You’ll remember how, as children, we sent what we fondly believed to be securely encrypted messages to our friends. The trick was using numbers or symbols in place of letters. Thus, “Dear Joe nice to have seen you yesterday. Are you getting a present for Anne?” could be written something like this: “23 11 4 7 15 19 11 3 22 8 11 14 19 55 4 66 11,” and so on.
We quickly learned how to decode this kind of message. In most English writing, the letter ‘e’ occurs far more frequently than, say, the letters ‘p’ or ‘y’. In my example above, out of 61 letters, the letter ‘e’ shows up a disproportionate 13 times. By substituting an ‘e’ for the most frequently occurring symbol, the message starts to reveal its meaning. The next most frequently found letter in English is T, followed by A. With a few more substitutions the message can be decoded. (more…)