Sunday, May 17, 2009

CLRTEOTSHTO!


Just finished reading The science of secrecy: the history of codebreaking by Simon Singh. I've read other works by this author and I think that he tends to lean a little too much towards the popular side of popular science - references and bibliographies are not his strong point - but his approach is tolerable in this instance. Written as a companion to a BBC series, Singh presents a (very) general overview of the history of cryptography.

He starts with the Babington Plot and the execution of Mary Queen of Scots. While Elizabeth the First was clearly a piece of work, I've always considered Mary to be suspiciously goody-two-shoes. (The story of her little lap dog that hid in her skirts as she faced the executioner, really it's just too twee. I would have cut her head off too.) Cryptography wise, it's not that interesting. At that time they mainly used a Caesar ciphers - a monoalphabetic substitution cipher where one letter of the alphabet (the cipher alphabet) represents another, according to a key. This method succumbed to frequency analysis. The characters e, t and a are the most common letters used in English. All you need to do to break the code is count how often certain letters appear and compare it to a table of frequency. You'll then be able to make a pretty good guess of what that character that letter may represent. Check out the title of this entry!

They then went on to use polyalphabetic cipher, a substitution cipher in which the cipher alphabet changes during the encryption. This was known as the Vigenère cipher - the 'unbreakable cipher'! Which was fine for a few centuries. Until it was broken. And it was broken by one Charles Babbage. The father of computing! The designer of the first programmable computer! (Except he never actually made a computer. After 10 years twiddling about he scrapped his plans and started on a new model. Not unsurprisingly his financial backers lost faith and his design remained incomplete - until 1991, when the Science Museum in London completed his plans.It worked perfectly.) Babbage's main contribution to science and mathematics was as a highly regarded cryptographer. In this he displayed a tendency to dilletantism typical of these code breakers. The story of cryptography is the story of people getting distracted, not bothering to publish their discoveries and generally wandering off to do other stuff.

Anyway, Babbage broke the unbreakable. My favourite story is about his correspondence with the poet Alfred Tennyson, regarding the poem "The vision of sin". This poem ends with the lines: Every moment dies a man, / Every moment one is born. To which Babbage objected:
It must be manifest that if this were true, the population of the world would be at a standstill... I would suggest that in the next edition of your poem you have it read - 'Every moment dies a man, Every moment 1 1/16th is born.'... The actual figure is so long I cannot get it onto a line, but I believe the figure 1 1/16th will be sufficiently accurate for poetry.
I am, Sir, yours, etc.,
Charles Babbage

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