public override ECPoint Add(ECPoint b)
{
if (this.IsInfinity)
return b;
if (b.IsInfinity)
return this;
if (this == b)
return Twice();
ECCurve curve = this.Curve;
Curve25519FieldElement X1 = (Curve25519FieldElement)this.RawXCoord, Y1 = (Curve25519FieldElement)this.RawYCoord,
Z1 = (Curve25519FieldElement)this.RawZCoords[0];
Curve25519FieldElement X2 = (Curve25519FieldElement)b.RawXCoord, Y2 = (Curve25519FieldElement)b.RawYCoord,
Z2 = (Curve25519FieldElement)b.RawZCoords[0];
uint c;
uint[] tt1 = Nat256.CreateExt();
uint[] t2 = Nat256.Create();
uint[] t3 = Nat256.Create();
uint[] t4 = Nat256.Create();
bool Z1IsOne = Z1.IsOne;
uint[] U2, S2;
if (Z1IsOne)
{
U2 = X2.x;
S2 = Y2.x;
}
else
{
S2 = t3;
Curve25519Field.Square(Z1.x, S2);
U2 = t2;
Curve25519Field.Multiply(S2, X2.x, U2);
Curve25519Field.Multiply(S2, Z1.x, S2);
Curve25519Field.Multiply(S2, Y2.x, S2);
}
bool Z2IsOne = Z2.IsOne;
uint[] U1, S1;
if (Z2IsOne)
{
U1 = X1.x;
S1 = Y1.x;
}
else
{
S1 = t4;
Curve25519Field.Square(Z2.x, S1);
U1 = tt1;
Curve25519Field.Multiply(S1, X1.x, U1);
Curve25519Field.Multiply(S1, Z2.x, S1);
Curve25519Field.Multiply(S1, Y1.x, S1);
}
uint[] H = Nat256.Create();
Curve25519Field.Subtract(U1, U2, H);
uint[] R = t2;
Curve25519Field.Subtract(S1, S2, R);
// Check if b == this or b == -this
if (Nat256.IsZero(H))
{
if (Nat256.IsZero(R))
{
// this == b, i.e. this must be doubled
return this.Twice();
}
// this == -b, i.e. the result is the point at infinity
return curve.Infinity;
}
uint[] HSquared = Nat256.Create();
Curve25519Field.Square(H, HSquared);
uint[] G = Nat256.Create();
Curve25519Field.Multiply(HSquared, H, G);
uint[] V = t3;
Curve25519Field.Multiply(HSquared, U1, V);
Curve25519Field.Negate(G, G);
Nat256.Mul(S1, G, tt1);
c = Nat256.AddBothTo(V, V, G);
Curve25519Field.Reduce27(c, G);
Curve25519FieldElement X3 = new Curve25519FieldElement(t4);
Curve25519Field.Square(R, X3.x);
Curve25519Field.Subtract(X3.x, G, X3.x);
Curve25519FieldElement Y3 = new Curve25519FieldElement(G);
Curve25519Field.Subtract(V, X3.x, Y3.x);
Curve25519Field.MultiplyAddToExt(Y3.x, R, tt1);
Curve25519Field.Reduce(tt1, Y3.x);
Curve25519FieldElement Z3 = new Curve25519FieldElement(H);
if (!Z1IsOne)
{
Curve25519Field.Multiply(Z3.x, Z1.x, Z3.x);
}
if (!Z2IsOne)
{
Curve25519Field.Multiply(Z3.x, Z2.x, Z3.x);
}
uint[] Z3Squared = (Z1IsOne && Z2IsOne) ? HSquared : null;
// TODO If the result will only be used in a subsequent addition, we don't need W3
Curve25519FieldElement W3 = CalculateJacobianModifiedW((Curve25519FieldElement)Z3, Z3Squared);
ECFieldElement[] zs = new ECFieldElement[] { Z3, W3 };
return new Curve25519Point(curve, X3, Y3, zs, IsCompressed);
}
public override ECFieldElement Negate()
{
uint[] z = Nat256.Create();
Curve25519Field.Negate(x, z);
return(new Curve25519FieldElement(z));
}
