2-21. Authentication is a security measure designed to protect a communications system against
fraudulent transmissions (imitative deception). The authentication system used within a
command is specified in accordance with the SOI. The most effective method of preventing
_______________ deception is by using _______________ systems.
(2-21. Answer: imitative, authentication)
2-22. If an enemy has intercepted our communications, he can then attempt to introduce false
traffic into the system or to imitate a station in the net. The most effective method of preventing
imitative deception is by using _______________ _______________.
(2-22. Answer: authentication systems)
2-23. In the challenge and reply authentication system, the called party always makes the first
challenge. When the caller desires authentication, he must first invite a challenge from the called
party by stating that he is prepared to authenticate. If an incorrect reply is received, the called
party should rechallenge with another randomly selected challenge.
The first challenge will always be made by the _______________ _______________.
(2-23. Answer: called party)
2-24. The challenging party randomly selects two letters from a prepared authentication table
and transmits them as a challenge. The party being challenged locates the first letter of the
challenge in the left-hand column, then proceeds horizontally across this line to the second letter
of the challenge. The reply is the cipher letter directly below the second letter of the challenge.