/// <summary>
/// Derives an unpredictable element of the group, using the input. Each
/// construction defines its own derivation mechanism, but each takes an
/// optional context string and an index, and returns a counter. Calling
/// this method with the same parameter values returns the same element
/// and counter, calling it with a different context or index value must
/// return a different element.
/// </summary>
/// <param name="context">An optional context used by the derivation
/// mechanism, can be null.</param>
/// <param name="index">An 8-bit integer index value.</param>
/// <param name="counter">A counter value indicating the state at which the
/// derivation mechanism stopped.</param>
/// <returns>A random group element.</returns>
public override GroupElement DeriveElement(byte[] context, byte index, out int counter)
{
// concatenate context and curve name
BouncyCastle.FpCurve curve = this.domainParams.Curve as BouncyCastle.FpCurve;
int count = 0;
BouncyCastle.ECFieldElement x = null, y = null;
while (y == null)
{
x = GetX(context, curve, index, count);
BouncyCastle.ECFieldElement alpha = x.Multiply(x.Square().Add(curve.A)).Add(curve.B);
if (alpha.ToBigInteger() == BCBigInt.Zero)
{
y = alpha;
}
else
{
y = alpha.Sqrt(); // returns null if sqrt does not exist
}
count++;
}
// determine which sqrt to return, i.e., Min(y, -y)
BouncyCastle.ECFieldElement minusY = y.Negate();
counter = count - 1;
return(new ECGroupElementBCImpl(new BouncyCastle.FpPoint(curve, x, y.ToBigInteger() < minusY.ToBigInteger() ? y : minusY)));
}
private void CheckOnCurve(GroupElement e)
{
ECGroupElementBCImpl ecge = e as ECGroupElementBCImpl;
if (ecge == null)
{
throw new ArgumentNullException();
}
BouncyCastle.FpPoint p = ecge.Point;
BouncyCastle.ECFieldElement x = p.AffineXCoord, y = p.AffineYCoord;
if (!y.Square().Equals(x.Multiply(x.Square().Add(domainParams.Curve.A)).Add(domainParams.Curve.B)))
{
throw new InvalidUProveArtifactException("point is not on curve");
}
}