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

Ilari Liusvaara <ilariliusvaara@welho.com> Fri, 06 May 2016 10:17 UTC

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Date: Fri, 6 May 2016 13:17:33 +0300
From: Ilari Liusvaara <ilariliusvaara@welho.com>
To: Simon Josefsson <simon@josefsson.org>
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References: <87bn543id1.fsf@alice.fifthhorseman.net> <87zisk94bg.fsf@latte.josefsson.org> <871t5wxfs4.fsf@alice.fifthhorseman.net> <87d1ozvfak.fsf@latte.josefsson.org>
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Subject: Re: [Cfrg] draft-irtf-cfrg-eddsa -- one final proposal for domain separation (context labels) for ed25519
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On Fri, May 06, 2016 at 09:03:47AM +0200, Simon Josefsson wrote:
> Daniel Kahn Gillmor <dkg@fifthhorseman.net> writes:
> 
> > A recent example is
> > https://tools.ietf.org/html/draft-ietf-curdle-dnskey-eddsa-00, which
> > defines DNSKEY records for both Ed25519 and Ed448.  I spoke to the
> > authors about using a context label to avoid RRSIGs from being re-usable
> > in non-RRSIG contexts.
> 
> Instead of introducing complexity (i.e., contexts), I suggest that you
> recommend to not re-use the same key for different purposes!
> 
> Key re-use for different purposes is the fundamental problem as I see
> it.  The idea behind context may have good intentions, but I believe
> contexts is only a band-aid for the problem of cross-domain key-reuse,
> and I believe that overall contexts introduce more harm than they
> address.  I believe it is better to address the problem at its source.

Actually, the problem is that context have serious fundamential usabilty
problems.

If you have self-inconsistent protocol, nothing will save you. However,
contexts suffer from serious usability problems (API and signature
layering), making them effectively unusable, even with protocols that
are self-consistent.

So yeah, just use separate keys. Don't cause problems for everybody
by using contexts.



-Ilari