From 5f08569f4a2565c0c7a47255428cfe2204467c45 Mon Sep 17 00:00:00 2001
From: Lukas Kurz <lukas@shortdev.de>
Date: Sun, 26 Jan 2025 19:01:02 +0100
Subject: [PATCH] Remove `HasPadding` check (#190)

Rely on paylod size in encrypted payload
---
 .../Encryption/CdpCryptor.cs                  | 22 ++-----------------
 .../CryptorTest.cs                            |  2 +-
 2 files changed, 3 insertions(+), 21 deletions(-)

diff --git a/lib/ShortDev.Microsoft.ConnectedDevices/Encryption/CdpCryptor.cs b/lib/ShortDev.Microsoft.ConnectedDevices/Encryption/CdpCryptor.cs
index e52df167..61ffedae 100644
--- a/lib/ShortDev.Microsoft.ConnectedDevices/Encryption/CdpCryptor.cs
+++ b/lib/ShortDev.Microsoft.ConnectedDevices/Encryption/CdpCryptor.cs
@@ -1,7 +1,6 @@
 using ShortDev.Microsoft.ConnectedDevices.Exceptions;
 using ShortDev.Microsoft.ConnectedDevices.Messages;
 using ShortDev.Microsoft.ConnectedDevices.Transports;
-using System.Buffers;
 using System.Buffers.Binary;
 using System.Diagnostics;
 using System.Security.Cryptography;
@@ -59,10 +58,8 @@ public ReadOnlyMemory<byte> DecryptMessage(CommonHeader header, ReadOnlySpan<byt
 
         byte[] decryptedPayload = _aes.DecryptCbc(payload, iv, PaddingMode.None);
 
-        if (HasPadding(decryptedPayload, out var paddingSize))
-            return decryptedPayload.AsMemory()[0..^paddingSize];
-
-        return decryptedPayload;
+        var payloadSize = BinaryPrimitives.ReadUInt32BigEndian(decryptedPayload.AsSpan()[..sizeof(uint)]);
+        return decryptedPayload.AsMemory(sizeof(uint), (int)payloadSize);
     }
 
     void VerifyHMac(CommonHeader header, ReadOnlySpan<byte> payload, ReadOnlySpan<byte> hmac)
@@ -157,10 +154,6 @@ public void Read(ref EndianReader reader, CommonHeader header)
 
         var decryptedPayload = DecryptMessage(header, encryptedPayload, hmac);
         reader = new(Endianness.BigEndian, decryptedPayload.Span);
-
-        var payloadLength = reader.ReadUInt32();
-        if (payloadLength != decryptedPayload.Length - sizeof(Int32))
-            throw new CdpSecurityException($"Expected payload to be {payloadLength} bytes long");
     }
 
     public void Dispose()
@@ -169,15 +162,4 @@ public void Dispose()
         _aes.Dispose();
         _hmac.Dispose();
     }
-
-    static bool HasPadding(ReadOnlySpan<byte> buffer, out byte paddingSize)
-    {
-        paddingSize = buffer[^1];
-        for (int i = buffer.Length - paddingSize; i < buffer.Length; i++)
-        {
-            if (paddingSize != buffer[i])
-                return false;
-        }
-        return true;
-    }
 }
\ No newline at end of file
diff --git a/tests/ShortDev.Microsoft.ConnectedDevices.Test/CryptorTest.cs b/tests/ShortDev.Microsoft.ConnectedDevices.Test/CryptorTest.cs
index bdad91b6..851dc74f 100644
--- a/tests/ShortDev.Microsoft.ConnectedDevices.Test/CryptorTest.cs
+++ b/tests/ShortDev.Microsoft.ConnectedDevices.Test/CryptorTest.cs
@@ -32,7 +32,7 @@ public void Decrypt_ShouldYieldSameAsEncrypt()
 
         var decrypted = cryptor.DecryptMessage(header, encryptedPayload, hmac).Span;
 
-        Assert.True(payload.SequenceEqual(decrypted[sizeof(uint)..]));
+        Assert.True(payload.SequenceEqual(decrypted));
     }
 
     sealed class FragmentSenderSpy : IFragmentSender