1 files changed, 93 insertions, 0 deletions
diff --git a/crypto/src/math/ec/ECAlgorithms.cs b/crypto/src/math/ec/ECAlgorithms.cs
new file mode 100644
index 000000000..be4fd1b14
--- /dev/null
+++ b/crypto/src/math/ec/ECAlgorithms.cs
@@ -0,0 +1,93 @@
+using System;
+
+using Org.BouncyCastle.Math;
+
+namespace Org.BouncyCastle.Math.EC
+{
+	public class ECAlgorithms
+	{
+		public static ECPoint SumOfTwoMultiplies(ECPoint P, BigInteger a,
+			ECPoint Q, BigInteger b)
+		{
+			ECCurve c = P.Curve;
+			if (!c.Equals(Q.Curve))
+				throw new ArgumentException("P and Q must be on same curve");
+
+			// Point multiplication for Koblitz curves (using WTNAF) beats Shamir's trick
+			if (c is F2mCurve)
+			{
+				F2mCurve f2mCurve = (F2mCurve) c;
+				if (f2mCurve.IsKoblitz)
+				{
+					return P.Multiply(a).Add(Q.Multiply(b));
+				}
+			}
+
+			return ImplShamirsTrick(P, a, Q, b);
+		}
+
+		/*
+		* "Shamir's Trick", originally due to E. G. Straus
+		* (Addition chains of vectors. American Mathematical Monthly,
+		* 71(7):806-808, Aug./Sept. 1964)
+		*  
+		* Input: The points P, Q, scalar k = (km?, ... , k1, k0)
+		* and scalar l = (lm?, ... , l1, l0).
+		* Output: R = k * P + l * Q.
+		* 1: Z <- P + Q
+		* 2: R <- O
+		* 3: for i from m-1 down to 0 do
+		* 4:        R <- R + R        {point doubling}
+		* 5:        if (ki = 1) and (li = 0) then R <- R + P end if
+		* 6:        if (ki = 0) and (li = 1) then R <- R + Q end if
+		* 7:        if (ki = 1) and (li = 1) then R <- R + Z end if
+		* 8: end for
+		* 9: return R
+		*/
+		public static ECPoint ShamirsTrick(
+			ECPoint		P,
+			BigInteger	k,
+			ECPoint		Q,
+			BigInteger	l)
+		{
+			if (!P.Curve.Equals(Q.Curve))
+				throw new ArgumentException("P and Q must be on same curve");
+
+			return ImplShamirsTrick(P, k, Q, l);
+		}
+
+		private static ECPoint ImplShamirsTrick(ECPoint P, BigInteger k,
+			ECPoint Q, BigInteger l)
+		{
+			int m = System.Math.Max(k.BitLength, l.BitLength);
+			ECPoint Z = P.Add(Q);
+			ECPoint R = P.Curve.Infinity;
+
+			for (int i = m - 1; i >= 0; --i)
+			{
+				R = R.Twice();
+
+				if (k.TestBit(i))
+				{
+					if (l.TestBit(i))
+					{
+						R = R.Add(Z);
+					}
+					else
+					{
+						R = R.Add(P);
+					}
+				}
+				else
+				{
+					if (l.TestBit(i))
+					{
+						R = R.Add(Q);
+					}
+				}
+			}
+
+			return R;
+		}
+	}
+}