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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 the purpose of IFF is to identify friendly targets, and not replying to an IFF interrogation does not make one a foe.

There are many reasons for not replying to IFF by friendly aircraft: battle damage, loss of crypto, wrong crypto, 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.

The IFF of WWII and Soviet era systems used coded radar signals (called Cross-Band Interrogation, or CBI) to automatically trigger the aircraft's transponder in an aircraft "painted" by the radar. Modern crypto IFF systems use a separate specialized transponder beacon which can operate without radar. They are referred to as cross-band beacon or transponders. Each IFF transponder also has a KIR or KIT crypto computer associated with it. The KIR (designed for interrogators) and the KIT (designed for transponders) have an access port where the crypto codes are inserted. The military IFF system will not function without a valid code. No code means no IFF. Civilian SIF systems and Mode S do not require crypto codes.

An IFF transponder responds:

• in a military aircraft, vehicle, or unit by returning a coded reply signal only when the incoming interrogation is identified as part of the friendly forces network;
• if no IFF response is generated a civil (Selective Identification Feature - SIF) interrogation may then be generated and the aircraft, by returning various mode replies can then be identified or sorted.

In an IFF network both the interrogation and reply are verified as friendly.

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 non-crypto SIF interrogation format is used. In SIF, the interrogation is composed of two pulses spaced apart by a different amount for each mode, and a transponder replies with a long series of bits. In IFF, the interrogation is a long series of bits that contains the message and parity. The message is encrypted with a secret crypto key, and only IFF transponders with the same crypto key will decode the message. Once decoded, the IFF transponder will process the encoded message and send back a 3 pulse reply. The interrogator compares the reply to the challenge message and marks the target friendly, and determines the azimuth and range. A second possibility is the target being marked as a spoof target. That is, the target replied but failed to process the message correctly on a significant number of challenges. Targets marked as a spoof may be declared hostile and, inside a battle-space, are often destroyed when able.

Major military benefits of IFF include:

• Reducing "friendly-fire" or fratricidal attacks
• Positive identification of friendly forces.

There are no civilian uses of IFF. Civilian systems are all based on SIF and MODE S.

There are two military modes of operation designated for use by Allied forces:

• Mode 4 is crypto-secure mode (the only currently fielded true method of determining friend)
• Mode 5 is crypto-secure using a secure wave form (ie spread spectrum)

• Challenge-response authentication
• Shibboleth
• Transponder
• Squawk code
• Secondary surveillance radar

• U.S. Federal Standard 1037C
• U.S. Department of Defense Dictionary of Military and Associated Terms