What Bit Encryption does Apple Use on Mobile Devices?
As per page 11 of the iOS Security Guide:


  Every time a file on the data partition is created, Data Protection creates a new 256-bit
  key (the “per-file” key) and gives it to the hardware AES engine, which uses the key to
  encrypt the file as it is written to flash memory using AES CBC mode. (On devices with
  an A8 processor, AES-XTS is used.) The initialization vector (IV) is calculated with the
  block offset into the file, encrypted with the SHA-1 hash of the per-file key. 


Emphasis mine