Between Silk and Cyanide by Leo Marks Read Free Book Online

you?'
    I memorized the way he said it so that I could try it on the coders.
    'Thanks, Tommy. But it would take all night.'
    'I've got all night.'
    'It's the poem-code! It has to go.'
    'Tell me why. And then tell me what you think should replace it'
    'I spared him nothing. My worry had a technical name: transposition-keys. They were the code equivalent of an anxiety neurosis.
    Every agent had to work out his transposition-keys before he could either send a message or decode one from us. I wanted Tommy to see for himself the kind of effort this involved in the soothing atmosphere of occupied Europe.
    I asked him if there were any particular poem or phrase he would like to use; he left it to me. I wrote one out and told him it was based on an SOE opinion poll:
    Y E O T H O M A S I S A P A I N I N T H E A R S E
    The lowest letter in that phrase is A. I asked him to put the figure 1 beneath it.
    Y E O T H O M A S I S A P A I N I N T H E A R S E
1.
    Then the figure 2 beneath the second A, a 3 beneath the third, a 4 beneath the fourth:
    Y E O T H O M A S I S A P A I N I N T H E A R S E
1. 2. 3. 4.
    The next letter is E. I asked him to put a 5 beneath it. Then 6 and 7 beneath the remaining E's.
    Y E O T H O M A S I S A P A I N I N T H E A R S E
5. 1. 2. 3. 6. 4. 7.
    Without waiting to be asked, Tommy continued numbering the rest of the letters in alphabetical order until we were looking at:
    Y E O T H O M A S I S A P A I N I N T H E A R S E
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.
    That numbered phrase was called a transposition-key. I broke the good news that all messages were encoded on a pair of transposition-keys—so the agent now had to start numbering another one using, for security reasons, a different five words of his poem. An interesting repeat performance with an uninvited audience on the prowl outside.
    The slightest mistake in the numbering would render the entire message indecipherable. The smallest error in the spelling would also produce gibberish. The permutations of mistakes an agent could make ran into hundreds of millions—and he still hadn't started to encode his message.
    To do so, the agent used his transposition-keys to put his clear text through a series of complex convolutions not unlike Ozanne's mind, so that the message arrived in London in jumbled form where we (hopefully) could unscramble it because we (hopefully) were the only ones who knew what his poem was.
    Unless the Germans had tortured it out of him.
    Or unless their cryptographers had broken one of his messages and mathematically reconstructed the words of his poem.
    I told Tommy that the poem-code must go and be replaced by one which the agents could not possibly remember. Their transposition-keys must never again be based on words, poetic or otherwise. They must be mass-produced by hand by specially trained groups of coders shuffling numbered counters at random.
    We would give each agent a series of transposition keys already worked out for him—and printed on silk. To encode a message, he would simply have to copy out the keys we had prepared for him and immediately cut them away from the silk and burn them. There should be no way that he could possibly remember the figures he had used. They would all have been selected at random—and would be different for every single agent in the field.
    Each silk would contain sufficient keys for 200 messages—100 from the agent to us, 100 from us to him. The greatly increased security of these 'worked-out' keys would allow the messages to be shorter. One hundred letters could be sent instead of the existing minimum of 200.
    The cryptographic parlour-game would be closed for the season.
    Enemy cryptographers would no longer have poems to reconstruct and would have to tackle every single message individually—an undertaking which was anathema to all cryptographers. Every message to and from the field would confront them with a new code, and to sustain an 'absolute