Certified Information Systems Security Professional (CISSP) Practice Exam

A one-time pad in encryption is defined as a technique that combines plaintext with a random pad of equal length. This method uses a randomly generated key that is as long as the message itself and is used only once, ensuring that each bit or character of the plaintext is combined with a unique key component.

The essential feature of a one-time pad is that the key is truly random and is kept secret between the sender and the recipient. When the plaintext is combined with this random key using a simple operation such as XOR for binary data, the result is ciphertext that is theoretically impossible to decipher without knowing the key, providing perfect secrecy.

This technique is unique among encryption methods because it does not rely on complex algorithms and can achieve perfect security, unlike many other encryption schemes. The requirement that the key must never be reused is critical; if the key is reused, it can introduce vulnerabilities that can be exploited by an attacker. Thus, the one-time pad stands out as the only encryption method that can be proven to provide unbreakable encryption under ideal conditions.