public AffineNielsPoint BitXor(AffineNielsPoint a)
{
return(new AffineNielsPoint
{
Y_plus_X = Y_plus_X.BitXor(a.Y_plus_X),
Y_minus_X = Y_minus_X.BitXor(a.Y_minus_X),
XY2d = XY2d.BitXor(XY2d)
});
}
public CompletedPoint Add(AffineNielsPoint other)
{
var Y_plus_X = Y.Add(X);
var Y_minus_X = Y.Sub(X);
var PP = Y_plus_X.Mul(other.Y_plus_X);
var MM = Y_minus_X.Mul(other.Y_minus_X);
var Txy2d = T.Mul(other.XY2d);
var Z2 = Z.Add(Z);
return(new CompletedPoint
{
X = PP.Sub(MM),
Y = PP.Add(MM),
Z = Z2.Add(Txy2d),
T = Z2.Sub(Txy2d)
});
}
public AffineNielsPoint Select(sbyte x)
{
// Compute xabs = |x|
var xmask = x >> 7;
sbyte xabs = (sbyte)((x + xmask) ^ xmask);
// Set t = 0 * P = identity
var t = new AffineNielsPoint();
for (var i = 1; i < 9; i++)
{
// Copy `points[j-1] == j*P` onto `t` in constant time if `|x| == j`.
t.ConditionalAssign(affineNielsPoints[i - 1], xabs == i);
}
// Now t == |x| * P.
byte neg_mask = (byte)(xmask & 1);
t.ConditionalNegate(neg_mask == 1);
// Now t == x * P.
return(t);
}
public void ConditionalAssign(AffineNielsPoint a, bool choice)
{
Y_plus_X.ConditionalAssign(a.Y_plus_X, choice);
Y_minus_X.ConditionalAssign(a.Y_minus_X, choice);
XY2d.ConditionalAssign(a.XY2d, choice);
}