Cryptographic framework for authenticating and verifying remoting messages. Must be implemented as per the suggested implementation in the comments. Buyer beware - this code has not been peer reviewed by an expert cryptologist and needs to be done so before this security is relied upon for important transactions.

OpenSim.Framework/Remoting.cs

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+using System;
+using System.Collections.Generic;
+using System.Text;
+using System.Security.Cryptography;
+
+namespace OpenSim.Framework
+{
+    /// <summary>
+    /// NEEDS AUDIT.
+    /// </summary>
+    /// <remarks>
+    /// Suggested implementation
+    /// <para>Store two digests for each foreign host. A local copy of the local hash using the local challenge (when issued), and a local copy of the remote hash using the remote challenge.</para>
+    /// <para>When sending data to the foreign host - run 'Sign' on the data and affix the returned byte[] to the message.</para>
+    /// <para>When recieving data from the foreign host - run 'Authenticate' against the data and the attached byte[].</para>
+    /// <para>Both hosts should be performing these operations for this to be effective.</para>
+    /// </remarks>
+    class RemoteDigest
+    {
+        private byte[] currentHash;
+        private byte[] secret;
+
+        private SHA512Managed SHA512;
+
+        /// <summary>
+        /// Initialises a new RemoteDigest authentication mechanism
+        /// </summary>
+        /// <remarks>Needs an audit by a cryptographic professional - was not "roll your own"'d by choice but rather a serious lack of decent authentication mechanisms in .NET remoting</remarks>
+        /// <param name="sharedSecret">The shared secret between systems (for inter-sim, this is provided in encrypted form during connection, for grid this is input manually in setup)</param>
+        /// <param name="salt">Binary salt - some common value - to be decided what</param>
+        /// <param name="challenge">The challenge key provided by the third party</param>
+        public RemoteDigest(string sharedSecret, byte[] salt, string challenge)
+        {
+            SHA512 = new SHA512Managed();
+            Rfc2898DeriveBytes RFC2898 = new Rfc2898DeriveBytes(sharedSecret,salt);
+            secret = RFC2898.GetBytes(512);
+            ASCIIEncoding ASCII = new ASCIIEncoding();
+
+            currentHash = SHA512.ComputeHash(AppendArrays(secret, ASCII.GetBytes(challenge)));
+        }
+
+        /// <summary>
+        /// Authenticates a piece of incoming data against the local digest. Upon successful authentication, digest string is incremented.
+        /// </summary>
+        /// <param name="data">The incoming data</param>
+        /// <param name="digest">The remote digest</param>
+        /// <returns></returns>
+        public bool Authenticate(byte[] data, byte[] digest)
+        {
+            byte[] newHash = SHA512.ComputeHash(AppendArrays(AppendArrays(currentHash, secret), data));
+            if (digest == newHash)
+            {
+                currentHash = newHash;
+                return true;
+            }
+            else
+            {
+                throw new Exception("Hash comparison failed. Key resync required.");
+            }
+        }
+
+        /// <summary>
+        /// Signs a new bit of data with the current hash. Returns a byte array which should be affixed to the message.
+        /// Signing a piece of data will automatically increment the hash - if you sign data and do not send it, the 
+        /// hashes will get out of sync and throw an exception when validation is attempted.
+        /// </summary>
+        /// <param name="data">The outgoing data</param>
+        /// <returns>The local digest</returns>
+        public byte[] Sign(byte[] data)
+        {
+            currentHash = SHA512.ComputeHash(AppendArrays(AppendArrays(currentHash, secret), data));
+            return currentHash;
+        }
+
+        /// <summary>
+        /// Helper function, merges two byte arrays
+        /// </summary>
+        /// <param name="a">A</param>
+        /// <param name="b">B</param>
+        /// <returns>C</returns>
+        private byte[] AppendArrays(byte[] a, byte[] b)
+        {
+            byte[] c = new byte[a.Length + b.Length];
+            Buffer.BlockCopy(a, 0, c, 0, a.Length);
+            Buffer.BlockCopy(b, 0, c, a.Length, b.Length);
+            return c;
+        }
+
+    }
+}