private static GroupElementP4 Select(int pos, sbyte b)
{
GroupElementP4 t;
GroupElementP4 minust;
var bnegative = Negative(b);
var babs = (byte)(b - ((-bnegative & b) << 1));
t = new GroupElementP4
{
YplusX = FieldElementOperations.Set1(),
YminusX = FieldElementOperations.Set1(),
XY2D = FieldElementOperations.Set0()
};
var table = LookupTables.Base[pos];
Cmov(ref t, ref table[0], Equal(babs, 1));
Cmov(ref t, ref table[1], Equal(babs, 2));
Cmov(ref t, ref table[2], Equal(babs, 3));
Cmov(ref t, ref table[3], Equal(babs, 4));
Cmov(ref t, ref table[4], Equal(babs, 5));
Cmov(ref t, ref table[5], Equal(babs, 6));
Cmov(ref t, ref table[6], Equal(babs, 7));
Cmov(ref t, ref table[7], Equal(babs, 8));
minust.YplusX = t.YminusX;
minust.YminusX = t.YplusX;
minust.XY2D = FieldElementOperations.Negate(ref t.XY2D);
Cmov(ref t, ref minust, bnegative);
return(t);
}
private static GroupElementP1 Madd(ref GroupElementP3 p, ref GroupElementP4 q)
{
var t0 = FieldElementOperations.Add(ref p.Z, ref p.Z); /* D = 2*Z1 */
var r = new GroupElementP1();
/* YpX1 = Y1+X1 */
r.X = FieldElementOperations.Add(ref p.Y, ref p.X);
/* YmX1 = Y1-X1 */
r.Y = FieldElementOperations.Sub(ref p.Y, ref p.X);
/* A = YpX1*ypx2 */
r.Z = FieldElementOperations.Multiplication(ref r.X, ref q.YplusX);
/* B = YmX1*ymx2 */
r.Y = FieldElementOperations.Multiplication(ref r.Y, ref q.YminusX);
/* C = xy2d2*T1 */
r.T = FieldElementOperations.Multiplication(ref q.XY2D, ref p.T);
/* X3 = A-B */
r.X = FieldElementOperations.Sub(ref r.Z, ref r.Y);
/* Y3 = A+B */
r.Y = FieldElementOperations.Add(ref r.Z, ref r.Y);
/* Z3 = D+C */
r.Z = FieldElementOperations.Add(ref t0, ref r.T);
/* T3 = D-C */
r.T = FieldElementOperations.Sub(ref t0, ref r.T);
return(r);
}
private static void Cmov(ref GroupElementP4 t, ref GroupElementP4 u, byte b)
{
FieldElementOperations.Mov(ref t.YplusX, ref u.YplusX, b);
FieldElementOperations.Mov(ref t.YminusX, ref u.YminusX, b);
FieldElementOperations.Mov(ref t.XY2D, ref u.XY2D, b);
}
