Gilbert Vernam

Gilbert Sandford Vernam (1890 – 7 February 1960) was a AT&T Bell Labs engineer who, in 1917, invented the stream cipher and later co-invented the one-time pad cipher. Vernam proposed a teletype cipher in which a previously-prepared key, kept on paper tape, is combined character by character with the plaintext message to produce the cyphertext. To decipher the ciphertext, the same key would be again combined character by character, producing the plaintext. Vernam later worked for Postal Telegraph Co., and became an employee of Western Union when W.U. acquiredPostal in 1943. His later work was largely with automatic switching systems for teletypewriter networks.

Vernam's patent

The combining function Vernam specified in US patent|1310719, issued July 22, 1919, is the XOR operation, applied to the individual impulses or bits used to encode the characters in the Baudot teletype 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."( [http://www.nsa.gov/publications/publi00017.pdf] p.3).

One-time pad

Shortly thereafter, Joseph Mauborgne, at that time a captain in the US Army Signal Corps, proposed, in addition, that the paper tape key contain random information. The two ideas, when themselves combined, implement anautomatic form of the one-time pad, though neither inventor used the name then. It was patented in the mid-1920sFact|date=February 2007.

Claude Shannon, also at Bell Labs, proved that the one-time pad is unbreakable in his World War II research that was later published in October 1949. It is the first and only encryption method for which there is such a proof.

The Vernam cipher

In modern terminology, a Vernam cipher is a stream cipher in which the plaintext is XORed with a random or pseudorandom stream of data the same length to generate the ciphertext. If the stream of data is truly random and used only once, this is the one-time pad. Substituting pseudorandom data generated by a cryptographically secure pseudo-random number generator is a common and effective construction for a stream cipher. RC4 is an example of a Vernam cipher that is widely used on the Internet.

Other patents

Other cryptographic patents filed by Vernam include:
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References

* Gilbert S. Vernam, "Cipher Printing Telegraph Systems For Secret Wire and Radio Telegraphic Communications", Journal of the IEEE, Vol 55, pp109-115 (1926).
* Gilbert S. Vernam, "Automatic Telegraph Switching System Plan 55-A", AIEE Transactions on Communication andElectronics, May 1958, p. 239. Also in Western Union Technical Review Vol 12 No 2, April 1958, p. 37.
* Gilbert S. Vernam, "Printing Telegraph Operation of Way Wires", AIEE Transactions vol 57, July 1938, p. 365.
* Gilbert S. Vernam, "An Automatic Concentration Unit for Printing Telegraph Circuits", Electrical Communication,April 1932, p. 200.
* C.E. Shannon, “Communication Theory of Secrecy Systems,” Bell System Technical Journal, Vol. 28, No. 4 (October 1949), pp. 656-715.