The Allies used the Rockex, an electronic valve-based one-time tape system, from 1943 onwards. This online machine connected to a teleprinter, encrypted, and sent letters in a random sequence. The Rockex could also decrypt messages immediately upon receipt.
A one-time tape system is a form of one-time pad. The random keystring is punched onto paper-tape as a five-bit code for each key letter rather than the letters being printed onto a paper pad. It operates by addition of the five bits representing the key letters with the five bits for the plaintext letters to produce the cipher characters for transmission.
Like all one-time systems, the Rockex was very secure and used to protect the most highly classified messages. However, it also demanded the generation and management of large volumes of keys. Various versions of the machine were used up to 1960s and beyond. Australia’s Department of External Affairs was one such user.
From its inception, the Defence Signals Bureau was responsible for providing the Rockex keys for Australian users by either generating keys or distributing foreign-sourced punched paper tapes.
