The poem code is a simple and insecure, cryptographic method which was used during World War II by the British Special Operations Executive (SOE) to communicate with their agents in Nazi-occupied Europe.
For instance, suppose the poem is the first stanza of Jabberwocky: ’Twas brillig, and the slithy toves Did gyre and gimble in the wabe: All mimsy were the borogoves, And the mome raths outgrabe.
We could select the five words THE WABE TOVES TWAS MOME, which are at positions 4, 13, 6, 1, and 21 in the poem, and describe them with the corresponding indicator group DMFAU.
For example, there might be an intentional error in every 18th letter to ensure that, if the agent was captured or the poem was found, the enemy might transmit without the security checks.
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?").
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.