The History and Mathematics of Codes and Code Breaking

Tag: Vigenère

Is cryptanalysis really as easy as Singh makes it seem?

As Singh deciphered the example of the Vigènere cipher on page 116, and also other ciphers previously, I contemplated just how simple he was making them. He makes a lot of assumptions, and he also never points out some flaws that I have seen in his messages. In the example on page 16, Singh uses a message that makes his technique work very well. In this example, he uses a keyphrase that is as long as the message. Normally, this should be almost impossible to crack, because none of the cipher alphabets would be repeated in a pattern. He proposes a solution, by placing common words (he uses “the”) in random locations in the plaintext. In his example, he gets it right on the first try. This is not that unlikely with such a short message, but a full paragraph of a long letter would take many more tries. He also makes the assumption that the cryptographer encrypting the message would use the word “the”, or “and”, or whatever word. If a cryptographer knew their code could be broken that way, they could simply refrain from using common words often. Once there are fewer common words present,  it becomes much more difficult to crack. In addition, using the method he proposed can cause false positives. It’s possible that the letters “the” in the plaintext produce a discernible string of three letters in the ciphertext. If the cryptographer was smart, they could place a few traps, so that random keywords would show up in the cipher text. This would completely confuse the person deciphering the code, and may just make it extremely difficult to crack. Singh fails to address these flaws in his examples, and it makes it cryptanalysis seem easier than it really is.

The Race Between Cryptanalysis and Encryption

The status quo of cryptography can be accurately represented by a game of tennis between two equally good players. When a strong cipher is developed, the ball moves to cryptanalysis. Upon development of better decryption techniques, the ball returns back to the court of the encryptors. The period in which an event happens in the world of cryptography is heavily influenced by who has the power between cryptanalysis and encryption.

During the time of Mary Queen of Scots, the users of cryptography had little to no faith in the abilities to decrypt, causing them to have  undue faith on their abilities to encrypt. By not giving sufficient credit to cryptanalysis, they did not bother with either reinforcing the difficulty of the cipher  or any sort of counter measures in case the cipher was broken, leaving them in a worse position had they chosen not to encrypt. On the other hand, the situation before the Vigenère cipher was the exact opposite as the strength of any cipher was presumed to be weak. Encryptors were motivated to fortify their ciphers and even after encryption, they would communicate in  ways that would seem senseless without context. Some would even avoid cryptography altogether and find other ways to convey the desired message.

I also believe that during the period of Mary Queen of Scots, cryptography itself was fairly new and unheard of. This meant that almost no one had any idea how to encrypt (and naturally, decrypt) ciphers. After cryptography became more popular, more people explored the avenue and cipher breaking became more ubiquitous. This was another reason for encryptors to strive to strengthen their ciphers.

I Don’t Really Think There Was Difference

What was the difference?

There was none! The environment that Mary Queen of Scots experienced is quite the same compared to the environment leading up to the development of the Vigenère cipher. In fact, it even states in the book that Vigenère’s publication of Traicté des chiffres happened “ironically… [in] the same year that Thomas Phelippes was breaking the cipher of Mary Queen of Scots.”

Even Mary herself was aware of how insecure the code was. It was just that (1) she could not come up with a better method of encryption and (2) she never questioned if her beloved messenger was a double-agent for Thomas Phelippes which he in fact was.

All of this just goes to show how much of an edge the cryptanalyst had over the cryptographers. No matter how much variation the cryptographers tried to invest into their monoalphabetic, the cryptanalyst could break them easily with one tool — frequency analysis.

However, this was only until Vigenère took the works of three other men to create a cipher that gave the edge back to cryptographers. With it, cryptanalyst would be left to sit hunch-back over their desk for years. It is only a shame however that his amazing cipher would not be appreciated until another two hundred years.

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