The poem code is a simple, and insecure, cryptographic method.
The method works by 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 some 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 is sent at the start of the message.
The method is insecure for, if one message is broken by any means (including threat, torture, or even cryptanalysis), future messages will be readable if the source poem has been identified. Since the poems used must be memorable for ease of use by an agent, there is a temptation to use well-known poems or poems from well-known poets. (e.g. SOE agents often used verses by Shakespeare, Racine, Tennyson, Molière, Keats, etc.).
Every poem code message commenced with an indicator-group of five letters, which showed which five words of an agent's poem had been used to encrypt the message. If the agent used the same poem code words to send a number of messages, these words could be discovered easily by enemy cryptographers. If the words could be identified as coming from a famous poem or quotation, then all of the future traffic submitted in that poem code could be read.
When Leo Marks was appointed codes officer of the Special Operations Executive (SOE) in London during World War II, he very quickly recognized the weakness of the technique, and the consequent damage to agents and to their organizations on the Continent, and began to press for changes. Eventually, the SOE began using original compositions (thus not in any published collection of poems from any poet) to give added protection (see The Life That I Have, an example), but also adopted other more secure methods such as Worked-out Keys (WOKs) and the one-time pad.
A project of Marks', named by him "Operation Gift-Horse", was a deception scheme aimed to disguise the more secure WOK code traffic as poem code traffic, so that German cryptographers would think "Gift-Horsed" messages were easier to break than they actually were. This was done by adding false duplicate indicator groups to WOK-keys, to give the appearance that an agent had repeated the use of certain words of their code poem. The aim of Gift Horse was to waste the enemy's time, and was deployed prior to D-Day, when code traffic increased dramatically.
- Between Silk and Cyanide by Leo Marks, HarperCollins (1998) ISBN 0-00-255944-7; Marks was the Head of Codes at SOE and this book is an account of his struggle to introduce better encryption for use by field agents; it contains more than 20 previously unpublished code poems by Marks, as well as descriptions of how they were used and by whom.