Poem code
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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 cypher to conceal the plaintext of the message. The cypher used was often double transposition. To indicate to the receiver which words had been chosen an indicator group is sometimes 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, there is a temptation to use well known poems or poems from well known poets, eg Racine, Moliere, Keats, etc.
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, though perhaps without so much literary merit) to give added protection, but also adopted other more secure methods such as the one-time pad.