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.

Today’s Math Monday begins a multi-part series on the columnar transposition cipher.

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.

Her duress code was to use a double transposition key with precisely 18 letters, so we know this message was not sent under duress.