Andrzej Dabrowa, Ph.D.
2. Historical background
The history of message encryption goes back to ancient Greece where messages
were written on a tape wound around a stick. When the tape was unwound, the
message could not be read. Since then many generals and diplomats have used
various means of encrypting messages, such as letter shifting or letter
replacement. However, these methods had a very limited number of combinations
and were fairly simple to decrypt. The first person to treat encryption
scientifically was an Italian, Gerolamo Cardiano (1501-1576), who formulated
encryption in terms of probabilities. The invention of telegraphy, and
especially radio which can be easily intercepted, forced diplomats and military
commanders to look for better methods of encrypting messages. These are based
upon the theory of probability, increasing the number of combinations by which
each letter could be represented. Ideally, encryption of a given letter should
never repeat!
First to patent a modern encrypting machine was a Dutchman H. A. Koch, but it
was never manufactured. At the same time a German national, A. Scherbius, was
working on an electromechanical encrypting machine. By repeated refinements and
the inclusion of Koch's ideas, Scherbius was able to produce a practical
machine. It consisted of five rotors: one fixed entry/exit rotor to connect the
keyboard to the machine, three moving rotors to perform the
encryption/decryption and one stationary rotor to reflect the electrical
signals back through the three moving rotors and the entry/exit rotor to the
display panel which displayed the encrypted/decrypted letters.
Each rotor had the
26 letters of the German alphabet and, as a particular letter was pressed on
the keyboard, the three rotors rotated to give a different encrypted letter
each time. The encrypting and decrypting processes depended upon the principle
of machine reciprocity. During encryption, keying an unencrypted letter
displayed the corresponding encrypted letter and, during decryption, keying the
encrypted letter displayed the corresponding decrypted letter. In addition to
the rotors the Enigma machine had a patch panel in which letters could be
patched around, further increasing the number of combinations by which a letter
could be represented. By changing rotor selection, rotor sequence, initial
rotor setting and the patch panel interconnections, the number of possible
combinations by which any given letter could be represented was raised to the
incredible number of 150,000,000,000,000,000,000!!! This meant that it
was practically impossible to decrypt such a message. For a better
understanding of the magnitude of the length of the code, assuming each
possible step in the code is represented by a length of one centimetre, it
would take light 167 years to travel the entire code length - a really
astronomical length!
Scherbius founded a small company to improve, produce and distribute a
commercial machine but by the time the machine was refined and patented, in
1929, he had died. Initially the Enigma was offered on the open market and
France, Britain, the United States and Poland each bought one or two to
investigate the new technology.
The German military planners realized that the next war would be highly mobile,
covering large areas. This required rapid and secure means of communication
between the command and field units. The appearance of the Enigma machine, in
the twenties, and the rapid development of radio technology, gave Germany the
means to plan modern warfare tactics which became known as
"Blitzkrieg".
Operation of the machine was fairly simple. It required identical machines with identically wired rotors, identical rotor selection, identical rotor setting, and identical patch panel interconnections. Initially this information was provided in quarterly or monthly bulletins. As long as some of these Enigma parameters were kept constant, the cryptologists had an opportunity to decrypt messages - assuming that a sufficient number of intercepts were available. Later, to achieve even higher security, the Germans changed the machine settings daily and then with each new message. However, frequent changes in the machine set up made it necessary to transmit this information to the recipients. The chosen method was to send this information twice ahead of the main message, first in plain text and again in encrypted text. This turned out to be one of their greatest mistakes since it was quickly spotted by the Polish cryptologists and gave them an additional means to reduce the number of possible solutions.
By that time Polish
cryptologists had developed an electromechanical device consisting of two
Enigmas which could go rapidly through large number of combinations to find the
proper solution. It was called "Cyclometer".
The Enigma machine, after various improvements, was first tested and adopted by
the German Navy in 1926 then by the Army in 1928 and by the Air Force in 1935.
It was also used by other government departments. Depending on the service and
the need for high security, various refinements were made throughout the war,
the highest degree of sophistication being achieved by the Naval Enigma. The
total number of Enigma machines built was estimated to be between 30,000 and
100,000 units.
(c) 2003 A R Dabrowa