In telecommunications, identification, friend or foe (IFF) is a cryptographic identification system designed for command and control. A system that enables military, and national (civilian-located ATC) interrogation systems to distinguish friendly aircraft, vehicles, or forces, and to determine their bearing and range from the interrogator.
IFF was first developed during World War II. The term is a bit of a misnomer, as IFF can only positively identify friendly targets but not hostile ones. If an IFF interrogation receives no reply, the object can only be treated as suspicious but not as a positively identified foe.
There are many reasons for not replying to IFF by friendly aircraft: battle damage, loss of encryption keys, wrong encryption keys, or equipment failure. Aircraft hugging terrain are very often poor candidates for microwave line-of-sight systems such as the IFF system. Microwaves can't penetrate mountains, and very often atmosphere effects (referred to as anomalous propagation) cause timing, range, and azimuth issues.
An IFF transponder responds:
In an IFF network both the interrogation and the reply are verified as friendly.
Each IFF transponder also has a KIR or KIT cryptography computer associated with it. The KIR (designed for interrogators) and the KIT (designed for transponders) have an access port where the encryption keys are inserted. The military IFF system will not function without a valid key. Civilian SIF systems and Mode S do not require encryption keys.
An IFF transponder receives interrogation pulses at one frequency (1030 MHz), and sends the reply pulses at a different frequency (1090 MHz). Just the opposite of a SIF interrogation, which is composed of two pulses spaced apart by a different amount for each mode, with the transponder reply being a long series of bits; the IFF interrogation is instead a long series of bits that contains the encrypted message and parity, and the reply is just three pulses.
The IFF message is encrypted with a secret key. IFF transponders with the same secret key will be able to decode the IFF message. Once decoded, the IFF transponder will execute the message and send back a 3 pulse reply. The interrogator then compares each reply to the challenge messages, and marks these targets friendly while also storing their azimuth and range.
A second possibility is a target being marked as a spoof target. That is, the target replies, but fails to process the IFF message correctly on a significant number of challenges. Targets marked as a spoofer can be declared hostile and, inside a battle-space, are often destroyed when able.
If no reply is received from the IFF transponder, the target continues to be an unknown. The IFF system is not used to declare a target hostile if they do not reply. Very often the pilot can have the wrong code (encryption key) selected, or the code is expired, and they will have an audible and visual alarm every time they are interrogated by IFF. If they can't clear the alarm they follow the pre-briefed safe passage procedures.
There are no civilian uses of IFF. Civilian systems are all based on SIF and MODE S.