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Re: [Cfrg] Handling invalid points

Michael Hamburg <mike@shiftleft.org> Sat, 22 November 2014 02:34 UTC

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From: Michael Hamburg <mike@shiftleft.org>
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Date: Fri, 21 Nov 2014 18:34:54 -0800
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Subject: Re: [Cfrg] Handling invalid points
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> On Nov 21, 2014, at 4:57 PM, Michael Hamburg <mike@shiftleft.org> wrote:
> 
> 2)
> 
> For curves with a cofactor, ECDH is not 1:1.  This affects Trevor Perrin’s
> TripleDH protocol, which attempts to work around concerns about patents on
> session-hashing (don’t ask me what those are, I haven’t studied them).  As
> a result, TripleDH implementations are forbidden to clear the cofactor, and
> secret keys must be invertible mod the cofactor (i.e. odd).  If this requirement
> were violated, it probably wouldn’t lead to an attack, but it wouldn’t guarantee
> that the sessions seen by both sides were exactly the same.

Trevor has corrected me: TripleDH no longer forbids clearing the cofactor, because
that’s weird.  Instead it deals with this issue in a different way.

— Mike

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