Worcester Polytechnic Institute


Gilbert S. Vernam, WPI Class of 1914

The era of modern cryptography dawned 90 years ago with a brainstorm by Vernam. While working for AT&T in New York, he figured out how to guarantee the security of private messages transmitted over telegraph wires. It was known that even when multiple messages were speeding through a wire in both directions, a savvy hacker with an oscilloscope could monitor the frequency changes and transcribe the messages. After mulling the problem over, Vernam developed an ingenious solution.

Printing telegraph machines typically used paper tapes for translating messages into electrical pulses and turning the pulses back into text. Vernam suggested using a second tape with a set of random pulses – a private key, in cryptographic terms – that could be added to the pulses of the text to create an encrypted message. An identical tape at the other end enabled the added pulses to be subtracted to reveal the message. Anyone intercepting the message would see only a meaningless jumble of pulses.

The first automatic, real-time method for coding and decoding messages, the system made cryptography, once a labor-intensive process that had to be done off-line, something that could be easily added to any communications system, from telephone calls, to radio transmissions, to e-mail messages flashed over the Internet.

Vernam’s invention also led to another seminal achievement in cryptography – the one-time pad, known as the only unbreakable cryptosystem. Not long after he learned of Vernam’s discovery, Major Joseph Mauborgne, head of the U.S. Army Signal Corps, discovered that if one employed a unique random key of the type developed by Vernam for each message and used each key only once, the result would be a system that even the most sophisticated code cracker could never break.