What is best practice for number combinations?

• A. Even numbers provide symmetrical patterns.
• B. Odd numbers work best because the person receiving cannot return the same number.
• C. Use the same number repeatedly.
• D. Use a simple, easily remembered sequence.

Answer: (B)

In number combination practices for secure communications, the goal is to prevent a simple echo or replay from proving a valid interaction. Using an odd number as the challenge helps because it prevents the recipient from simply returning the same number as a valid response. The protocol or procedure is set up so the response must differ in value, or be derived in a way that changes parity, making a straight mirror reply ineffective. This reduces the chance that an attacker could spoof or replay a credential by just copying the input.

The other approaches introduce vulnerabilities: using even numbers can create symmetry that an adversary could exploit; repeating the same number invites straightforward replay; and a simple, easy-to-remember sequence is predictable and easier to guess or compromise.