Why are RijndaelManaged and AesCryptoServiceProvid

Question

Here is the example that I have run. It has the same Mode, Padding, BlockSize, KeySize. I am using the same init vector, key and data.

Using the RijndaelManaged produces an encrypted value of: 0x8d,0x81,0x27,0xc6,0x3c,0xe2,0x53,0x2f,0x35,0x78,0x90,0xc2,0x2e,0x3b,0x8a,0x61, 0x41,0x47,0xd6,0xd0,0xff,0x92,0x72,0x3d,0xc6,0x16,0x2b,0xd8,0xb5,0xd9,0x12,0x85

Using the AesCryptoServiceProvider produces an encrypted value of: 0x8d,0x9f,0x6e,0x99,0xe9,0x54,0x8b,0x12,0xa9,0x88,0x1a,0x3d,0x65,0x23,0x9c,0x4e, 0x18,0x5a,0x89,0x31,0xf5,0x75,0xc5,0x9e,0x0d,0x43,0xe9,0x86,0xd4,0xf3,0x64,0x3a

Here is the code I used to generate these results

   public partial class AesTest
   {
      private SymmetricAlgorithm mEncryptionType;
      private byte[] mPrivateKey;
      private byte[] mInitializationVector;
      private byte[] mData;

      public AesTest()
      {
         mPrivateKey = new byte[32] 
         { 
            0x22, 0x22, 0x22, 0x22, 
            0x22, 0x22, 0x22, 0x22, 
            0x22, 0x22, 0x22, 0x22, 
            0x22, 0x22, 0x22, 0x22,
            0x22, 0x22, 0x22, 0x22, 
            0x22, 0x22, 0x22, 0x22, 
            0x22, 0x22, 0x22, 0x22, 
            0x22, 0x22, 0x22, 0x22
         };

         mInitializationVector = new byte[16]
         { 
            0x33, 0x33, 0x33, 0x33,
            0x33, 0x33, 0x33, 0x33,
            0x33, 0x33, 0x33, 0x33,
            0x33, 0x33, 0x33, 0x33
         };

         mData = new byte[16]
         {
            0x44, 0x44, 0x44, 0x44,
            0x44, 0x44, 0x44, 0x44,
            0x44, 0x44, 0x44, 0x44,
            0x44, 0x44, 0x44, 0x44
         };

         mEncryptionType = new RijndaelManaged();
         mEncryptionType.Mode = CipherMode.CFB;
         mEncryptionType.Padding = PaddingMode.PKCS7;
         mEncryptionType.BlockSize = 128;
         mEncryptionType.KeySize = 256;

         byte[] rij_encrypted_data = Encrypt(mData);

         mEncryptionType = new AesCryptoServiceProvider();
         mEncryptionType.Mode = CipherMode.CFB;
         mEncryptionType.Padding = PaddingMode.PKCS7;
         mEncryptionType.BlockSize = 128;
         mEncryptionType.KeySize = 256;

         byte[] aes_encrypted_data = Encrypt(mData);
      }

      public virtual byte[] Encrypt(byte[] unencryptedData)
      {
         return TransformData(unencryptedData, mEncryptionType.CreateEncryptor(mPrivateKey, mInitializationVector));
      }

      private byte[] TransformData(byte[] dataToTransform, ICryptoTransform cryptoTransform)
      {
         byte[] result = new byte[0];
         if (dataToTransform != null && cryptoTransform != null && dataToTransform.Length > 0)
         {
            // Create the memory stream to store the results
            MemoryStream mem_stream = new MemoryStream();
            // Create the crypto stream to do the transformation
            CryptoStream crypto_stream = new CryptoStream(mem_stream, cryptoTransform, CryptoStreamMode.Write);
            // bytes are transformed on a write
            crypto_stream.Write(dataToTransform, 0, dataToTransform.Length);
            // Flush the final block
            crypto_stream.FlushFinalBlock();
            // Convert the transformed memory stream back to a byte array
            result = mem_stream.ToArray();
            // Close the streams
            mem_stream.Close();
            crypto_stream.Close();
         }
         return result;
      }
   }

I guess I'm just wondering if I missed something.

Update: Turns out that AesManaged will throw a CryptographicException ("The specified cipher mode is not valid for this algorithm") if you try and set the CipherMode to CFB. I feel that the AesCryptoServiceProvider should do that same, but it doesnt. Seems funny that the FIPS Certified class allows invalid cipher modes.

