Why are RijndaelManaged and AesCryptoServiceProvider returning different results?

Answer 1

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 2

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)?