What kind of attack does a nonce primarily protect against?

• A. Man-in-the-middle attacks
• B. Replay attacks
• C. Phishing attacks
• D. DDoS attacks

Answer: (B)

A nonce, which stands for "number used once," is a unique, random value that is generated for a specific session or transaction, ensuring that it is used only once. Its primary purpose is to protect against replay attacks, where a malicious actor intercepts valid data transmissions and then attempts to re-send that data in order to gain unauthorized access or perform unauthorized actions.

In the context of a replay attack, if an attacker were to capture a message that included a valid session token or authentication data, they could attempt to resend this captured message to impersonate the original sender. By including a nonce in communication, each request is made unique and cannot be reused, as the nonce will be specific to that particular transaction. When the receiving system checks for the uniqueness of the nonce, it can quickly identify and reject any duplicate requests, thus thwarting potential replay attacks.

Other types of attacks, such as man-in-the-middle, phishing, and DDoS, are mitigated through different means, such as encryption, user education, and network security measures, rather than directly through the use of nonces. Consequently, the functionality of a nonce is specifically tailored to address the risks associated with replay attacks, making it the most appropriate answer in this context.