Decrypt AES/OFB/NoPadding

Question

I need to decrypt content encoded using the AES/OFB/NoPadding combo in C#. This doesn't seem to be supported natively: the following code won't do it as

var aes = new AesManaged
{
   Padding = PaddingMode.None,
   Mode = CipherMode.OFB,
   KeySize = 128,
   BlockSize = 128
};

Produces:

System.Security.Cryptography.CryptographicException:
Specified cipher mode is not valid for this algorithm.

The closest issue that I could find here on SO is this one, which uses BouncyCastle to do encryption:

Public Function DoEncryption(ByVal KeyArray() As Byte, ByVal IVArray() As Byte, ByVal Buffer     As Byte()) As Byte()

    Dim ae As New CipherKeyGenerator()
    ae.Init(New KeyGenerationParameters(New SecureRandom(), 256))
    Dim aesKeyParam As KeyParameter = ParameterUtilities.CreateKeyParameter("AES",     KeyArray)
    Dim aesIVKeyParam As ParametersWithIV = New ParametersWithIV(aesKeyParam, IVArray)
    Dim cipher As IBufferedCipher = CipherUtilities.GetCipher("AES/OFB/NoPadding")
    cipher.Init(True, aesIVKeyParam)
    Dim encrypted() As Byte = cipher.DoFinal(Buffer)
    Return encrypted

End Function

Other issues (like this one) contain more information but also lots of custom code - I'd rather use BouncyCastle. Can anybody help me with the decryption, or point me to some helpful documentation?

Answer 1

From the code in the actual question, the only step necessary to trigger decryption instead of encryption was to change the boolean parameter in the cipher.Init invocation:

cipher.Init(False, aesIVKeyParam) // False == decryption, True == encryption

The final snippet in C# is

private static byte[] DoDecryption(byte[] keyArray, byte[] ivArray, byte[] encoded)
{
    var aesKeyParam = ParameterUtilities.CreateKeyParameter("AES", keyArray);
    var aesIvKeyParam = new ParametersWithIV(aesKeyParam, ivArray);
    var cipher = CipherUtilities.GetCipher("AES/OFB/NOPADDING");
    cipher.Init(false, aesIvKeyParam);
    var decrypted = cipher.DoFinal(encoded);
    return decrypted;
}