Best encryption algorithm server to mobile app client?

I am looking for a encryption algorithm for my app server. Data will be encrypted on server and decrypted on a ios application.

I tried aes256ctr, but it is clearly not suitable for this, as the encryption key needs to be the same, hard coded in the app and server. With CTR, the IV is supposed to change with each encrypt, and that is simply not possible in this situation, right?

Thanks in advance