fix: replace SequenceEqual with SecureMemory.SecureCompare on secret key comparisons

Eliminates timing side-channels in SessionManager key lookup and
Sodium.ValidateX25519PublicKey small-order point checks by using
the constant-time sodium_memcmp wrapper instead of short-circuiting
SequenceEqual. Adds SecureCompare_EqualKeys_ReturnsTrue test.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: rbenzing

LibEmiddle.Tests.Unit/SecurityTests.cs

@@ -779,5 +779,25 @@ public void SecureMemoryClear_ShouldEffectivelyClearMemory()
         }
 
         #endregion
+
+        #region SecureCompare Tests
+
+        /// <summary>
+        /// Tests that SecureCompare correctly identifies equal and differing keys.
+        /// </summary>
+        [TestMethod]
+        public void SecureCompare_EqualKeys_ReturnsTrue()
+        {
+            var key = new byte[32];
+            System.Security.Cryptography.RandomNumberGenerator.Fill(key);
+            var key2 = (byte[])key.Clone();
+            Assert.IsTrue(LibEmiddle.Core.SecureMemory.SecureCompare(key, key2),
+                "SecureCompare must return true for byte-identical keys");
+            key2[0] ^= 0xFF;
+            Assert.IsFalse(LibEmiddle.Core.SecureMemory.SecureCompare(key, key2),
+                "SecureCompare must return false for differing keys");
+        }
+
+        #endregion
     }
 }

LibEmiddle/Core/Sodium.cs

@@ -1391,7 +1391,7 @@ public static bool ValidateX25519PublicKey(ReadOnlySpan<byte> x25519PublicKey)
         // Check against known small-order points
         foreach (var smallOrder in SMALL_ORDER_POINTS)
         {
-            if (x25519PublicKey.SequenceEqual(smallOrder))
+            if (SecureMemory.SecureCompare(x25519PublicKey, smallOrder))
                 return false;
         }
 

LibEmiddle/Sessions/SessionManager.cs

@@ -591,7 +591,7 @@ public async Task<IChatSession> GetOrCreateChatSessionAsync(
             foreach (var kv in _activeSessions)
             {
                 if (kv.Value is Messaging.Chat.ChatSession cs &&
-                    cs.RemotePublicKey.AsSpan().SequenceEqual(recipientPublicKey))
+                    SecureMemory.SecureCompare(cs.RemotePublicKey, recipientPublicKey))
                 {
                     return cs;
                 }