When poetry was conscripted

The life that I have Is all that I have And the life that I have Is yours.

This is the beginning of a short love poem, which was one of the last things a young woman learnt before dropping by parachute as a Special Operations Executive agent into occupied France in 1944; she was captured and executed by the Gestapo. Her name was Violette Szabo, and her adventures were later made into a film. This book tells the complicated story of how that poem was composed by a young man named Leo Marks. But to understand the story we need to go back to 1941.

Churchill set up a clandestine organisation, later known as SOE, to send agents into the occupied countries of Europe to encourage the resistance and provide them with the means to engage in sabotage. The agents smuggled into the Continent had to be equipped with a local identity and forged papers, but to keep in touch with London they needed also radio operators with a portable transmitter. This was risky, because transmission could be located by direction-finding apparatus and thus needed to be kept as short as possible.

Messages were sent in code, but the theory was that if nothing were in writing, the agent might pass even an intensive search. So codes were devised that could be constructed from memory. They were based on a short poem from which words were chosen as the basis of a transposition key. The security of such codes was poor, and SOE asked for a cryptographer to take charge of coding. At this time a school of cryptography had been set up, and one promising young man who was sent there instead of being conscripted into the army was Leo Marks. He was the son of an antiquarian bookseller, who at the age of eight had broken his father's code for recording the price paid for the books in his stock. His capacity for independent thought and disrespect for authority had already given him a hard time at school. Discipline was anathema to him, and he failed his course in cryptography. Rejected by Bletchley Park, the cryptographic Mecca, he was assigned to SOE.

His first experience there was not encouraging. He was given a coded message to solve, and his superiors were surprised to find it took him all day; but surprise turned to astonishment when they found he had not noticed a copy of the code on his desk and had taken the instruction to "break" it in the usual cryptographic sense. They assured him this code was no longer in use, but he had shown up a weakness in the system.

He next discovered that when an "indecipherable" message was received, the agent was instructed to encode and transmit it again, thus doubling the amount of time spent on the air. He found it fairly easy to solve these indecipherables, and taught a team of coders how to do this, often by trying several thousand combinations.

But there was a group of signals he was forbidden to touch, a special code used by the Free French. These were passed undeciphered to their HQ in London, but they too contained a proportion of indecipherables. Marks, strictly against orders, broke the code, solved the faulty signals and re-encoded them correctly before passing them on. His disregard of orders did much to improve the agents' chances of surviving.

Another weakness was the agents' habit of choosing well-known poems for their personal code. For once the Germans had succeeded in breaking a message, they could reconstruct the code words, which might well enable them to guess the poem. Marks had the bright idea of getting agents to write poems of their own to memorise, and supplied them with his poems if they failed to think of one. Most were doggerel, some obscene, but a few had real poetic merit, like the one quoted. Even poetry could be conscripted in wartime.

Marks then invented a better code, the ideal being one that was never used twice, but such a code could not be committed to memory. He solved this problem by having the codes printed on silk, which could be cut away and burnt as soon as used, and could easily be concealed in the lining of clothes. But this idea was too radical for his masters. When asked what was the difference between the old system and his new idea, he replied "between silk and cyanide", referring to the lethal pill all agents carried. Only when this sank in was he allowed to organise the production and printing of his new code-books.

Another weakness was that every message had to be at least 200 letters long to preserve security. So Marks devised a different type of code based on letter substitution, which was also printed on silk. This enabled the agent to send very short messages, again lessening the risk of discovery. But the poems remained in use as a back-up in case of emergencies.

The details of these codes will fascinate all with a taste for puzzles, but there is more to this book. It is a brief history of the behind-the-scenes battles over SOE, which the regular services jealously tried to take over. But even more interesting is the human angle. Marks had to brief the agents before they left, and he gives us vignettes of some very brave men and women, who risked and in many cases lost their lives on these operations. We get glimpses of his superior officers, some of whom he admired, some he loathed for their incompetence, but his quirky sense of humour kept him hard at work. SOE had its failures and successes, and Marks did much to improve the chances of its agents' survival.

John Chadwick is emeritus reader in Greek, University of Cambridge. He worked as a cryptographer during the second world war.