Re: [TLS] draft-ietf-tls-curve25519-01: Is public key validation necessary or helpful?

Brian Smith <brian@briansmith.org> Thu, 31 December 2015 00:47 UTC

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Date: Wed, 30 Dec 2015 14:47:20 -1000
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From: Brian Smith <brian@briansmith.org>
To: Watson Ladd <watsonbladd@gmail.com>
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Cc: Karthikeyan Bhargavan <karthik.bhargavan@gmail.com>, "<tls@ietf.org>" <tls@ietf.org>
Subject: Re: [TLS] draft-ietf-tls-curve25519-01: Is public key validation necessary or helpful?
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Watson Ladd <watsonbladd@gmail.com> wrote:

> Why not hash the public values into the result of the key exchange? I
> don't want security to depend on omittable checks.
>

One would need an omittable check in the code to decide whether to do that
extra hashing, so that wouldn't solve the (non-)problem of "omittable
checks".

Similarly, one would need an omittable check to decide whether to require
the session hash extension, so it wouldn't solve the (non-)problem of
"omittable checks".

Actually, because the check for non-zero result can/should/is in the
X25519/X448 functions themselves, the check for non-zero result is the
least likely of all these possible solutions to be omitted. And, it is also
the easiest to test.

Cheers,
Brian
-- 
https://briansmith.org/