The TSEC/KL-7, also known as Adonis was an off-line non-reciprocal rotor encryption machine. The KL-7 had rotors to encrypt the text, most of which moved in a complex pattern, controlled by notched rings. The non-moving rotor was fourth from the left of the stack. The KL-7 also encrypted the message indicator. History and development In 1952, the machine was introduced by AFSA's successor, the U.S. National Security Agency, in the US Army, Navy and Air Force. In the early 1960s, the AFSAM-7 was renamed TSEC/KL-7, following the new standard crypto nomenclature. It was the most widely used crypto machine in the US armed forces until the mid-1960s and was the first machine capable of supporting large networks that was considered secure against known plaintext attack. Some 25,000 machines were in use in the mid-1960s. The KL-7 was also used by several NATO countries until 1983. An 8-rotor version sent indicators in the clear and was code named POLLUX. The encrypted or decrypted output of the machine was printed on a small paper ribbon. It was the first cipher machine to use the re-entry (re-flexing) principle, discovered by Albert W. Small, which re-introduces the encryption output back into the encryption process to re-encipher it again. The research for the new cipher machine, designated MX-507, was initiated in 1945 by the Army Security Agency (ASA) as a successor for the SIGABA and the less secure Hagelin M-209. Its development was turned over to the newly formed Armed Forces Security Agency (AFSA) in 1949. The machine was renamed AFSAM-7, which stands for Armed Forces Security Agency Machine No 7. It was the first crypto machine, developed under one centralized cryptologic organisation as a standard machine for all parts of the armed forces, and it was the first cipher machine to use electronics (vacuum tubes), apart from the British ROCKEX, which was developed during World War 2. Description The KL-7 was designed for off-line operation. It was about the size of a Teletype machine and had a similar three-row keyboard, with shift keys for letters and figures. The KL-7 produced printed output on narrow paper strips that were then glued to message pads. When encrypting, it automatically inserted a space between five-letter code groups. One of the reasons for the five letter groups was messages might be given to a morse code operator. The number of five letter groups was easily verified when transmitted.