refactor(battle-node): dedupe NodeCrypto AES setup into BuildAes helper

SVSim.BattleNode/Wire/NodeCrypto.cs

@@ -41,12 +41,7 @@ public static class NodeCrypto
     {
         if (key.Length != 32)
             throw new ArgumentException($"Key must be exactly 32 chars, got {key.Length}", nameof(key));
-        using var aes = Aes.Create();
-        aes.KeySize = 256;
-        aes.Mode = CipherMode.CBC;
-        aes.Padding = PaddingMode.PKCS7;
-        aes.Key = Encoding.UTF8.GetBytes(key);
-        aes.IV = Encoding.UTF8.GetBytes(key.Substring(0, 16));
+        using var aes = BuildAes(key);
         using var encryptor = aes.CreateEncryptor();
         var plainBytes = Encoding.UTF8.GetBytes(plaintext);
         var cipherBytes = encryptor.TransformFinalBlock(plainBytes, 0, plainBytes.Length);
@@ -60,14 +55,25 @@ public static class NodeCrypto
             throw new ArgumentException("Encrypted blob is shorter than the 32-char key prefix", nameof(encrypted));
         var key = encrypted.Substring(0, 32);
         var cipherBytes = Convert.FromBase64String(encrypted.Substring(32));
-        using var aes = Aes.Create();
+        using var aes = BuildAes(key);
+        using var decryptor = aes.CreateDecryptor();
+        var plainBytes = decryptor.TransformFinalBlock(cipherBytes, 0, cipherBytes.Length);
+        return Encoding.UTF8.GetString(plainBytes);
+    }
+
+    /// <summary>
+    /// Configure an AES-256-CBC instance with the node's IV derivation (first 16 chars
+    /// of the key, UTF-8). Callers own disposal. Assumes <paramref name="key"/> is the
+    /// 32-char ASCII key the encrypt / decrypt path has already validated.
+    /// </summary>
+    private static Aes BuildAes(string key)
+    {
+        var aes = Aes.Create();
         aes.KeySize = 256;
         aes.Mode = CipherMode.CBC;
         aes.Padding = PaddingMode.PKCS7;
         aes.Key = Encoding.UTF8.GetBytes(key);
         aes.IV = Encoding.UTF8.GetBytes(key.Substring(0, 16));
-        using var decryptor = aes.CreateDecryptor();
-        var plainBytes = decryptor.TransformFinalBlock(cipherBytes, 0, cipherBytes.Length);
-        return Encoding.UTF8.GetString(plainBytes);
+        return aes;
     }
 }