dh fully works, refactor packet handling stuff

Novaria.Common/Crypto/DiffieHellman.cs

@@ -0,0 +1,41 @@
+using Novaria.Common.Util;
+using System.Numerics;
+
+namespace Novaria.Common.Crypto
+{
+    public class DiffieHellman : Singleton<DiffieHellman>
+    {
+        private BigInteger p = BigInteger.Parse("1552518092300708935130918131258481755631334049434514313202351194902966239949102107258669453876591642442910007680288864229150803718918046342632727613031282983744380820890196288509170691316593175367469551763119843371637221007210577919");
+
+        private BigInteger g = 2;
+
+        private BigInteger spriv = new BigInteger(new byte[] { 1, 2, 3, 4 }); // hardcoded server priv key
+
+        public BigInteger ServerPublicKey { get; set; }
+
+        public DiffieHellman()
+        {
+            //g** Spriv mod p
+            ServerPublicKey = BigInteger.ModPow(g, spriv, p);
+        }
+
+        public byte[] CalculateKey(byte[] clientPubKey) // server calculates key like this 
+        {
+            BigInteger clientPubKeyInt = new BigInteger(clientPubKey.Reverse().ToArray());
+
+            //Cpub**Spriv mod p
+            return BigInteger.ModPow(clientPubKeyInt, spriv, p).ToByteArray(true, true)[..32];
+        }
+
+        // this is for pcap parsing use only, officalServerPubKey is in the first IKE response, client priv will be pcaped too
+        public byte[] CalculateKey(byte[] officalServerPubKey, byte[] officialClientPriv)
+        {
+            BigInteger officalServerPubKeyInt = new BigInteger(officalServerPubKey.Reverse().ToArray());
+            BigInteger officialClientPrivInt = new BigInteger(officialClientPriv.Reverse().ToArray());
+
+            BigInteger result = BigInteger.ModPow(officalServerPubKeyInt, officialClientPrivInt, p);
+
+            return result.ToByteArray(true, true)[..32];
+        }
+    }
+}