Useful article from the .NET Security blog about AES and Rijndael. Essentially, Rijndael is more flexible than it’s use as defined in the AES standard. Can’t say I’ve ever come across a situation where I’d want to less than 256 bit, but then I’ve never had to do time critical encryption. Might benchmark it, that might be an interesting task.

The short of it is:

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
3. The key size is 128, 192 or 256 bits (Added by Andy)