Poem code facts for kids

The poem code was a simple way to send secret messages during World War II. The British Special Operations Executive (SOE) used it to talk to their agents. These agents were working in parts of Europe controlled by Nazi Germany.

This method used a poem that both the sender and receiver knew beforehand. The sender would pick a few words from the poem. Each letter in these words was then given a number. These numbers became a special key. This key was used with a transposition cipher to mix up the real message, called the plaintext. Often, they would mix it up twice, which is called a double transposition. To tell the receiver which words were chosen, a special group of letters was sent at the very start of the message.

How the Poem Code Worked

To make a message secret, an agent would choose words from their pre-arranged poem. Every poem code message began with a special group of five letters. These letters, based on their position in the alphabet, told the receiver which five words from the poem were used as the key.

For example, imagine the poem started with these lines from "Jabberwocky": ’Twas brillig, and the slithy toves      Did gyre and gimble in the wabe: All mimsy were the borogoves,      And the mome raths outgrabe.

If the agent chose the words THE, WABE, TOVES, TWAS, and MOME, these words are at specific spots in the poem. Let's say they are at positions 4, 13, 6, 1, and 21. The special indicator group might be DMFAU. This group told the receiver which words to use.

Creating the Secret Key

The five chosen words were written down one after another. Then, each letter in these words was given a number. This was done in a specific order: all the 'A's were numbered first, then all the 'B's, and so on. If a letter wasn't there, it was skipped.

Using our example words (THE WABE TOVES TWAS MOME), the letters would be numbered like this:

• The 'A's would get numbers 1 and 2.
• The 'B' would get number 3.
• The 'E's would get numbers 4, 5, 6, and 7.
• This process continued for all letters.

This numbering created a special order for scrambling the message.

Scrambling the Message

First, the real message was written into a grid. This grid had columns equal to the length of the secret key. For instance, if the message was "THE OPERATION TO DEMOLISH THE BUNKER IS TOMORROW AT ELEVEN RENDEZVOUS AT SIX AT FARMER JACQUES", it would look like this on a grid:

T H E O P E R A T I O N T O D E M O L I S H T H E B U N K E R I S T O M O R R O W A T E L E V E N R E N D E Z V O U S A T S I X A T F A R M E R J A C Q U E S X

Then, the columns of the grid were read out in the order given by the numbered key. This mixed up the letters. For example, the first column to be read might be the one under the number 1 from the key, then the column under number 2, and so on.

After scrambling, the message would look like a jumble of letters, like this: PELAD OZCEB ETETT IRUVF OREEI OAXHH ASMOO ULRSS TKNRO RUENI NREMV QTSWT ANEAT SDJIE RMOHE XOTEA

Finally, the indicator group (like DMFAU) was added to the very beginning of this scrambled message. This complete jumble was the secret message sent to the receiver.

Extra Security Steps

Most of the time, this scrambling process was done twice (double transposition). This made the message even harder to break. Agents also added small, planned mistakes into the message. For example, every 18th letter might be wrong on purpose. This was a safety check. If an agent was caught, the enemy might send messages without these special errors. This would tell the SOE that the agent was in trouble.

Improving the Code

When Leo Marks became the codes officer for the SOE during World War II, he quickly saw that the poem code wasn't very strong. He knew it put agents in danger. So, he pushed for changes.

New Poems and Keys

Eventually, the SOE started using poems that were made up just for the agents. These weren't from any published books. This made them even more secret. An example is a poem called The Life That I Have. Another improvement was to use a different poem for each message. These poems were often written on special fabric, not just memorized.

Stronger Methods

Over time, the SOE replaced the poem code with much safer ways to send messages. The first big improvement was called Worked-out Keys (WOKs). Leo Marks invented these. WOKs were ready-made secret keys given to agents. This meant they didn't need a poem anymore. Each message was encrypted using one key, which was written on special silk. After sending the message, the agent would tear off and destroy the part of the silk with the key.

Tricking the Enemy

Marks also created "Operation Gift-Horse." This was a clever trick. It made the more secure WOK messages look like the older, weaker poem code messages. The idea was to make German code-breakers think the messages were easy to break. This wasted the enemy's time. It was used before D-Day, when many secret messages were being sent.

The One-Time Pad

The poem code was finally replaced by the one-time pad. Specifically, they used the letter one-time pad (LOP). With LOP, an agent got a long string of random letters and a special grid called a substitution square. The real message was written under the random letters. Pairs of letters (one from the message, one from the random string) pointed to a unique letter on the square. This unique letter was the secret letter. The random letter string was never used again, but the substitution square could be reused safely. This made sending messages very fast and very secure.

See also

• Book cipher
• The Life That I Have (a famous code poem)