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One-time tape cipher machine - this page is a stub

Telekrypton was an electromechanical one-time tape cipher machine, built around 1933 in the United States by the Western Union Telegraph Company. Built in small quantities it was based on the so-called Vernam Cipher, invented in 1918 by Gilbert Sandford Vernam (1890-1960) and described in US Patent 1310719 [2]. It is further described in an AIEE journal in February 1926 [3].

In order to prove the concept, the Western Union Telegraph Company built a limited number of machines that were marketed for several years uner the name Telekrypton. The machine was commercially not very successful and had two major weaknesses. First of all it was extremely large. Consisting of a table full of equipment, it would in practice be very difficult to maintain. More importantly however, was the fact that the keystream tape was looped, presenting a major cryptographic flaw. Due to its repetitive nature, the keystream tape could be reconstructed.

Photograph from Vernam's paper of 1926 [2] describing the machine

In his paper of 1926 [3 p. 16] Vernam descibes a way of improving security by using two looped keystream tapes of approx. 7 feet, each of which has a different length (that should not share a common factor with the number of characters on the other tape). The image above shows the system that was described in his paper. Although the increased cipher period greatly improves the cryptographic strength of the system, it still is not a true one-time pad system.

In 1940, The British Security Coordination (BSC) was looking for a machine that could be used to send secure messages in pre-war America, between the BSC in Washington and the BSC in New York. Canadian communications expert Benjamin deForest Bayly was asked to find a solution.

Bayly took the existing Telekrypton machine, of which Western Union still had two left in its warehouse, and reworked them for the BSC. Unnecessary parts were removed and a much longer random tape was introduced. By using a random tape that had the same length as the message (or more) and allowing only two identical tapes to exist, Bayly had in fact created an unbreakable OTP cipher. He also instructed the key tapes at both ends to be destroyed immediately after use.

Using hand-punched keystream tapes, the first Telekrypton line between the BSC in New York and the BSC in Washington became operational in January 1942. In May 1942 an additional line was opened between the BSC and Ottawa and finally in July between the BSC and the transatlantic transmitter at Camp X in Whitby 1 (Ontario, Canada) [1].

  1. During WWII, Camp X was the unofficial name of a paramilitary, clandestine and commando training installation of the British Security Coordination (BSC), located in Ontario (Canada) between Whitby and Oshawa. It is known today as Intrepid Park.

In the following year, Bayly fixed a number of security weaknesses in the Telekrypton design and modified the combining logic so that only the 26 letters of the Latin alphabet appeared in the encrypted output, allowing the ciphertext to be nicely formatted in groups of five letters each, separated by an automatically inserted space. The modified machine became known as Rockex and was first used in 1943 on the transatlantic link between Camp X and the UK.

 More about Rockex

  1. Jerry Proc and contributors, Telekrypton
    Website. Retrieved January 2015

  2. G.S. Vernam, US Patent 1310719
    Secret Signaling System, Filed 13 September 1918.

  3. G.S. Vernam, Cipher Printing Telegraph Systems
    AIEE (IEEE) VOL. XI.V. pp. 109-115. February 1926 1

  4. Wikipedia, Camp X
    Retrieved January 2015.
  1. The American Institute of Electrical Engineers (AIEE) was merged with the Institute of Electrical and Electronics Engineers (IEEE) on 1 January 1963 and is now known as IEEE. The document cited here is located off-site and is reproduced there by permission of the IEEE (read the enclosed copyright notice).

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Crypto Museum. Created: Tuesday 27 January 2015. Last changed: Sunday, 25 February 2018 - 18:00 CET.
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