Maybe you should ask the server guy? When ever you have encryption between too parties you have to have some kind of agreement on the format of that data, the raw primitives don't handle that alone, not to mention it's easy to mess things up security wise dealing with just the primitives and the desire to just send the aes ciphertext alone is going to cause mistakes.
RNCryptor, which you mention, is a high level encryption library it defines a simple format that others would have to conform too, it's simple thus helps going cross platform, but it has that extra that you need to do AES properly. There are other libraries like that too (NaCL, GPGME, and Keyczar), that are not as simple in format, but simple in usage, so you'd need to be able to use the library on both ends, but I'd highly recommend that you uses something like that, if you can, rather than rolling your own.
Keyczar specifically exists for java, python, c++, c# and go, so if you can use the c++ version on the iOS (or Mac, which ever you are targeting on the client) you might be good on the server as there are several choices.