This fact gives us a way to think about the order of a columnar transposition cipher: (1) Determine its cycle decomposition, and then (2) find the LCM of the cycle lengths. Let’s briefly begin ...

To decrypt a double transposition, construct a block with the right number of rows under the keyword, blocking off the short columns. Write the cipher in by columns, and read it out by rows.

It’s Math Monday! In this third post in a series on columnar transposition ciphers, we look at a formula for the underlying permutation of a CTC and specialize it to the rail fence cipher.

The instructions link to hints, which in turn link to each cipher's solution. Over a fortnight in November 1999, we offered a contest to see if anyone could crack any or all of the three ciphers.