Which mode of the IFF transponder system can be cryptographically keyed for secure operations?

• A. Mode 4
• B. Mode 1
• C. Mode S
• D. Mode A

Answer: (A)

The key idea is identifying which IFF mode provides cryptographic authentication for secure identification. Mode 4 is the cryptographic mode: it uses a shared secret key between the interrogator and the aircraft transponder. When challenged, the transponder generates a cryptographic reply proof that it knows the key, without revealing it, so only friendly aircraft with the correct key respond correctly. This cryptographic challenge–response eliminates spoofing and ensures secure identification in operations that require it.

The other modes do not use cryptographic keying for secure identification. They rely on non-cryptographic codes or data (such as civilian or mission codes, or broader air-surface surveillance data) and do not provide the same cryptographic authentication mechanism.