RFC 7748, "Elliptic Curves for Security", January 2016Source of RFC: IRTF
Errata ID: 4730
Reported By: Adam Langley
Date Reported: 2016-07-05
Verifier Name: Lars Eggert
Date Verified: 2016-07-05
Section 4.1 says:
V(P) 147816194475895447910205935684099868872646061346164752889648818 37755586237401
It should say:
V(P) 431144251710685529207648989359339670393703861982038067307639101 66200978582548
The Montgomery form of the curve is generally used with a ladder, where the v coordinate is unused and unspecified. Thus I picked the smaller of the two possible values for v.
However, the curve is birationally equivalent to edwards25519, where both coordinates of the base point are used and are already in widespread use. Sadly, picking the smaller of the values for v ends up mapping to the negative of the base point on edwards25519.
This change replaces v with -v so that it matches up.