So last time I visited the Simon Singh’s Code Book was just as Charles Babbage was breaking Vigenére’s cipher by applying frequency analysis to the individually addressed letters in the key.
As Singh later describes, repetition is the enemy of security, and this holds true in pretty much all cases. It is the reason Babbage was able to break the Vigenére cipher and the reason, as we see later, that Allied cryptographers used to break the Enigma.
One way to make a perfect cipher is to prevent repetition, with for example a one-time pad of random keys. This is as Singh suggest, a “perfect cipher”, impregnable to any cryptanalysis. It is not easy to maintain though, and producing random one-time pads is very difficult.
So short cuts are taken, especially considering that around this time period (late 19th/early 20th century) the invention of the telegraph and radio means that more and more communication is being done out in the open, available for any to capture. As a result, cryptography becomes part of the mainstream as many members of the public try to encrypt their private correspondence to prevent prying eyes and ears from reading their messages.
All the meanwhile, government-run cryptanalysis centers beef up their staff and talent as politics gets heated. When war finally breaks out in Europe, cryptanalysis plays a large role in determining the outcome of the battles and the war as a whole.
During the war some really interesting insights are developed. Since this is the first time radio is used to communicate during wartime, analysts realize the power of traffic and signal analysis, both very interesting fields in of themselves. For example, analysts are able to determine the location of message senders through triangulation and are able to uniquely identify the message sender by analyzing how the message is being sent.
The real gem of cryptanalysis during the First World War comes from the British. British cryptanalysts at Room 40 had managed to completely decipher German communications at some point and were openly reading all of the messages being sent by German command. Singh doesn’t really go over the ciphers and how they were decrypted, but it appears that this had been done mostly through spycraft and intelligence gather rather than through cryptanalysis.
Nonetheless, the intelligence they gained was gold. At one point the German command had decided to renege on a deal that had kept the United States out of the war in order to ensure themselves a swift victory against the British. The British decrypted the message with the order to begin unrestricted U-Boat warfare as well as the invitation for Mexico to make war against the United States in order to keep America too busy to engage in Europe. With some slick spycraft, they were able to publicize the message without even letting the German’s suspect their cipher had been cracked, which in turn let them continue to read all communiqué unhindered.
Even after the war, the Germans didn’t know their cipher had been broken. Years after the war when it slowly became public knowledge that the British had been reading everything they had broadcasted, the Germans decided to take steps to protect themselves with a neat little machine developed by a German inventor named Arthur Scherbius, the Enigma Machine…
The next post will cover the Enigma Machine, the mechanism and the goliath task of cracking it.