Anoraks Corner
by Tony Sale

Welcome to Anoraks Corner, a quick revision of the Lorenz machine, its physical and operational characteristics.

1. The physical characteristics.

The German Lorenz Company was asked by the German High Command to produce an absolutely secure method for enciphering communications between the top leaders of the German Forces.

The Lorenz engineers based their system on a method for enciphering teleprinter transmissions invented in 1918 in the USA by Gilbert Vernam. It was known as an additive cipher and worked by adding obscuring characters to the input plain language of the message to produce the cipher text to be transmitted to the intended recipient either by telephone land lines or by Radio. At the receiving end exactly the same obscuring characters added to the cipher characters cancelled out the original obscuring characters and revealed the plain language message.

Gilbert Vernam had proposed that the obscuring characters should be pre- punched onto teleprinter paper tape in a completely random sequence and consumed one by one to be added to the message text. This required that an exact copy of the obscuring characters tape would be at the receiving end would be exactly synchronised with the tape used at the sending end.

The Lorenz engineers decided that it would be operationally simpler to use a machine to generate the obscuring characters. The same machine at both ends of a link would then encipher and decipher the messages provided that both machines had been configured to exactly the same position at the start of sending and receiving a message.

To increase the security of the cipher, the Lorenz engineers also decided to add not one but two obscuring characters at the sending end, requiring the same two characters to be added back at the receiving end to reveal the message text.

The Lorenz machine wheels.

The Lorenz cipher machine used two sets of five wheels to generate the two five element obscuring characters. These wheels, which were all of different diameters were geared together to rotate as the message or cipher text was entered. Small cams round the peripheral of the wheels operated on switches to produce the five bit patterns of the obscuring characters.

But the cipher machines had to be able to handle very long messages of up to 20,000 characters. To ensure that the sequences of generated characters would not repeat in such long messages, the Lorenz engineers arranged that one of the sets of five wheels would index round intermittently thus extending the repetition period of the machine.

The resultant Lorenz cipher machine therefore had one set of five wheels which all indexed one position upon every incoming character. A second set of five wheels indexed round but intermittently under the control of two further wheels. The first of these two additional wheels indexed on every character input but the second of the two only indexed depending on the cam settings on the first wheel. The intermittently moving set of five wheels the only indexed depending on cams on this second control wheel.

The sets of wheels were fixed in axial position in the Lorenz machine. The security of the cipher, the key, was in two parts. Firstly there was the pattern of the cams round the periphery of each wheel, then there was the relative start positions of each wheel at the start of enciphering or deciphering. Since this was a relatively simple operation these wheel start positions could be changed for each message by using finger notches around the wheels. Changing cams was a more difficult job and most wheel cams were only changed infrequently. Each wheel had a different number of cams arround its periphery and these numbers varied from 23 to 61 with only mutually prime numbers being used, 501 cams in all.

The electrical connections to the Lorenz machine.

Electrically the Lorenz cipher machine operated as a cipher atatchment to a teleprinter. For transmitting a message, the serial electrical signal from a teleprinter went into the Lorenz, was converted from serial to five bit parallel then operated on by the wheel cam patterns, finally being converted back to serial form to be transmitted on by land line or Radio.

For receiving a ciphered message, the cipher text came from a Radio receiver or from a land line in serial form. It went into the Lorenz, was converted to five bit parallel form, had the obscuring characters stripped off by the wheel cams then was converted back to serial form to go into a teleprinter for printing of the deciphered message text.

This page was originally created by the late Tony Sale, the original curator of the Bletchley Park Museum,

Anoraks Corner by Tony Sale

Welcome to Anoraks Corner, a quick revision of the Lorenz machine, its physical and operational characteristics.

1. The physical characteristics.

The Lorenz machine wheels.

The electrical connections to the Lorenz machine.

Anoraks Corner
by Tony Sale