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. There was an adaptor available, the HL-1/X22, 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 for 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 a certain offset. The ring and the offset to use for each position were 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 basket and set of rotors, 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 keyboard itself was a large sliding switch, also called permutor board. A signal, coming from a letter key, went through the rotors, back to the permutor board to continue to the printer. The KL-7 was non-reciprocal. Therefore, depending on the Encipher or Decipher position of the permutor board, the direction of the signal through the rotors was changed.
The rotor basket had two sets of connectors, two with 26 pins and two with 10 pins, at each end that mated with the main assembly. Both 26 pin connectors were connected to the keyboard to enable the switching of the signal direction through the rotors. Both 10 pin connectors on each side were hard-wired with each other. If a signal that entered on one of the 26 pins left the rotor pack on one of these 10 pins, that signal was redirected back into the rotors on the entry side to perform a new pass through the rotors. This loop-back, the so-called re-entry, created complex scrambling of the signal and could result in multiple passes through the rotor pack, depending on the current state of the rotor wiring.
There was also a switch pile-up under each movable rotor that was operated by cams on its plastic outer ring. Different outer rings had different arrangements of cams. The circuitry of the switches controlled solenoids which in turn enabled the movement of the rotors. The combination of cam rings and the controlling of a rotor by several switches created a most complex and irregular stepping. The exact wiring between switches and solenoids is still classified.
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. The KL-7 was withdrawn from service in June 1983, and Canada's last KL-7-encrypted message was sent on June 30, 1983, "after 27 years of service."
The successor to the KL-7 was the KL-51, an off-line, paper tape encryption system that used digital electronics instead of rotors.
- Jerry Proc's page on the KL-7, retrieved 2012-08-28.
- NSA Crypto Almanac 50th Anniversary - The development of the AFSAM-7, retrieved 27 February 2011.
- Technical details and history of the TSEC/KL-7, from Dirk Rijmenants' Cipher Machines & Cryptology, retrieved 27 February 2011.
- Patent for Rotor Re-entry by Albert W Small, filed 1944 from Free Patents On-line, retrieved 27 February 2011.
- "Cryptology", Encyclopædia Britannica. Retrieved 22 June 2005 from Encyclopædia Britannica Online [dead link].
- Card attached to KL-51 on display at the National Cryptologic Museum, 2005.
- TSEC/KL-7 with detailed information and many