Gilbert Vernam

Gilbert Sandford Vernam (3 April 1890 – 7 February 1960) was a Worcester Polytechnic Institute 1914 graduate and AT&T Bell Labs engineer who, in 1917, invented an additive polyalphabetic stream cipher and later co-invented an automated one-time pad cipher. Vernam proposed a teleprinter cipher in which a previously prepared key, kept on paper tape, is combined character by character with the plaintext message to produce the ciphertext. To decipher the ciphertext, the same key would be again combined character by character, producing the plaintext. Vernam later worked for the Postal Telegraph Company, and became an employee of Western Union when that company acquired Postal in 1943. His later work was largely with automatic switching systems for telegraph networks.

The combining function Vernam specified in U.S. Patent 1,310,719, issued July 22, 1919, is the XOR operation, applied to the individual impulses or bits used to encode the characters in the Baudot code. Vernam did not use the term "XOR" in the patent, but he implemented that operation in relay logic. In the example Vernam gave, the plaintext is A, encoded as "++---" in Baudot, and the key character is B, encoded as "+--++". The resulting ciphertext will be "-+-++", which encodes a G. Combining the G with the key character B at the receiving end produces "++---", which is the original plaintext A. The NSA has called this patent "perhaps one of the most important in the history of cryptography.".

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