Data processor with flexible data encryption

A data processor (20) which flexibly encrypts data within different address ranges includes an encryption determination circuit (50) to monitor an address conducted on an internal address bus (22) and when the address is within certain predefined ranges, perform encryption or decryption of address and/or data. For example the encryption determination circuit (50) may be used to selectively enable a data encryption-decryption circuit (60). When the data encryption-decryption circuit (60) is disabled, data conducted on an internal data bus (23) becomes "cleartext", i.e., non-encrypted. In one embodiment, the data encryption-decryption is performed in partial dependence on the address itself, and the address conducted to the external address bus is itself selectively encrypted as well.