SOLUTION to Challenge 13: Fractionating

Congratulations to Euchre Mutt for being the first person to solve Challenge 13!  It remained unsolved for 45 days, which means he is awarded 45 points on the leaderboard.

If you haven’t tried your hand at solving it, read this first.

Here’s original description and ciphertext:

Today, I wanted to learn about ciphers that use fractionation. I decided on a method created by a French amateur cryptographer who spent his days in working in a customs warehouse.

Three hints:

  1. J=I
  2. You need to figure out the keyword and the period.
  3. “Now Morn her rosie steps in th’ Eastern Clime, Advancing, sow’d the earth with Orient Pearle.”

And the ciphertext is:

xmwuv hnmhv hderq
tbkht newsv hnmhv
fpeks ngifq rlqv


The solution:

This puzzle uses the bifid cipher, invented by Félix Delastelle around 1901. Delastelle was a bonded warehouseman by day and an amateur cryptographer by night. The bifid cipher uses a numbered grid in a very clever way to fractionate the plaintext, making it harder to crack.

The third clue is a line from Milton’s Paradise Lost.  That should help you figure out that the keyword is MILTON. Incidentally, this is the opening line from Book 5, which should lead you to a block length of 5.

The decoded text reads:  “Who overcomes by force, hath overcome but half his foe, ” another quote from Paradise Lost.

You can use this online Biphid Decoder to test all this out.


With CodeAWeek.com, I hope to release one cipher, puzzle, or mystery each week. Anyone can attempt to solve. The winner is the first person to send a correct solution and a description of the solve method to codemaster@codeaweek.com. Once a correct solution is e-mailed, I will publish a follow-up post, congratulating the winner and revealing the secrets of the code.

You may post questions or theories in the comments, but DO NOT POST SOLUTIONS. E-mail them to codemaster@codeaweek.com.

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