An ASIC or ASSP has processor circuitry (110), a predetermined initialization program (100) for execution by the processor circuitry at power up, and a non-volatile key table (120) readable by the initialization program, and not accessible otherwise by the processor circuitry. The initialization program reads a key index associated with encrypted data, from external memory, and uses the key index to read a corresponding key from the table, to decrypt the encrypted data for use by the processor circuitry. Optionally another key is first decrypted and used for the decryption of the encrypted data. By keeping the key on board the chip and restricting access in this way, the key and therefore the encrypted data can be protected from software based reverse engineering. This means the encrypted data can cheaper memory chips or other storage. Thus the processor circuitry can be formed on a smaller integrated circuit.