A system for processing packet streams, the system comprising: a system data-bus for transferring packets of data; a system memory connected to the data-bus for storing packets; a first processor connected to the data-bus, arranged to retrieve secure packets from the system memory via the data-bus; and a second processor connected to the data-bus, programmed to perform security processing by applying one or more security algorithms to the secure packets to generate at least partially security-processed packets. The first processor is programmed so as to control the security processing of a previously retrieved packet by the second processor, the security processing involving a delay whilst the first processor awaits the return of a security-processed packet from the second processor; and the first processor is arranged to further process a previously security-processed packet to generate a corresponding output data during the delay.