KL-7 on display at USAF Communications Agency museum. Rotors have been removed.

The TSEC/KL-7, code named ADONIS, was a rotor machine encryption system introduced in the 1950s by the U.S. National Security Agency. It had eight rotors, seven of which moved in a complex pattern. The non-moving rotor was in the middle of the stack. It replaced the SIGABA system developed during World War II.

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 placed a space between each five-letter code group. There was an adaptor available, the KLX-7, that allowed 5-level Baudot punched paper tape from teletype equipment to be read for decryption. The standard KL-7 had no ability to punch tapes. A variant of the KL-7, the KL-47, could also punch paper tape, allowing direct input to teleprinters.

Each rotor had 36 contacts. To establish a new encryption setting, operators would select a rotor and place it in a plastic outer ring at the correct offset. The rings to use for each position and the offset was specified in a printed key list. This process would be repeated eight times until all rotor positions were filled. Key settings were usually changed every day at midnight, GMT. The basket containing the rotors was removable, and it was common to have a second set of rotors and basket, allowing the rotors to be set up prior to key change. The old basket could then be kept intact for most of the day to decode messages sent the previous day, but received after midnight.

The rotor basket had two sets of connectors at each end that mated with the main assembly. One pair of connectors, with 26 pins each, connected to the keyboard and printer. Another pair, with 10 pins each, connected through the mechanism used to control the stepping of the rotors. There was also a microswitch under each movable rotor that was operated by cams on its plastic outer ring. Different outer rings had different arrangements of cams. The exact way all these features worked together is not publicly known, but it is likely they advanced the rotors in a pseudorandom fashion, a design principle that had proved successful with SIGABA. One former KL-7 operator relates that the rotor stepping was independent of the plaintext or ciphertext input [1] (http://www.jproc.ca/crypto/kl7.html).

There was a sliding permutor board under the keyboard that may have been used to switch the input and output of the basket, so that the same rotor setup could be used both to encrypt and decrypt messages.

The KL-7 was largely replaced by electronic systems such as the KW-26 ROMULUS and the KW-37 JASON in the 1970s, but KL-7s were kept in service as backups and for special uses. In 1967, when John Anthony Walker (a sailor in the U.S. Navy) walked into the embassy of the Soviet Union in Washington, DC seeking employment as a spy, he carried with him a copy of a key list for the KL-47. KL-7s were compromised at other times as well. A unit captured by North Vietnam is on display at NSA's National Cryptologic Museum. After the Walker spy ring was uncovered in 1985, remaining KL-7's and KL-47's were withdrawn from service and decommissioned. Canada's last KL-7-encrypted message was sent on June 30, 1983, "after 27 years of service."

See also

• NSA encryption systems
• Typex

Sources

• Jerry Proc's page on the KL-7 (http://www.jproc.ca/crypto/kl7.html)
• Information collected on the KL-7 from various sources (http://www-users.cs.york.ac.uk/matthew/KL-7.html)