A programmable crypto processing system (10) includes several processing resources (14, 16, 26) implemented on a single ULSI die. The processing system is both key and algorithm agile allowing for simultaneous execution of a variety of cryptographic programs through the use of background staging of the next program and context (key and state) during execution of a current program. The programmable crypto processing system includes a programmable crypto processor (17) for processing data units in accordance with a channel program, a crypto controller (11) for identifying a channel program, two interface processors (13, 15) for asynchronously receiving and transferring data units from and from an external host. Data units identify a particular channel program, and are processed in a selected processing engine in accordance the identified channel program. The interface processors are capable of full duplex operation and provide full physical data interface isolation for processing plain-text and cipher-text data.