Conditional move: t = u, if and only if b != 0.
calculates a * A + b * B B is the Ed25519 base point (x,4/5) with x positive.
h = a * B where a = a[0]+256*a[1]+...+256^31 a[31] B is the Ed25519 base point (x,4/5) with x positive.
ge_p1p1 (completed): ((X:Z),(Y:T)) satisfying x=X/Z, y=Y/T
ge means group element.
ge_p3 (extended): (X:Y:Z:T) satisfying x=X/Z, y=Y/Z, XY=ZT
ge_precomp (Duif): (y+x,y-x,2dxy)