Vernam

Vernam refers to a cipher system developed by Gilbert S. Vernam in 1917 that forms the basis of the one-time pad concept. The Vernam cipher is a symmetric key, stream-style cipher in which a plaintext message is combined with a random key stream of equal length using a bitwise exclusive OR (XOR). When the key is truly random, as long as the plaintext, and used only once, the resulting ciphertext reveals no information about the original message without knowledge of the key.

The encryption process is simple in principle: C = P XOR K, where P is the plaintext, K is the key, and C is the ciphertext. The security relies on the key being completely random, kept secret, and never reused. If the key is reused or is not random, the cipher loses its theoretical security, reducing to other forms of cryptanalysis vulnerability. In practice, a perfectly random, non-reusable key of sufficient length is difficult to manage, which has limited the widespread use of the Vernam/one-time pad outside specialized contexts.

Historically, Vernam’s idea led to the terminology of the one-time pad, and Claude Shannon later proved the principle of perfect secrecy for the scheme. The Vernam cipher is sometimes described as a specific implementation of the one-time pad using XOR, distinguishing it from repeating-key or alphabetic variants such as the Vigenère cipher. In modern cryptography, the one-time pad is rarely used for general communication due to logistical challenges, but it remains a foundational concept in discussions of theoretical security.