John Herivel - Operator Error

Operator Error

At the start of each day, the operator would set the "ring settings" (German: Ringstellung) on the Enigma rotors; that is, the position of the ring of letters (or numbers) around the rotor. The ring settings were taken from a codebook, but changed daily, and had to be altered at the start of each day before any messages could be sent. The ring settings could be adjusted before or after inserting the rotors into the machine. Herivel assumed that at least some of the operators would adjust them after. In the normal course of things, adjusting the rotors inside the machine would likely leave the correct ring setting at the top, or near the top, of the rotors.

Furthermore, for each message, the sending operator would follow a standard procedure. He would first select a starting position for the rotors, the ground setting (German: Grundstellung): GKX, for example. He would then use Enigma with the rotors set to GKX to encrypt a second starting position, the message setting, which he might choose to be RTQ; RTQ might encrypt to LLP. (Before May 1940 the message setting would be repeated then encrypted, but this makes no difference to Herivel's observation.) The operator would then turn his rotors to RTQ and encrypt the actual message. Included in the preamble to the message, unencrypted, would be the ground setting (GKX) as well as the encrypted message setting (LLP). A receiving Enigma operator could use this information to recover the message setting and then decrypt the message.

The ground setting (GKX in the above example) should have been chosen at random, but Herivel reasoned that if an operator were lazy, or in a hurry, or otherwise under pressure, he might simply use whatever rotor setting was currently showing on the machine. If this was the first message of the day, and the operator had set the ring settings with the rotors already inside the machine, then the rotor position currently showing on the machine could well be the ring setting itself, or else very close to it. (If this situation occurred in the above example, then GKX would be the ring setting, or very close to it). Moreover, the ground setting was sent unencrypted in the preamble to the message, which could then be easily spotted by the codebreakers at Bletchley Park.