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.

Description

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 YOE 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.

Analysis

The code's advantage is to provide relatively strong security while not requiring any codebook. The encryption process is error-prone and for security required messages at least 200 words long.

It is vulnerable to attacks. 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.).

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. The German cryptologic units were successful in decoding many of the poems by searching through collections of poems.

Development

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.

Worked-out Keys (WOKs) was the first major improvement. WOKs are pre-arranged transposition keys given to the agents and which made the poem unnecessary. Each message would be encrypted on one key, which was written on special silk. They key was disposed off when the message was sent.

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.

The poem code was ultimately replaced with the one-time pad.

Bibliography

• 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.

See also

• Book cipher
• The Life That I Have (also known as Yours, arguably the most famous code poem)