diff --git a/SVSim.BattleNode/Protocol/MsgPayloadCodec.cs b/SVSim.BattleNode/Protocol/MsgPayloadCodec.cs
new file mode 100644
index 0000000..5e5095c
--- /dev/null
+++ b/SVSim.BattleNode/Protocol/MsgPayloadCodec.cs
@@ -0,0 +1,26 @@
+using MessagePack;
+using SVSim.BattleNode.Wire;
+
+namespace SVSim.BattleNode.Protocol;
+
+/// <summary>
+/// Full chain between an envelope and the bytes that ride as a SocketIO binary attachment.
+/// Inbound:  bytes → msgpack-string → NodeCrypto.Decrypt → JSON → MsgEnvelope
+/// Outbound: MsgEnvelope → JSON → NodeCrypto.Encrypt → msgpack-bytes
+/// </summary>
+public static class MsgPayloadCodec
+{
+    public static MsgEnvelope Decode(byte[] msgpackBytes)
+    {
+        var encryptedString = MessagePackSerializer.Deserialize<string>(msgpackBytes);
+        var json = NodeCrypto.DecryptForNode(encryptedString);
+        return MsgEnvelope.FromJson(json);
+    }
+
+    public static byte[] Encode(MsgEnvelope envelope, string key)
+    {
+        var json = MsgEnvelope.ToJson(envelope);
+        var encryptedString = NodeCrypto.EncryptForNode(json, key);
+        return MessagePackSerializer.Serialize(encryptedString);
+    }
+}
diff --git a/SVSim.UnitTests/BattleNode/Protocol/MsgPayloadCodecTests.cs b/SVSim.UnitTests/BattleNode/Protocol/MsgPayloadCodecTests.cs
new file mode 100644
index 0000000..afbfb75
--- /dev/null
+++ b/SVSim.UnitTests/BattleNode/Protocol/MsgPayloadCodecTests.cs
@@ -0,0 +1,53 @@
+using NUnit.Framework;
+using SVSim.BattleNode.Protocol;
+
+namespace SVSim.UnitTests.BattleNode.Protocol;
+
+[TestFixture]
+public class MsgPayloadCodecTests
+{
+    private static string FreshKey()
+    {
+        var seq = 0;
+        return SVSim.BattleNode.Wire.NodeCrypto.GenerateKey(() => (seq++ * 7) % 16);
+    }
+
+    [Test]
+    public void Roundtrip_PreservesEnvelope()
+    {
+        var env = new MsgEnvelope(
+            Uri: NetworkBattleUri.Loaded,
+            ViewerId: 906243102,
+            Uuid: "udid",
+            Bid: "1234",
+            Try: 0,
+            Cat: EmitCategory.Battle,
+            PubSeq: 3,
+            PlaySeq: null,
+            Body: new Dictionary<string, object?>());
+
+        var bytes = MsgPayloadCodec.Encode(env, key: FreshKey());
+        var back = MsgPayloadCodec.Decode(bytes);
+
+        Assert.That(back.Uri, Is.EqualTo(NetworkBattleUri.Loaded));
+        Assert.That(back.PubSeq, Is.EqualTo(3));
+        Assert.That(back.Bid, Is.EqualTo("1234"));
+    }
+
+    [Test]
+    public void Decode_KnownEnvelope_ReturnsExpectedUriAndBody()
+    {
+        // The captures only contain decoded JSON, so we build the encrypted-msgpack representation
+        // ourselves with the same JSON and a known key — this confirms the full chain end-to-end.
+        var key = FreshKey();
+        var originalJson = "{\"uri\":\"InitNetwork\",\"viewerId\":1,\"uuid\":\"u\",\"try\":0,\"cat\":99,\"resultCode\":1}";
+        var encrypted = SVSim.BattleNode.Wire.NodeCrypto.EncryptForNode(originalJson, key);
+        var bytes = MessagePack.MessagePackSerializer.Serialize(encrypted);
+
+        var env = MsgPayloadCodec.Decode(bytes);
+
+        Assert.That(env.Uri, Is.EqualTo(NetworkBattleUri.InitNetwork));
+        Assert.That(env.Cat, Is.EqualTo(EmitCategory.General));
+        Assert.That(env.Body["resultCode"], Is.EqualTo(1L));
+    }
+}