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.
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
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
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.