The KL-7, codenamed ADONIS or POLLUX, is an off-line rotor cipher machine that was developed in the late 1940's by the American National Security Agency (NSA) as a successor of the SIGABA. The machine entered service in 1952. ADONIS and POLLUX were two different encryption procedures for the KL-7. The American ADONIS procedure applied an encrypted message key to pre-set the initial start position of the rotors, whereas the ‘export version’ POLLUX procedure used far less secure non-encrypted message keys. The KL-7 was compromised by John Walker who sold technical information and key lists to the Soviets. The KL-7 was used by the US and many of its Allies and retired in 1983.

Output of the KL-7 was printed on a paper ribbon and some versions had a paper puncher for 5-bit code output. The KL-7 has eight rotors (the fourth from the left was stationary) with 36 contacts each. During its service time, the rotors were recalled and re-wired regularly. The rotors are placed in a rotor cage called KLK-7 which can be removed from the machine. Each rotor has an exchangeable plastic outer ring with cams. Switches, controlled by these cams, engage electromagnets which in turn enable the motor to step certain rotors. This resulted in a highly irregular stepping of the rotors.

The 26 inputs and outputs of the rotor cage are used to encrypt the letters. The 10 remaining inputs and outputs are looped back through the rotors, resulting in a very complex signal path for the 26 letters. The machine was non-reciprocal. This was achieved by a sliding permuter board underneath the keyboard which swapped all input and output contacts of the rotor cage. Details about rotor and stepping unit wiring are still classified. Today, all publicly availably machines, such as this machine from the Royal Dutch Signals Museum, are carefully sanitized and stripped of any wiring, related to the rotors and stepping unit.