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Re: [Cfrg] (flaws with Curve25519 DH function, if one does not check the output) Re: Elliptic Curves - curve form and coordinate systems

Viktor Dukhovni <ietf-dane@dukhovni.org> Tue, 17 March 2015 00:48 UTC

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From: Viktor Dukhovni <ietf-dane@dukhovni.org>
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Subject: Re: [Cfrg] (flaws with Curve25519 DH function, if one does not check the output) Re: Elliptic Curves - curve form and coordinate systems
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On Mon, Mar 16, 2015 at 08:39:06AM -0700, Watson Ladd wrote:

> Of course, we've always described what's required to ensure contributory
> behavior in Curve25519, and it's always the case that implementors
> should know what the protocol requires as far as assuring contributory
> behavior, namely rejecting 8 inputs. TLS is really the odd man out in
> requiring contributory behavior.

If I recall correctly it is simpler to reject the corresponding
single output (namely 0) than the 8 inputs that map to zero.
Admittedly my memory of the details is a bit fuzzy on this...

Since the output is sensitive, all one has to do is a constant-time
memcmp of the output with a 256-bit zero.

Does TLS need to defend against this?  It is not obvious that it
does.

-- 
	Viktor.

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