ECDH.cs

using System;
 
namespace Waher.Security.EllipticCurves
{
    public static class ECDH
    {
        public static PointOnCurve GetSharedPoint(byte[] LocalPrivateKey, byte[] RemotePublicKey,
            bool RemoteBigEndian, EllipticCurve Curve)
        {
            PointOnCurve PublicKey = Curve.Decode(RemotePublicKey, RemoteBigEndian);
            return Curve.ScalarMultiplication(LocalPrivateKey, PublicKey, true);
        }
 
        public static byte[] GetSharedKey(byte[] LocalPrivateKey, byte[] RemotePublicKey, 
            bool RemoteBigEndian, HashFunctionArray HashFunction, EllipticCurve Curve)
        {
            PointOnCurve P = GetSharedPoint(LocalPrivateKey, RemotePublicKey, RemoteBigEndian, Curve);
 
            byte[] B = P.X.ToByteArray();   // Little endian
 
            if (B.Length != Curve.OrderBytes)
                Array.Resize(ref B, Curve.OrderBytes);
 
            Array.Reverse(B);   // Most significant byte first.
 
            return HashFunction(B);
        }
    }
}