/// <summary>
/// Returns the inverse of k modulo p.
/// </summary>
/// <param name="k">Must be non-zero</param>
/// <param name="p">Must be a prime</param>
/// <returns></returns>
public BigInteger InverseMod(BigInteger k, BigInteger p)
{
if (k == 0)
{
return(BigInteger.Zero);
//throw new DivideByZeroException(nameof(k));
}
if (k < 0)
{
return(p - this.InverseMod(-k, p));
}
(BigInteger gcd, BigInteger x, BigInteger y)result = EllipticCurveHelpers.ExtendedGcd(p, k);
if (result.gcd != 1)
{
throw new ArgumentException("Gcd is not 1");
}
if (EllipticCurveHelpers.MathMod(k * result.x, p) != 1)
{
throw new ArgumentException("(k * result.x) % p != 1");
}
return(EllipticCurveHelpers.MathMod(result.x, p));
}
/// <summary>
/// Returns k * point computed using the double and <see cref="PointAdd"/> algorithm.
/// </summary>
/// <param name="k"></param>
/// <param name="point"></param>
/// <returns></returns>
public BigIntegerPoint ScalarMult(BigInteger k, BigIntegerPoint point)
{
if (EllipticCurveHelpers.MathMod(k, this.N) == 0 || point.IsEmpty())
{
return(BigIntegerPoint.GetEmpty());
}
if (k < 0)
{
return(this.ScalarMult(-k, this.NegatePoint(point)));
}
BigIntegerPoint result = BigIntegerPoint.GetEmpty();
BigIntegerPoint addend = point;
while (k > 0)
{
if ((k & 1) > 0)
{
result = this.PointAdd(result, addend);
}
addend = this.PointAdd(addend, addend);
k >>= 1;
}
return(result);
}
public (BigInteger privateKey, BigIntegerPoint publicKey) GenerateKeyPair()
{
BigInteger privateKey = EllipticCurveHelpers.RandomIntegerBelow(this.Curve.N);
BigIntegerPoint publicKey = this.Curve.ScalarMult(privateKey, this.Curve.G);
return(privateKey, publicKey);
}
/// <summary>
/// Returns <c>true</c> if the given point lies on the elliptic curve.
/// </summary>
/// <param name="point"></param>
/// <returns></returns>
public bool IsOnCurve(BigIntegerPoint point)
{
if (point.IsEmpty())
{
return(true);
}
BigInteger x = point.X;
BigInteger y = point.Y;
return(EllipticCurveHelpers.MathMod(y * y - x * x * x - this.A * x - this.B, this.P) == 0);
}
/// <summary>
/// Returns -point.
/// </summary>
/// <param name="point"></param>
/// <returns></returns>
public BigIntegerPoint NegatePoint(BigIntegerPoint point)
{
if (point.IsEmpty())
{
return(point);
}
BigInteger x = point.X;
BigInteger y = point.Y;
BigIntegerPoint result = new BigIntegerPoint(x, EllipticCurveHelpers.MathMod(-y, this.P));
return(result);
}
public IEnumerable <BigIntegerPoint> GenerateAllPoints()
{
for (BigInteger x = 0; x < this.P; x++)
{
BigInteger ySquare = BigInteger.ModPow(x, 3, this.P) + this.A * x + this.B;
bool isSquare = EllipticCurveHelpers.Legendre(ySquare, this.P) != -1;
if (isSquare)
{
var y = EllipticCurveHelpers.ShanksSqrt(ySquare, this.P);
yield return(new BigIntegerPoint(x, y));
yield return(new BigIntegerPoint(x, EllipticCurveHelpers.MathMod(-y, this.P)));
}
}
}
/// <summary>
/// Returns the result of lhs + rhs according to the group law.
/// </summary>
/// <param name="lhs"></param>
/// <param name="rhs"></param>
/// <returns></returns>
public BigIntegerPoint PointAdd(BigIntegerPoint lhs, BigIntegerPoint rhs)
{
if (lhs.IsEmpty())
{
return(rhs);
}
if (rhs.IsEmpty())
{
return(lhs);
}
BigInteger x1 = lhs.X, y1 = lhs.Y;
BigInteger x2 = rhs.X, y2 = rhs.Y;
if (x1 == x2 && y1 != y2)
{
return(BigIntegerPoint.GetEmpty());
}
BigInteger m;
if (x1 == x2)
{
m = (3 * x1 * x1 + this.A) * this.InverseMod(2 * y1, this.P);
}
else
{
m = (y1 - y2) * this.InverseMod(x1 - x2, this.P);
}
BigInteger x3 = m * m - x1 - x2;
BigInteger y3 = y1 + m * (x3 - x1);
BigIntegerPoint result = new BigIntegerPoint(EllipticCurveHelpers.MathMod(x3, this.P), EllipticCurveHelpers.MathMod(-y3, this.P));
return(result);
}