Editing Poem code (section)

== 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). Frequently, the poems were humorous or overtly sexual to make them memorable ("Is de Gaulle's prick//Twelve inches thick//Can it rise//To the size//Of a proud flag-pole//And does the sun shine//From his arse-hole?"). Another improvement was to use a new poem for each message, where the poem was written on fabric rather than memorized.

Gradually, the SOE replaced the poem code with more secure methods. Worked-out Keys (WOKs) were the first major improvement—an invention of Marks.  WOKs are pre-arranged transposition keys given to the agents, which made the poem unnecessary. Each message would be encrypted on one key, which was written on special silk. The key was disposed of by tearing a piece off the silk, 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]], specifically the letter one-time pad (LOP).  In LOP, the agent was provided with a string of letters and a substitution square. The plaintext was written under the string on the pad. The pairs of letters in each column (such as P and L) indicated a unique letter on the square (Q). The pad was never reused while the substitution square could be reused without loss of security. This enabled rapid and secure encoding of messages.