public static byte[] Hash(byte[] data, int offset, int count)
        {
            var hasher = new Sha512();

            hasher.Update(data, offset, count);
            return(hasher.Finish());
        }
        public static void KeyExchange(ArraySegment <byte> sharedKey, ArraySegment <byte> publicKey, ArraySegment <byte> privateKey)
        {
            if (sharedKey.Array == null)
            {
                throw new ArgumentNullException("sharedKey.Array");
            }
            if (publicKey.Array == null)
            {
                throw new ArgumentNullException("publicKey.Array");
            }
            if (privateKey.Array == null)
            {
                throw new ArgumentNullException("privateKey");
            }
            if (sharedKey.Count != 32)
            {
                throw new ArgumentException("sharedKey.Count != 32");
            }
            if (publicKey.Count != 32)
            {
                throw new ArgumentException("publicKey.Count != 32");
            }
            if (privateKey.Count != 64)
            {
                throw new ArgumentException("privateKey.Count != 64");
            }

            FieldElement montgomeryX, edwardsY, edwardsZ, sharedMontgomeryX;

            FieldOperations.fe_frombytes(out edwardsY, publicKey.Array, publicKey.Offset);
            FieldOperations.fe_1(out edwardsZ);
            MontgomeryCurve25519.EdwardsToMontgomeryX(out montgomeryX, ref edwardsY, ref edwardsZ);
            byte[] h = Sha512.Hash(privateKey.Array, privateKey.Offset, 32);//ToDo: Remove alloc
            ScalarOperations.sc_clamp(h, 0);
            MontgomeryOperations.scalarmult(out sharedMontgomeryX, h, 0, ref montgomeryX);
            CryptoBytes.Wipe(h);
            FieldOperations.fe_tobytes(sharedKey.Array, sharedKey.Offset, ref sharedMontgomeryX);
            MontgomeryCurve25519.KeyExchangeOutputHashNaCl(sharedKey.Array, sharedKey.Offset);
        }