Poem code

The poem code is a simple, and insecure, cryptographic method which was used by SOE to communicate with their agents in Nazi-occupied Europe.

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.

To encrypt a message, the agent would select words from the poem as the key. 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.

The words would be written sequentially, and their letters numbered to create a transposition key to encrypt a message. For example, if the words are YEO THOMAS IS A PAIN IN THE ARSE, then the transposition key is: 25 5 16, 23 8 17 13 1 20, 10 21, 2, 18 3 11 14, 12 15, 24 9 6, 4 19 22 7. These are the locations of the first appearances of A's, B, etc. in the sentence.

This defines a permutation which is used for encryption (25->1, 5->2 etc.). First, the plaintext message is arranged in columns. Then the columns are permuted, and then the rows are permuted.

For example, the text "THE OPERATION TO DEMOLISH THE BUNKER IS TOMORROW AT ELEVEN" would be written on grid paper as:

TPTTMSEKSOWLN
HEIOOHBETRAEA
ERODLTURORTVX
OANEIHNIMOEET

(The above transposition key requires longer messages which would have at least 25 columns and 25 rows).

Security checks: As an additional security measure, the agent would add prearranged errors into the text as security checks. For example, there might be an intentional error in every 18th letter. This was to ensure that, if the agent was captured or the poem was found, the enemy might transmit without the security checks.

The code's advantage is to provide relatively strong security while not requiring any codebook.

However, the encryption process is error-prone when done by hand, and for security reasons, messages should be at least 200 words long. The security check was usually not effective: if a code was used once intercepted and decoded, any security checks were revealed and could often be tortured out from the agent.

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