Breaking Caesar’s Cipher (part III)

In the last installment of this series, we looked at Markov chains as a mean of estimating the likelihood of a given piece of text of actually being a message, written in English, rather than mere gibberish.

This week, we finally piece everything together to obtain a program to crack Caesar’s cipher without (much) human intervention.

Breaking Caesar’s Cipher (Caesar’s Cipher, part II)

In the last installment of this series, we had a look at Caesar’s cipher, an absurdly simple encryption technique where the symmetric encryption only consists in shifting symbols places.

While it’s ridiculously easy to break the cipher, even with pen-and-paper techniques, we ended up, last time, surmising that we should be able to crack the cipher automatically, without human intervention, if only we had a reasonable language model. This week, let us have a look at how we could build a very simple language model that does just that.

Caesar’s Cipher

Julius Caesar, presumably sometimes during the war in Gaul, according to Suetonius, used a simple cipher to ensure the privacy of his communications.

Caesar’s method can hardly be considered anything close to secure, but it’s still worthwhile to have a look at how you can implement it, and break it, mostly because it’s one of the simplest substitution ciphers.

Suggested Reading: Cryptography: The science of secret writing

Laurence Dwight Smith —Cryptography: The science of secret writing — Dover, 1943, 164 pp. ISBN 0-486-20247-X

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