Revert "use Argon2 KDF for key derivation"

This reverts commit 2a87990.

Author: bfour

src/QuickUnlockKeyProv.cs

@@ -572,29 +572,16 @@ private static ProtectedBinary DecryptKey(ProtectedString QuickUnlockKey, Protec
 
     private static byte[] AdjustQuickUnlockKey(ProtectedString QuickUnlockKey, byte[] salt)
     {
-      byte[] bUtf8 = QuickUnlockKey.ReadUtf8();
-      byte[] derivedKey = null;
-
-      try
-      {
-          KdfParameters kdfParams = new KdfParameters(KeePassLib.Cryptography.KeyDerivation.KdfUuid.Argon2);
-          
-          // these should be set to values that provide a good balance between security and performance for quick unlock scenarios
-          kdfParams.SetUInt64(KdfParameters.KeyArgon2Iterations, 3);
-          kdfParams.SetUInt64(KdfParameters.KeyArgon2Memory, 64 * 1024 * 1024); // memory cost (64 MiB)
-          kdfParams.SetUInt32(KdfParameters.KeyArgon2Parallelism, 2);           // thread count
-          kdfParams.SetBytes("S", salt); 
-
-          var argon2 = new KeePassLib.Cryptography.KeyDerivation.Argon2Kdf();
-          derivedKey = argon2.Generate(bUtf8, kdfParams);
-      }
-      finally
+      byte[] bUtf8 = QuickUnlockKey.ReadUtf8(); // must be wiped
+      byte[] derivedKey;
+      
+      // use standard PBKDF2 to mitigate brute-forcing of short PINs
+      using (var rfc2898 = new Rfc2898DeriveBytes(bUtf8, salt, 100000))
       {
-          // Ensure the plaintext PIN is scrubbed from memory immediately
-          MemUtil.ZeroByteArray(bUtf8);
+          derivedKey = rfc2898.GetBytes(32); // 256 bits required for ChaCha20
       }
-
-      // The returned derivedKey will naturally be 256 bits (32 bytes)
+      
+      MemUtil.ZeroByteArray(bUtf8);
       return derivedKey;
     }
   }