A hardware accelerator is arranged to perform cipher operations and comprises a first memory area arranged to store a first bit string and a second memory area arranged to store a second bit string. A calculation block is arranged to receive a round key and to perform a function on the first bit string. The function comprises combining the first bit string with the round key to produce a combined bit string and performing a non-linear mapping from the combined bit string to a mapped bit string. An addition block is arranged to add the mapped bit string to the second bit string to produce a resultant bit string. A controller is arranged to receive a control signal and, depending on the state of the control signal, provides the first bit string and the resultant bit string to the appropriate memory area.