Vigenère’s cipher: Crypto gets serious

I’ve been reading Simon Singh’s Code Book which reads as part cryptography lesson and part historical thriller. The last time I updated we read about the cipher of Mary Queen of Scots and how it was broken through excellent spy craft and cryptanalysis. Even though most us mere mortals wouldn’t be able to break her cipher, to a trained analyst like those who served Queen Elizabeth and Walsingham her spy master, the cipher was easily broken.

Mary was at the time under house arrest in England, watched over by a stern guardian who monitored all of her communication. As Singh tells the story, Mary thought herself forgotten by the world until a young catholic named Gilbert Gifford offered his services as a courier between Mary and a young radical named Babington. It turns out that Gifford was in fact a double agent who was asked by Walsingham (Queen Elizabeth’s spymaster) to become the courier. Needless to say during the entire time he was ferreting letters back and forth between Babington and Mary, he was letting Walsingham copy the letters character by character, giving Walsingham and his cryptanalysts time to decipher them, all the while leaving Babington and Mary to believe they were communicating securely. These encrypted letters which later decrypted by Walsingham, condemn Mary Queen of Scots to death for her role in the Babington Plot against Queen Elizabeth.

Now what Singh fails to mention is of critical importance here. Singh fails to mention how Babington and Mary, who had never met and as we know never really had a secure channel of communications, were able to coordinate their initial cipher. The challenge nowadays in cryptography is that initial handshake, how did Mary and Babington manage to overcome this problem?

Around the same time another cipher was developed that Singh suggests would have saved Mary’s life had she used it, the Vigenère cipher. Vigenére was a mathematician and cryptographer who developed a new cipher that used more than one caesar cipher alphabet. According to Singh, no one really thought to use the cipher until decades later, but the polyalphabetic cipher were considerably more difficult to crack than the monoalphabetic cipher. The problem that monoalphabetic ciphers like Mary’s had was that they were susceptible to frequency analysis. Polyalphabetic ciphers were also susceptible as Charles Babbage later proves.

Babbage is more famously known for his ingenious designs that predicted the computer over a century before the first transistor was invented at Bell Labs. Babbage saw the Vigenére cipher as a challenge and used mathematics and statistics, much like before the earlier cryptanalysts did when using frequency analysis against monoalphabetic ciphers, but this time around the mathematics gets considerably more complicated.

As we can see from Babbage and the Vigenére cipher, math is becoming more and more important in cryptography. Babbage’s Victorian-era story, the story of the “father of computing” tackling a challenging puzzle of mathematics and linguistics only to break yet another cryptographic protocol foreshadows our contemporary story of hackers and cryptopunks constantly tweaking away at the systems that secure the internet and our data.

As always, I’ll have more soon. Stay tooned!