/**
* Decompresses a compressed point P = (xp, yp) (X9.62 s 4.2.2).
*
* @param yTilde
* ~yp, an indication bit for the decompression of yp.
* @param X1
* The field element xp.
* @return the decompressed point.
*/
protected override ECPoint DecompressPoint(int yTilde, BigInteger X1)
{
ECFieldElement x = FromBigInteger(X1), y = null;
if (x.IsZero)
{
y = B.Sqrt();
}
else
{
ECFieldElement beta = x.Square().Invert().Multiply(B).Add(A).Add(x);
ECFieldElement z = SolveQuadraticEquation(beta);
if (z != null)
{
if (z.TestBitZero() != (yTilde == 1))
{
z = z.AddOne();
}
switch (this.CoordinateSystem)
{
case COORD_LAMBDA_AFFINE:
case COORD_LAMBDA_PROJECTIVE:
{
y = z.Add(x);
break;
}
default:
{
y = z.Multiply(x);
break;
}
}
}
}
if (y == null)
{
throw new ArgumentException("Invalid point compression");
}
return(this.CreateRawPoint(x, y, true));
}
protected override ECPoint DecompressPoint(int yTilde, BigInteger X1)
{
ECFieldElement eCFieldElement = FromBigInteger(X1);
ECFieldElement eCFieldElement2 = null;
if (eCFieldElement.IsZero)
{
eCFieldElement2 = B.Sqrt();
}
else
{
ECFieldElement beta = eCFieldElement.Square().Invert().Multiply(B)
.Add(A)
.Add(eCFieldElement);
ECFieldElement eCFieldElement3 = SolveQuadradicEquation(beta);
if (eCFieldElement3 != null)
{
if (eCFieldElement3.TestBitZero() != (yTilde == 1))
{
eCFieldElement3 = eCFieldElement3.AddOne();
}
switch (CoordinateSystem)
{
case 5:
case 6:
eCFieldElement2 = eCFieldElement3.Add(eCFieldElement);
break;
default:
eCFieldElement2 = eCFieldElement3.Multiply(eCFieldElement);
break;
}
}
}
if (eCFieldElement2 == null)
{
throw new ArgumentException("Invalid point compression");
}
return(CreateRawPoint(eCFieldElement, eCFieldElement2, withCompression: true));
}
protected override ECPoint DecompressPoint(int yTilde, BigInteger X1)
{
ECFieldElement x = FromBigInteger(X1);
ECFieldElement alpha = x.Square().Add(A).Multiply(x).Add(B);
ECFieldElement beta = alpha.Sqrt();
//
// if we can't find a sqrt we haven't got a point on the
// curve - run!
//
if (beta == null)
{
throw new ArithmeticException("Invalid point compression");
}
if (beta.TestBitZero() != (yTilde == 1))
{
// Use the other root
beta = beta.Negate();
}
return(new SecP521R1Point(this, x, beta, true));
}
protected override ECPoint DecompressPoint(int yTilde, BigInteger X1)
{
ECFieldElement b = this.FromBigInteger(X1);
ECFieldElement y = null;
if (b.IsZero)
{
y = this.B.Sqrt();
}
else
{
ECFieldElement beta = b.Square().Invert().Multiply(this.B).Add(this.A).Add(b);
ECFieldElement element4 = this.SolveQuadraticEquation(beta);
if (element4 != null)
{
if (element4.TestBitZero() != (yTilde == 1))
{
element4 = element4.AddOne();
}
switch (this.CoordinateSystem)
{
case 5:
case 6:
y = element4.Add(b);
goto Label_00A9;
}
y = element4.Multiply(b);
}
}
Label_00A9:
if (y == null)
{
throw new ArgumentException("Invalid point compression");
}
return(this.CreateRawPoint(b, y, true));
}