From cddbb7badbf63a5a9a83e8483462ad9ffcca5fc1 Mon Sep 17 00:00:00 2001
From: Justin Baur <19896123+justindbaur@users.noreply.github.com>
Date: Sun, 16 Jul 2023 15:46:04 -0400
Subject: [PATCH] Add MAC Validation

---
 .../src/Types/EncryptedString.cs              | 19 +++++++++++++++++--
 .../src/Types/SymmetricCryptoKey.cs           |  8 ++++----
 2 files changed, 21 insertions(+), 6 deletions(-)

diff --git a/extensions/Bitwarden.Core/src/Types/EncryptedString.cs b/extensions/Bitwarden.Core/src/Types/EncryptedString.cs
index 3dfbab6..4468a53 100644
--- a/extensions/Bitwarden.Core/src/Types/EncryptedString.cs
+++ b/extensions/Bitwarden.Core/src/Types/EncryptedString.cs
@@ -61,8 +61,6 @@ private static byte[] DecryptAes256(ReadOnlySpan<char> encryptionPart, Symmetric
             throw new FormatException();
         }
 
-        // TODO: Validate mac
-
         Span<byte> data = stackalloc byte[dataPart.Length];
         converted = Convert.TryFromBase64Chars(dataPart, data, out bytesWritten);
         if (!converted)
@@ -71,6 +69,15 @@ private static byte[] DecryptAes256(ReadOnlySpan<char> encryptionPart, Symmetric
         }
 
         data = data[..bytesWritten];
+
+        Span<byte> validatedMac = stackalloc byte[32];
+        ValidateMac(symmetricCryptoKey.Mac, iv, data, validatedMac);
+
+        if (!validatedMac.SequenceEqual(mac))
+        {
+            throw new FormatException("Invalid MAC");
+        }
+
         Span<byte> output = stackalloc byte[data.Length];
 
         using var aes = Aes.Create();
@@ -105,6 +112,14 @@ public static EncryptedString Parse(ReadOnlySpan<char> s)
         return new EncryptedString(type, encryptionPart);
     }
 
+    private static void ValidateMac(Span<byte> macKey, ReadOnlySpan<byte> iv, ReadOnlySpan<byte> data, Span<byte> destination)
+    {
+        var hmac = IncrementalHash.CreateHMAC(HashAlgorithmName.SHA256, macKey);
+        hmac.AppendData(iv);
+        hmac.AppendData(data);
+        hmac.TryGetCurrentHash(destination, out _);
+    }
+
     public static bool TryParse(ReadOnlySpan<char> s, [MaybeNullWhen(false)] out EncryptedString result)
     {
         try
diff --git a/extensions/Bitwarden.Core/src/Types/SymmetricCryptoKey.cs b/extensions/Bitwarden.Core/src/Types/SymmetricCryptoKey.cs
index 6d21860..fc52cc4 100644
--- a/extensions/Bitwarden.Core/src/Types/SymmetricCryptoKey.cs
+++ b/extensions/Bitwarden.Core/src/Types/SymmetricCryptoKey.cs
@@ -16,10 +16,10 @@ public class SymmetricCryptoKey : ISpanParsable<SymmetricCryptoKey>
     const int KeyLength = 32;
     const int MacLength = 32;
 
-    private readonly ReadOnlyMemory<byte> _rawData;
+    private readonly byte[] _rawData;
 
-    public ReadOnlyMemory<byte> Key => _rawData[..KeyLength];
-    public ReadOnlyMemory<byte> Mac => _rawData.Length == KeyLength + MacLength ? _rawData[KeyLength..] : default;
+    public byte[] Key => _rawData[..KeyLength];
+    public byte[]? Mac => _rawData.Length == KeyLength + MacLength ? _rawData[KeyLength..] : default;
 
     private SymmetricCryptoKey(ReadOnlySpan<byte> rawData)
     {
@@ -99,6 +99,6 @@ public static bool TryParse(ReadOnlySpan<char> s, [MaybeNullWhen(false)] out Sym
 
     public override string ToString()
     {
-        return Convert.ToBase64String(_rawData.Span);
+        return Convert.ToBase64String(_rawData);
     }
 }