, then I think the wording needs some editing.

the "empty string" message in my message was not part of the proposed
wording change to the draft, but i can see how it might be confusing if
it were to make it into an edit.

If we need additional clarification in the draft to avoid confusion, i
propose:

  If no context label is supplied, it is treated as an octet string of
  zero length; that is, (context || x) is the same as x.

Regards,

  --dkg