I hope you enjoyed Ed’s guest posts on NP-complete problems on TV the last couple of Mondays. It’s always great to hear from others on math that they are thinking about. This week it’s me again, and ...

Welcome to Math Monday! Each Monday here at Casting Out Nines, we feature a mathematics-themed article. Today’s is a new installment in an ongoing virtual seminar on columnar transposition ciphers.

This was one of the most secure hand ciphers used in the Second World War. It was used by both the Allies and the Axis, and served both well. Its main weakness was that if the attacker intercepted two ...