Where this idea comes from
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.