Which mode of IFF interrogation requires a cryptographic code to identify "FRIENDLY" aircraft?

The mode of IFF interrogation that requires a cryptographic code to identify "FRIENDLY" aircraft is indeed Mode 4. This mode is specifically designed for secure operations and utilizes encrypted codes to ensure that only validated and authorized friendly units can respond. Mode 4 is particularly important in military operations, where the identification of friendly aircraft is critical to prevent friendly fire incidents and maintain operational security.

Mode 4's reliance on cryptographic codes makes it distinct from other modes. For example, Mode 1 and Mode 3 do not utilize such encryption; these modes are based on simpler altitude reporting and selective identification techniques that may not provide the same level of security as Mode 4. Mode S also differs in its approach by utilizing advanced signal processing techniques and can provide more detailed information than the traditional modes, but it does not inherently require a cryptographic code for identification purposes like Mode 4.

Understanding Mode 4's cryptographic nature is vital for anyone involved in air defense and operations, reinforcing the need for secure identification of friendly forces in the battlefield environment.