I don’t intend the material in this course to be a textbook, and, frankly speaking, I am not going to take you through the entire history and methodology, except for encryption. I can’t help it because first, this is an interesting topic, and second, it’s rather useful as there are still people out there who believe that by substituting letters or words they are securely encrypting their information.
Already in BC the then emperor Gaius Julius Caesar, when writing an important message, used a substitution cipher where he replaced each letter in the plaintext by a different letter some fixed number of positions down the alphabet with a left or right shift of three symbols. If his enemies intercepted the encrypted message, they couldn’t read it, probably seeing it as a foreign language. This method of encryption is named a “Caesar cipher” and belongs to the group of substitution ciphers that rely on symbol replacement.
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To illustrate how Caesar cipher works, take a look at the alphabet below. The first line is the alphabet as it is, and the second line – the transformed version with a rotation of three symbols.
a b c d e f g h i j k l m n o p q r s t u v w x y z
x y z a b c d e f g h i j k l m n o p q r s t u v w
For instance, the word anonymitywill be transformed into bopojnjuz.This method of encryption is very easily decrypted by a quick check of all possible combinations. Already in A.D. Caesar cipher was regarded unreliable. It was superseded by more sophisticated substitution methods where letters were replaced with symbols.
Come to think of it, how do you break a cipher where letters are replaced by unknown symbols? Check all possible combinations? But if you replaced popular words with one symbol in your encrypted messages, you left no chance to substitution.
This method was used by Mary Stuart imprisoned in Sheffield Castle in her correspondence with Anthony Babington who conspired and plotted the assassination of Elizabeth I of England. Here’s how their cipher looked like.
However, Elizabeth’s secret service led by Francis Walsingham intercepted the letter, and her best cryptanalyst Thomas Phelippes easily decrypted it. How did he manage it? He employed frequency analysis.
All letters in a language occur with varying frequencies. For instance, in the Russian language the occurrence of the letter “o” is 11% of all the letters, that of the letter “r” – around 5%, and of the letter “f” – only 0,26%.
Therefore, you simply work out the frequency percentage for the occurrences of the symbols in the text, and it makes it possible to suggest which letter this or that symbol replaces. Then the process requires some time for substitution and testing of the assumptions. That basically sums up what frequency analysis is. It can be applied only to relatively long texts, and the longer the text, the more effective it proves.
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Anthony Babington was eventually hanged, drawn and quartered. Maria Stuart was beheaded, and the encryption method that used letter substitution since then has been seen as ineffective. However, the frequency analysis was quickly overcome. It is enough to use several ways of encryption: encrypt one line with one method, another line – with another one, and the frequency analysis will be useless.
Since that time there has been a constant race between devising encryption and cracking ciphers. The most prominent success of cracking encryption algorithms is the breaking of the Enigma machine used by Nazi Germany for protecting its secret wartime communication codes. Enigma is considered a perfect encryption machine at the time that was designed by Germany’s most talented specialists.
But it took just as much talent to break the Enigma Code: an entire team of British cryptologists headed by a young scientist Alan Turing relentlessly worked to crack it. Despite the mystery around Turing, he is said to devise a means for obtaining the key for breaking Enigma. And his breakthrough was prompted by an ordinary Nazi salute that was mandatory for use at the end of every encrypted message.
Alan Turing made the impossible by providing his country with an invaluable advantage in the Second World War. In return he was “rewarded” by a chemical castration homosexuals in Great Britain were subject to at the time. That led Turing to end his life.