Identification: Friend or Foe (IFF) is an identification system designed for command and control. It enables military and national (civilian-located ATC) interrogation systems to identify aircraft, vehicles or forces as friendly and to determine their bearing and range from the interrogator. IFF may be used by both military and civilian aircraft. IFF was first developed during World War II. The term is something of a misnomer, as IFF can positively identify friendly targets but not hostile ones. If an IFF interrogation receives no reply or an invalid reply, the object cannot be identified as friendly but is not positively identified as a foe. The major military benefits of IFF include preventing “friendly fire” and being able to positively identify friendly forces. Taken from Wikipedia.
Telling friends from foes isn’t just a problem for warriors in combat — it is a major problem in cyberwarfare, too. Email is a key battlefield of cyberspace — SP Guard is Email IFF.