Which IFF transponder mode is encrypted for secure identification?

• A. Mode 4
• B. Mode 3A
• C. Mode S
• D. Mode C

Answer: (A)

Encrypted identification in IFF is provided by a mode that uses a cryptographic algorithm with a shared key to generate a challenge-response reply. This makes it impossible for an unauthorized transmitter to impersonate an aircraft because the correct encrypted response can only be produced with the proper key.

The other modes do not provide this encryption. One mode simply transmits a civilian four-digit identity code with no encryption, another adds altitude information but still isn’t encrypted, and the third offers selective addressing and digital data, but encryption for secure identification isn’t its standard feature. So the mode designed for secure, encrypted identification is the encrypted mode.