[Cfrg] draft-irtf-cfrg-eddsa - Implementation failure for 448

Jim Schaad <ietf@augustcellars.com> Wed, 20 July 2016 12:19 UTC

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From: Jim Schaad <ietf@augustcellars.com>
To: <draft-irtf-cfrg-eddsa@ietf.org>
Date: Wed, 20 Jul 2016 14:18:52 +0200
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Subject: [Cfrg] draft-irtf-cfrg-eddsa - Implementation failure for 448
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There is a difference between the document and the code for point addition
of 448.  Which is the correct one?

Text in section 5.2.4

    X3 = A*G*(H-C-D)
    Y3 = A*G*(D-C)

Python Code
        tmp.x=zcp*F*((self.x+self.y)*(y.x+y.y)-xcp-ycp)
        tmp.y,tmp.z=zcp*G*(ycp-xcp),F*G

Note that the first uses A*G the section uses A*F in computing the new x
value.

Jim