Also check authhash

Robust.Shared/Network/AuthManager.cs

@@ -13,6 +13,7 @@ internal interface IAuthManager
         string? ServerPublicKey { get; set; }
         string? UserPublicKey { get; set; }
         string? UserJWT { get; set; }
+        string? SharedSecretBase64 { get; set; }
 
         void LoadFromEnv();
     }
@@ -22,6 +23,7 @@ internal sealed class AuthManager : IAuthManager
         public string? ServerPublicKey { get; set; }
         public string? UserPublicKey { get; set; }
         public string? UserJWT { get; set; }
+        public string? SharedSecretBase64 { get; set; }
 
         public void LoadFromEnv()
         {
@@ -40,6 +42,11 @@ public void LoadFromEnv()
                 UserJWT = userJWT;
             }
 
+            if (TryGetVar("ROBUST_SHARED_SECRET", out var sharedSecretBase64))
+            {
+                SharedSecretBase64 = sharedSecretBase64;
+            }
+
             static bool TryGetVar(string var, [NotNullWhen(true)] out string? val)
             {
                 val = Environment.GetEnvironmentVariable(var);

Robust.Shared/Network/NetManager.ClientConnect.cs

@@ -155,8 +155,30 @@ private async Task CCDoHandshake(NetPeerData peer, NetConnection connection, str
                 var encRequest = new MsgEncryptionRequest();
                 encRequest.ReadFromBuffer(response, _serializer);
 
-                var sharedSecret = new byte[SharedKeyLength];
-                RandomNumberGenerator.Fill(sharedSecret);
+                byte[] sharedSecret;
+
+                if (string.IsNullOrEmpty(_authManager.SharedSecretBase64))
+                {
+                    // Generate new shared secret in robust client
+                    sharedSecret = new byte[SharedKeyLength];
+                    RandomNumberGenerator.Fill(sharedSecret); // In order for this to work, server must not be verifying JWT
+                }
+                else
+                {
+                    // Use shared secret from launcher
+
+                    // (Robust client does not have direct access to the user's private key for safety.  The JWT needs to
+                    // include the authhash to avoid a MITM.  Thus, the launcher must generate the JWT and by extension
+                    // the shared secret.  Launcher will generate it and pass it to us in Base64 format.)
+                    sharedSecret = System.Convert.FromBase64String(_authManager.SharedSecretBase64);
+
+                    if (sharedSecret.Length != SharedKeyLength)
+                    {
+                        var msg = $"Invalid shared secret length from launcher. Expected {SharedKeyLength}, but was {sharedSecret.Length}.";
+                        connection.Disconnect(msg);
+                        throw new Exception(msg);
+                    }
+                }
 
                 if (encrypt)
                     encryption = new NetEncryption(sharedSecret, isServer: false);

Robust.Shared/Network/NetManager.ServerAuth.cs

@@ -138,6 +138,9 @@ private async void HandleHandshake(NetPeerData peer, NetConnection connection)
                     if (msgLogin.Encrypt)
                         encryption = new NetEncryption(sharedSecret, isServer: true);
 
+                    var authHashBytes = MakeAuthHash(sharedSecret, CryptoPublicKey!);
+                    var authHash = Base64Helpers.ConvertToBase64Url(authHashBytes);
+
                     // Validate the JWT
                     var userPublicKeyString = msgEncResponse.UserPublicKey ?? "";
                     var userJWTString = msgEncResponse.UserJWT ?? "";
@@ -219,6 +222,23 @@ private async void HandleHandshake(NetPeerData peer, NetConnection connection)
                             connection.Disconnect("JWT Validation Error\nJWT appears to be for another server.\nTry returning to launcher and reconnect.");
                             return;
                         }
+
+                        // Also verify authhash matches.
+                        // (This step helps deter a MITM/proxy attack, since even if traffic was proxied, it should also
+                        // be encrypted.)
+                        string authHashClaim = "";
+                        var authHashClaimNode = jsonNode["authhash"];
+                        if (authHashClaimNode == null)
+                        {
+                            connection.Disconnect("JWT Validation Error - No auth hash in JWT\n(Ensure you are using latest launcher version).");
+                            return;
+                        }
+                        authHashClaim = (string) authHashClaimNode.GetValue<string>();
+                        if (authHashClaim != authHash)
+                        {
+                            connection.Disconnect("JWT Validation Error - Wrong auth hash in JWT\n(Check server address is correct).");
+                            return;
+                        }
                     }
 
                     _logger.Verbose(