A data processing apparatus having a first secure area and a second secure area coupled by a monitor is provided. The monitor applies security credentials to processing circuitry transitioning from the first secure area to the second secure area to enable the processing circuitry to perform functions in the second secure area. A call gateway comprising a transition instruction and access parameters stored in a trusted storage device is used by the monitor to determine when to applying the security credentials to the processing circuitry. The access parameters comprising a target function or a memory location.