Poem code

The poem code is a simple and insecure, cryptographic method which was used during World War II by the British Special Operations Executive (SOE) to communicate with their agents in Nazi-occupied Europe.

The method works by having the sender and receiver pre-arranging a poem to use. The sender chooses a set number of words at random from the poem and gives each letter in the chosen words a number. The numbers are then used as a key for a transposition cipher to conceal the plaintext of the message. The cipher used was often double transposition. To indicate to the receiver which words had been chosen, an indicator group of letters is sent at the start of the message.