Encryption of data within a memory 6 is provided by key generation circuitry 12 which serves to generate a key as a function of the address within the memory 6 being accessed and then encryption circuitry 14 or decryption circuitry 16 which serve respectively to encrypt or decrypt the data as a function of the key that has been generated based upon the address. The encryption and the decryption may be performed using a bitwise XOR operation. The key generation circuitry may have the form of physically unclonable function circuitry, which varies from instance to instance of implementation and that operates to generate the same key for the same address upon both write and read operations within the same instance.