correct domain in the sadb a transmitter 25 sends messages back to the key management applications 29 it will be appreciated that the security association control module 24 may comprise a single processor or may be any arrangement associated with the key engine unit 21 that is suitable for carrying out the actions described one or more individual processors may be used to implement the security association control module 24 and it is also conceivable that the receiver 22 transmitter 25 and security association control unit 24 may be incorporated together or in any combination in one or more circuit in practice the key engine unit is likely to be operated by software and fig3 is an illustration of a key engine unit 31 illustrating how this may be implemented in this case the key engine unit again includes a receiver 32 security association control module 34 and transmitter 35 each of the receiver 32 sa control module 34 and transmitter 35 includes a processing unit 321 341 351 each processing unit is associated with a computer program product in the form of a memory 322 342 352 e g one or more rom harddisk flash memory or eeprom the memory 322 associated with the receiver 32 comprises a computer program 323 which can be run by the processing unit 321 to control the receipt of pf_key messages from key management applications in the.