In IFF transponder modes, which mode is cryptographically keyed for secure operations?

• A. Mode 2
• B. Mode A
• C. Mode S
• D. Mode 4

Answer: (D)

The main idea is secure authentication through cryptographic keying in IFF. Mode 4 is designed specifically to provide cryptographically protected replies, using a shared secret key between friendly aircraft and interrogators. When an interrogation is sent, the aircraft uses its secret key to compute a response that is tied to that challenge. Because the response depends on both the challenge and the key, it cannot be forged without the same key, and it changes with every challenge, protecting against replay and spoofing.

This is what makes Mode 4 the secure option among the IFF modes. The other modes are not built around this cryptographic challenge–response concept; they rely on fixed or non-secure codes rather than a cryptographic key, so they don’t provide the same level of authentication or protection. Mode 4’s cryptographic approach is the reason it’s considered secure for friend-or-foe operations.