Andrzej Dabrowa, Ph.D.
3. Breaking the Enigma Code
To understand the chronology of breaking the Enigma code we have to go to 1920 Poland.
After regaining independence following 123 years of German and Russian occupation, Poland was attacked in 1920 by an overwhelming Soviet army. The Polish military commander, Marshal J. Pilsudski, was able to hold the attack at Warsaw and then turn it back. This was due, to a large extent, to the Polish ability to intercept orders from the Soviet headquarters and quickly relay the contents to the Polish field commanders. This victory sealed the importance of intelligence in the Polish Army at the highest level.
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| Marshal Pilsudski | Colonel Langer | Major Ciezcki |
In 1926 German intelligence was able to intercept and decrypt about 6000
messages between headquarters and military airfields in Poland. This caused the
Polish high command to initiate steps to form a modern cryptoanalysis unit with
Colonel Guido Langer in charge and Major Maksymilian Ciezki as his deputy. The
first step was to establish a cryptoanalysis course at Poznan University, which
had a very good mathematical department. After several weeks into the course,
students were given encrypted German messages which, though already decrypted,
were considered very difficult to break. Several students were able to break
the code and amongst them were M. Rejewski, J. Rozycki and H. Zygalski, who
later became the nucleus of the modern Polish cryptology team.
After completing their studies the three were engaged on September 1st, 1932 to
work in the bureau of cryptology at the Army Headquarters in Warsaw. Initially
they were occupied in decrypting pre-Enigma coded messages using standard
statistical and linguistic methods. Since all of them were fluent in German,
they were remarkably successful.
In January 1929 Polish customs intercepted a package sent from Germany to a
German company in Warsaw. The recipients and the German consul were very
anxious, insisting that the package should be immediately returned to Germany.
Customs alerted Polish intelligence which, with the assistance of cryptologists
and taking the utmost care not to be spotted by German surveillance, opened the
package. They realized that it was an unknown crypto machine and took
photographs and made drawings for future use. The machine was repackaged with
the utmost care so that the Germans would not suspect that the package had been
opened. A report was made but, since nobody knew what the machine was or how to
use it, nothing further was done.
Later in the year French
intelligence officer Capt. G. Bertrand learned, from an independent
intelligence source that the German Navy had been introducing a new encrypting
machine called Enigma since 1926 and that more and more German messages could
not be decrypted. Initially French intelligence would not believe that secure
encryption was possible using a commercially available machine. However, Capt.
Bertrand realized that a military version would have additional elements,
refinements and procedures to make it unbreakable.
In the fall of 1931, a German national who worked in a German cryptology
department approached French intelligence with an offer to sell information
relating to military intelligence and cryptology. After an extensive and
independent verification of his background Hans-Thilo Schmidt, a younger
brother of an army general, was engaged and given the code name
"Asche". He was able to confirm that the Germans were rapidly
introducing an encrypting machine called Enigma and provided some construction
details. Using this information, the Polish cryptologists were able to
reconstruct the wiring of the Enigma rotors. Other important information were
the operational instructions and some keys for selecting rotors and their
positions. Furthermore, it was learned that the Germans were resetting rotor
positions every three months and the patch-panel daily. This allowed the Polish
cryptology team to sort German coded messages into quarterly and daily
groupings for analysis.
On December 7th, 1934, Capt. Bertrand arrived in Warsaw to exchange information
about the Enigma machine. On comparing his information with the earlier
interception of an encrypting machine in the Polish mail, it was concluded that
they were the same. On the basis of that conclusion it was decided to buy a
commercial version and to start reconstructing the military version. A small
unobtrusive company, AVA, was entrusted to make the copies since a small
commercial electrical component manufacturer would not arouse the interest of
German intelligence. On the second visit Capt. Bertrand brought tables of rotor
settings for the months of September and November, further reducing the number
of encryption combinations that had to be considered.
Mr. H. Zygalski developed his own method, based on perforated sheets, and
the Poles were then able to decrypt a substantial number of Enigma coded German
messages.
It is essential to understand the distinction between the machine set up and
the encrypting/decrypting processes. The machine set up consists of selecting
rotors, rotor sequence, rotor positions and patch panel interconnect. This
places the message to be encrypted in a particular range of an extremely long
encryption chain, which could be billions and billions of steps.
The encryption process may be very complex and consist of a very long combinational sequence but it still follows an established pattern defined by the mechanical and electrical construction of Enigma. Once the Enigma machine set up is known, the decryption process is simple and fast, but that was hardly the case throughout the war. In the majority of cases the machine set up had to be obtained by painstaking analyses of encrypted messages that contained the names of the sender and receiver, rank, address, as well as machine set up and peculiarities associated with a particular operator and organisation. These standard contents substantially increased the probability of a correct guess. Comparison with the previously decrypted messages, kept in a large library, gave an appreciable number of possible solutions.
All these bits of information were fed into "Bomba" until the proper solution was found. It must be emphasised that in order to have a reasonable chance of success a substantial number of messages with the same machine set-up were required for analysis. The solution time depended upon how many bits of information were collected and could vary from hours, through days, to weeks.
The decryption processes that were developed by Rejewski, Rozycki and Zygalski provided a very strong foundation for the successful Enigma code breaking at Bletchley Park. Furthermore, machines much more powerful than "Bomba" were built, such as "Colossus" which could rapidly check millions of combinations. This round the clock operation was supported by a staff of more than 10,000 people. The mathematical analysis developed by the Poles was omitted because of its complexity. For the mathematical treatment see the references.
(c) 2003 A R Dabrowa