Which is NOT an example of sign/countersign?

Which is NOT an example of sign/countersign?

• A. Biometric scan.
• B. Code words.
• C. Challenge/password.
• D. Number combinations.

Answer: (A)

Sign/countersign rests on a challenge–response exchange: one party presents a sign, and the other must provide the correct countersign to prove affiliation. Code words work as the sign with a known reply, and a challenge paired with a password serves the same purpose, since the correct password is the required response. Number combinations can function as a numeric countersign in the same way, as a secret that must be supplied in response to a challenge. A biometric scan does not fit this pattern. It verifies an inherent physical attribute of the person rather than requiring a reciprocal secret or a timed response to a prompt. It’s an identification/authentication check, not a signaling exchange between two parties. That’s why biometric scan is not an example of sign/countersign.

Sign/countersign rests on a challenge–response exchange: one party presents a sign, and the other must provide the correct countersign to prove affiliation. Code words work as the sign with a known reply, and a challenge paired with a password serves the same purpose, since the correct password is the required response. Number combinations can function as a numeric countersign in the same way, as a secret that must be supplied in response to a challenge.

A biometric scan does not fit this pattern. It verifies an inherent physical attribute of the person rather than requiring a reciprocal secret or a timed response to a prompt. It’s an identification/authentication check, not a signaling exchange between two parties. That’s why biometric scan is not an example of sign/countersign.