El papel de las instituciones educativas en el proceso de hospitalización

Until 1925 the rest of Europe were still receiving a large amount of intelligence from Germany via decrypted transmissions. After the German adoption of the Enigma machine this rapidly stopped. Previously British and French cryptanalysis had been tenacious in their efforts to decipher previously unbreakable codes, but when faced with Enigma they quickly gave up. After WW1 Germany’s military was had been largely neutralised and the country was in ruins. The French no longer feared her might. On the other side of Germany, her neighbours weren’t as complacent. In 1925

Poland was caught between a strengthened Germany and Russia, a nation bent on spreading communism. Faced with these threats Poland was desperate for intelligence and had a very strong cryptanalysis department called Biuro Szyfrów. There was little that they could do without first understanding the workings of the cipher.

On the 8th of November they got their first break. A German working in the department responsible for secure communication, Hans-Thilo Schmidt, sold the plans to the Enigma machine to a French agent for 20,000 Marks. The French were not particularly interested in their new found knowledge, they assumed that even if they understood how the Enigma worked they would still not be able to work out a way to break the cipher. They did however have a decade old treaty of military cooperation with Poland, who had expressed interest in anything to do with Enigma. Thinking it of little practical value the French gave the information to Poland. Using this information the Biuro was able to create a replica of the Enigma machine to study. As well as details of the Enigma machine the French intelligence contained the protocol that the Germans were using. Codebooks were distributed amongst the German radio operators. The books contained a month’s worth of plug-board settings and scrambler arrangement and orientations, one for each day, called day-keys. To make the system more secure messages were encrypted with different scrambler orientation settings. This was called the message-key and was encrypted twice with the day-key and transmitted at the start of the message. This was done to ensure that the message-key was received correctly and the message could be decoded without error, but it introduced an insecurity into the system as the attacker knew that the 1st plaintext character of the message was the same as the 4th although the ciphertext characters would be different. The difference between them would be determined by the scrambler settings.

This was studied by the Polish cryptanalyst Marian Rejewski. He had at his disposal hundreds of messages every day, the first six characters of each of which would be encrypted using the same settings. Although he did not know the plaintext characters he studied the way they changed, finding they formed chains with varying numbers of links. For example, if the first character of one message was ‘L’ and the fourth was ‘W’, in another message the first would be ‘W’ and the forth ‘G’ and then in a third ‘G’ would change back to ‘L’, forming a chain with three links. Rejewski realised that properties of these chains would be affected only by the scramblers and

not by the plug-board settings, they would only change the values individual letters in the chain. This meant that the chains could act as a fingerprint for the different scrambler settings. He spent a year cataloguing the chain lengths for all of the 105,456 possible scrambler arrangements, from this he would be able to determine the scrambler settings of the day-key, he could then use this to try and decrypt a message key and use that to decrypt a message, if the plug-board settings did not affect any of the letters in the message-key it would be possible to mostly decrypt the message. The plug-board setting could then be determined by looking at the generated message and changing letters until it made sense. Using this technique Rejewski could retrieve a day-key and read all of that day’s messages.

When the Germans adapted the way they transmitted messages it made Rejewski’s catalogue of chains obsolete. Instead of painstakingly recreating it he developed a mechanical device, based on an enigma machine, which was capable of trying lots of different scrambler settings until it spotted the correct one. As the scramblers could be arranged in six different ways six of the so called bombes were required. In December 1938 the Germans augmented the security of Enigma, increasing the number of different scramblers to five and the number of plug-board cables to ten. This vastly increased the number of possible plug-board permutations and increased the number of bombes required by a factor of ten. The cost of manufacturing the new bombes was beyond the resources of the Polish cryptographic department, and in 1939 the flow of German intelligence into Poland dried up. Sensing an imminent German invasion the Polish were willing to share their cryptanalysis breakthroughs with their allies. On the 24th of July senior cryptanalysts from France and Britain arrived in Poland where they were informed, to their surprise, of the Polish successes in reading secure German messages. Spare Enigma machines and blueprints for the bombes were shipped to London and Paris where the Polish work could continue. On the 1st of September Hitler invaded Poland and WW2 had begun.

In Britain the responsibility for breaking German codes had moved from Room 40 to Bletchley Park, a large Victorian mansion in Buckinghamshire. The British cryptanalysts quickly mastered the polish techniques and with greater resources had created the bombes necessary to break the encryption. The Polish technique hinged on the fact that the Germans always transmitted the message-key twice at the start of the

message, this repetition was the weakness that allowed the cryptanalysts to peer inside the Enigma code. Some cryptanalysts at Bletchley Park were responsible for continuing the research into weaknesses in the code, in case the Germans strengthened their transmission protocol and stopped sending the key twice. One of the researchers was Alan Turing, he realised that there was another potential avenue for attack due to the fact that Enigma was being used by the military. The military thrive on routine, the contents of some parts of the messages would be predictable, for example, a weather report would be transmitted shortly after 6 am every day. The section of plaintext that was known to the attackers was referred to as a crib. Using these cribs Turing developed a new technique for decoding Enigma messages. He also studied chains in encipherment of various letters, for example if a ‘w’ was ciphered as an ‘e’ and the next plaintext letter was ‘e’ that had been changed to a ‘t’ and later on in the message there was a plaintext ‘t’ that was converted to a ‘w’, this was the type of chain Turing was interested in. Turing imagined a machine that was a series of Enigma machines in parallel. Details of a chain would be entered into it by connecting the output of the first one to the input of the second and so on. In between the input of the first and the output of the third there was a lamp. The scrambler settings on the three machines would rotate. The lamp would only light when the circuit was complete, this would only happen when the scrambler settings were correct for all three machines. Again the study of these chains allowed the cryptanalysts to divorce the plug-board settings from the scramblers. This is because the plug-board settings are constant, although in the example chain above it is not known what letter the first ‘w’ is converted to by the plug-board, it is known that when the ‘t’ is converted to a ‘w’ the signal has travelled through the plug-board cable twice, cancelling out the effect. While the plug-board contributes the majority of the different combinations of settings an Enigma machine can have, it is only a mono-alphabetic substitution, and, as there were only ten plug-board cables, an incomplete one at that. By decoding the original message with the scrambler settings the plug-board settings can soon be determined. Turing’s decoding machine was built; it arrived on the 14th of March 1940 but was a lot slower than anticipated. The design was refined and a new one was ordered, but it was going to take four months to build. On the 10th of March 1940 the Germans changed their key transmission protocol so that the key was only sent once and the decoded Enigma messages dried up until the 8th of August when the new

machine arrived. This fulfilled all of Turing’s hopes and messages could be decoded for the rest of the war.

The information that was gained by cracking the Enigma code was of crucial value to the wartime effort of the allies. Lessons can be learnt for both cryptanalysts and the users of cryptographic algorithms. The Polish code-breakers, motivated by desperation, never gave up hope that Enigma could be broken, and through their tenacity found a technique that could retrieve German Keys. The real weakness in the enigma code wasn’t the code itself but the way it was used. The first method of breaking it used the fact that the message-key was enciphered twice and the second method relied on knowing sections of the plaintext. While the cipher isn’t secure by today’s standards, as it is important to assume that an attacker may have access to plaintexts as well as ciphertexts, any code can be made much weaker by using it improperly. It is important to minimise any additional information that is leaked to any potential attacker.