Question

I am building a Monotouch application which downloads data from the server encrypted using AES. I then need to decrypt this data when the file is accessed.

What is the best way for doing this using MonoTouch? iOS AES decryption is apparently hardware accelerated and so I would ideally like to call into CCCrypt. I am a bit of a n00b to MonoTouch so does anyone know how to do this?

Or alternatively is there a better approach to doing AES decryption in MonoTouch?

Answer 1

MonoTouch provides AES support inside it’s class library, e.g. the RijndaelManaged class.

However you need to know a bit more about how it was encrypted (cipher mode, padding mode, key size) to be able to decrypt a file. Also depending on the file size you might want to decrypt it in memory (safer) if it’s small or to a temporary file (if large).

Notes:

• Rijndael is the original name of the algorithm that got selected to be AES;

• AES is a subset of Rijndael (only one block size, 128 bits) so you can do everything AES supports using RijndaelManaged;

• At the moment MonoTouch does not use CommonCrypto (it uses the managed implementation from Mono) so you won’t get hardware acceleration. This will likely change in future releases (and will be compatible, i.e. simply re-compile, for people who used RijndaelManaged in their applications).

EDIT

MonoTouch 5.3.3 (alpha) now default to use CommonCrypto implementations, including hardware acceleration (when available) for AES and SHA1.