Re: [TLS] draft-ietf-tls-curve25519-01 and the X25519 significant bit.

Kurt Roeckx <kurt@roeckx.be> Wed, 30 December 2015 10:30 UTC

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Date: Wed, 30 Dec 2015 11:30:40 +0100
From: Kurt Roeckx <kurt@roeckx.be>
To: Brian Smith <brian@briansmith.org>
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Cc: Adam Langley <agl@imperialviolet.org>, Simon Josefsson <simon@josefsson.org>, "tls@ietf.org" <tls@ietf.org>
Subject: Re: [TLS] draft-ietf-tls-curve25519-01 and the X25519 significant bit.
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On Tue, Dec 29, 2015 at 09:02:25AM -1000, Brian Smith wrote:
> 
> Does that matter, though? The CFRG document doesn't allow the sender to set
> the high bit to 1, right? In particular, it says "All calculations are
> performed in GF(p), i.e., they are performed modulo p." and "For X25519,
> the unused, most-significant bit MUST be zero."
> 
> If the receiver can detect that the sender is non-conforming, then it
> should be able to stop talking to it on that basis alone.

I don't know enough about all the various draft to know if this
might be a problem or not, but I'm concerned about providing an
error oracle.


Kurt