Identification of friend or foe

Cryptography factors its way into defense in more than one way. History has seen everything from the ENIGMA to the Zimmermann Telegram be crucial turning points at times of war. But apart from just serving as a means of secretive communication, it also finds usage in detecting the enemy. Particularly used in Air Defense Operations, it is used to detect whether the approaching aircraft is a friend or a foe. With technology revolutionizing the defense sector, there has been such a substantial increase in the speed of aircrafts and missiles that it is virtually impossible to judge whether the enemy is approaching with merely the naked eye. In the vast skies, at such a high altitude from the ground, relying on intuition is a gamble. It is here that IFF (Identification of Friend or Foe) works its magic. It can detect the motive of an approaching aircraft faster than your computer at home!

IFF works with two frequencies, one for the 'interrogation signal' and the other for the reply. The interrogator sends code pulses at precisely defined intervals and the airborne transponder replies by selecting certain timings and rejecting others. The reply sent is also a sequence of pulses (12 such information pulses exist). These pulses are coded as 1's and 0's digitally. These replies are decoded by the source aircraft and they are hence able to monitor the altitude, location and nature of the approaching aircraft round about the clock.

The secure/cryptographic mode of operation of this device came about in order to avoid unauthorized access. This is done by attaching a prelude (of sorts) to the original message which notifies the transponder that it is on the brink of receiving a secure message. The message by itself is also encrypted by another device which is present in the interrogator. This encryption is done using complex mathematical algorithms, and the inverse of these algorithms is used to break the code on the transponders side. In case, a reply doesn't ensue, the aircraft is identified as a foe.

Another precautionary measure taken is that special keycodes are designed which should be entered from time to time by the person using the interrogating/transponding equipment, in order to avoid their unauthorized use. A set of challenges, each which should be dealt with in a different way, are supplied from time to time. Defense is a serious business. And cryptography helps not just you and me, but also war heroes feel just a little safer.

Reference

Identification of friend or foe systems
http://www.dean-boys.com/extras/iff/iffqa.html