Where this idea comes from
The cipher wheel
The Roman writer Suetonius says Julius Caesar wrote private letters with each letter moved three places along the alphabet. Around 1467 the Italian architect Leon Battista Alberti described a cipher disk — two rings of letters, the inner one turning — in a short book, De componendis cifris.
One thing to hold: Two discs of card or metal, one inside the other, pinned through the middle.
Why it matters: Alberti turned his disk part-way through a message, so one letter could be written many different ways. That idea shaped secret writing for the next 400 years.
What we don’t know: None of Caesar’s own coded letters survive; we know of his shift only because Suetonius wrote it down, about 150 years later.
Finding the keyword’s length (Kasiski’s trick)
In 1863 Friedrich Kasiski published a book in Berlin, Die Geheimschriften und die Dechiffrir-Kunst (Secret Writing and the Art of Deciphering). It showed how to break the Vigenère cipher, which uses a keyword to shift each letter by a different amount.
One thing to hold: A long coded message with the same three-letter group repeated in it, and a ruler to count the gaps.
Why it matters: When a common word lines up with the same part of the keyword twice, the same code letters appear twice. The gaps between those repeats are multiples of the keyword’s length. Once you know the length, the code splits into a few simple shift ciphers, each cracked by counting letters.
What we don’t know: The English mathematician Charles Babbage seems to have found the same idea on his own as early as 1846. Exactly how he worked it out is not known.
Dates:
- 1846 Babbage seems to find the trick, as early as this year.
- 1863 Kasiski publishes it.