Answer 1

Addition information from this post says:

Essentially, if you want to use RijndaelManaged as AES you need to make sure that:
1) The block size is set to 128 bits
2) You are not using CFB mode, or if you are the feedback size is also 128 bits

Ok, great. I added mEncryptionType.FeedbackSize = 128; to my above example and I get an CryptographicExecption:

System.Security.Cryptography.CryptographicException was unhandled
  Message="Bad Data.\r\n"
  Source="System.Core"
  StackTrace:
       at System.Security.Cryptography.CapiNative.SetKeyParameter(SafeCapiKeyHandle key, KeyParameter parameter, Byte[] value)
       at System.Security.Cryptography.CapiNative.SetKeyParameter(SafeCapiKeyHandle key, KeyParameter parameter, Int32 value)
       at System.Security.Cryptography.CapiSymmetricAlgorithm.SetupKey(SafeCapiKeyHandle key, Byte[] iv, CipherMode cipherMode, Int32 feedbackSize)
       at System.Security.Cryptography.CapiSymmetricAlgorithm..ctor(Int32 blockSize, Int32 feedbackSize, SafeCspHandle provider, SafeCapiKeyHandle key, Byte[] iv, CipherMode cipherMode, PaddingMode paddingMode, EncryptionMode encryptionMode)
       at System.Security.Cryptography.AesCryptoServiceProvider.CreateEncryptor(SafeCapiKeyHandle key, Byte[] iv)
       at System.Security.Cryptography.AesCryptoServiceProvider.CreateEncryptor(Byte[] key, Byte[] iv)
       at AESTest.Form1.Encrypt(Byte[] unencryptedData) in C:\Documents and Settings\nschoonmaker\My Documents\Visual Studio 2005\Projects\AESTest\AESTest\Form1.cs:line 79
       at AESTest.Form1..ctor() in C:\Documents and Settings\nschoonmaker\My Documents\Visual Studio 2005\Projects\AESTest\AESTest\Form1.cs:line 73
       at AESTest.Program.Main() in C:\Documents and Settings\nschoonmaker\My Documents\Visual Studio 2005\Projects\AESTest\AESTest\Program.cs:line 17

Is there something wrong with the System.Core dll that wouldnt support this, or do I need to change something else?

On a side note, if I change the FeedbackSize to 8 for both, its seems to work! Even for CFB mode. So I guess my next question is, how do I get 128 to work (and hopefully this will put an end to this question)?

Answer 2

I think it has to do with the CipherMode.CFB. See this post describing AesManaged:

AesManaged is actually just a wrapper around RinjdaelManaged with some code added to make sure that you do not setup the algorithm to operate in a non-AES compatible way. For instance, AesManaged does not allow you to change the block size. (It will also disallow the use of CFB and OFB mode because of the way that RijndaelManaged works with those modes).

Note that if you use CipherMode.ECB or CipherMode.CBC, you'll see identical results. Any reason why you need CFB and not CBC?

Answer 3

Response from Microsoft:

RijndaelManaged class and AesCryptoServiceProvider class are two different implementations. RijndaelManaged class is a kind of implementation of Rijndael algorithm in .net framework, which was not validated under NIST (National Institute of Standards and Technology) Cryptographic Module Validation Program (CMVP).

However, AesCryptoServiceProvider class calls the Windows Crypto API, which uses RSAENH.DLL, and has been validated by NIST in CMVP. Although Rijndael algorithm was the winner of the NIST competition to select the algorithm that would become AES, there are some differences between Rijndael and official AES. Therefore, RijndaelManaged class and AesCryptoServiceProvider class have subtle differences on implementation.

In addition, RijndaelManaged class cannot provide an equivalent implementation with AES. There is another class implemented in .net framework, AesManaged class. This class just wrapped RijndaelManaged class with a fixed block size and iteration count to achieve the AES standard. However, it does not support the feedback size, especially, when the mode is set as CFB or OFB, the CryptographicException will be thrown.

For more information, please refer to the following MSDN documents.

AesManaged Class and AesManaged.Mode Property

If you want to pick up standard AES as security algorithm in your application, we recommend using the AesCryptoServiceProvider class. If you want to mix the RijndaelManged class and AesCryptoServiceProvider class in your application, we suggest using CBC mode instead of CFB mode in your program, since the implementation of the CBC mode in both classes is the same.