by Leo Marks
‘Yes, sir.’ I nearly said ‘only too well’.
Bletchley worked closely with C, and the less the ‘bastards of Broadway’ knew about our plans for expansion the better our chances of survival.
‘And there’s one other thing. Heffer and I don’t insist on this but neither of us think you should discuss one-time pads with him’ – he turned to Heffer for confirmation.
The Guru looked at me thoughtfully. ‘The timing’s wrong,’ he said.
He then explained between puffs that letter one-time pads were an important new concept but their use by SOE might involve Bletchley in a conflict of interests, and I’d be well advised not to refer to them until a special meeting could be arranged with Tiltman present.
The wizard of Bletchley had been present ever since I’d met him, but Heffer misread my expression. ‘I need hardly remind you,’ he said, doing just that, ‘that he is SOE’s coding adviser, that he did support your concept of WOKs and that if letter pads make Bletchley’s task more difficult, then the least we can do is …’
I switched off at this point because I’d prefer to see German agents sitting in the War Cabinet or even running 84 than allow SOE’s agents to be deprived of LOPs for a day longer than was absolutely necessary, providing that the system had no flaws.
Nick pointed his finger at me as if he didn’t want it back. ‘You must learn to concentrate on one code at a time, and WOKs are more than good enough to be going on with.’
But good enough wasn’t good enough if we could produce something better, though I had the sense not to say so.
Thirty minutes later my visitor arrived.
Nick was right in saying that Commander Dudley-Smith was ‘exceptionally bright’. His gold braid glistened, his eyes sparkled and his mind was a torpedo looking for a target.
Firing practice began at once. He regretted he couldn’t lunch with me, as he’d heard ‘excellent accounts of Baker Street cuisine’, but he’d be glad of some coffee if we had any going (it was already on its way). He congratulated me on the charts for breaking indecipherables which I’d shown Tiltman, and was then kind enough to explain the various uses to which Bletchley put them, none of which had even occurred to me, though I nodded knowingly. It was a subtle way of reminding me that amateurs don’t belong in the same ring as professionals. His round on points.
I returned the compliment by thanking him for Bletchley’s machinemade keys, and he at once asked what was wrong with them.
I decided not to dissimulate until it was absolutely necessary and pointed out that the long spaces between the keys would make them difficult to reproduce. I also pointed out some mistakes in them, including duplicated numbers. He thanked me for bringing them to his attention and assured me that the mistakes wouldn’t occur again, at which point I began to suspect that they were deliberate.
He asked whether country sections shared their forward planning with the Signals directorate, and I replied that as far as I could see they had none to share.
I was saved from further questions by the arrival of the coffee and black-market sandwiches.
I waited until he was involved with the problem of selection before presenting the bill: ‘Commander, I’ve a favour to ask.’
‘Go ahead,’ he said with smoked-salmon-induced euphoria.
I told him that I’d been working on a new code which I hadn’t thought of when Tiltman was here. Could he possibly spare the time to discuss it with me?
He said that he’d be glad to help in any way he could, though it might be better if I waited for Tiltman’s next visit.
‘I’d value your opinion, Commander.’
‘What sort of code is it?’ he enquired with ill-concealed reluctance.
He stopped chewing when I said it was a new form of one-time pad which used letters instead of figures and required no code book.
I produced a LOP which Joan Dodd had managed to have printed on silk as a very special favour.
‘Brief me as if I were an agent,’ he said, ‘or a country-section head.’ He sat next to me just as Tiltman had when we broke Skinnarland’s indecipherable.
I asked him to choose a message but he preferred to leave it to me.
‘ “Long live Tiltman”?’ I suggested.
‘I second that,’ he said.
I’d have preferred ‘Vivat Tiltman’ to show off my Latin, but I wanted a phrase at least fifteen letters long.
I asked him to copy out the first fifteen letters of his one-time pad, and write our benediction beneath them.
His handwriting was a flotilla of small ships setting out for Dunkirk. Twelve seconds later (it was important to time him) he completed the first part of his message.
One-time pad:
OPXCA
PLZDR
BHTEJ
Message:
LONGL
IVETI
LTMAN
‘You now have to encode each pair of letters in turn, starting with the first pair O over L, and ending with the last J over N …’
‘How do I do that?’ he asked, as if his future depended on it.
‘With the help of a substitution square, which may look complicated but couldn’t be simpler.’
I produced one for him [see opposite].
‘You see the alphabet which runs along the top? Glance down the o column until you come to the l. What does o over l give you?’
‘Q.’
‘Then that’s your first pair of letters encoded. Your next pair is p over o. What does p over o give you?’
‘E,’ he replied.
‘Then that’s your next code group. No great hardship, is it? The third pair is X over N … what does X over N give you?’
‘M’
Eighteen seconds later his task was complete.
One-time pad:
OPXCA
PLZOR
BHTEJ
Message:
LONGL
IVETI
LTMAN
Code groups:
OEMXK
XOTKR
JVYFG
In the absence of any comment, such as ‘Rise, Sir Leo’, I explained that the decoding process was just as simple. All he had to do was write the code groups under the one-time pad groups and the substitution square would decode each pair in turn.
Pretending that he really was an agent, I pointed out that o over l had produced q, and that in the decoding process o over q would produce l, and the same principle applied to all the other pairs of letters.
He accepted my word for it without bothering to check, which made me uneasy.
But his frown worried me even more. It was clear that something was puzzling him.
Wondering what I’d overlooked, I explained that agents would use LOPs as their main code, have WOKs in reserve and fall back on poems if they lost both. I then pointed out that the pads and substitution squares would be different for each agent, and that they’d be given high-grade security checks. I started to describe them.
‘Hold on a minute.’
I did – to the desk.
‘I agree that the pads must be different, but why the substitution squares? Surely it wouldn’t matter a damn how many of them were captured? What use would they be without the pads? Or have I missed something?’
‘No, Commander – I have. Of course they can be the same – you’ve saved us an enormous amount of work.’
But he was still frowning.
What other idiocy had he spotted?
‘Look here, Marks …’
I prepared for the worst.
‘… did I understand you to say that you didn’t think of this code until after Tiltman had left?’
‘Yes! What’s wrong with it?’
‘Nothing at all … As a matter of fact letter one-time pads have been working very successfully for quite a long time.’
I’d forgotten how to handle relief, and he was closer to being hugged than he’d ever know. It was the equivalent to hearing that Hitler had choked t
o death on a piece of gefilte fish.
‘My God, sir’ – I’d decided to promote him – ‘that’s the best news you could possibly have given me.’
And so it was – but why was he looking at me in such an extraordinary way? Surely he didn’t think I was disappointed because I couldn’t take the credit for LOPs? Surely to God it couldn’t be that?*
‘What’s the problem, Commander?’
He hesitated for fully ten seconds.
Again, I prepared for the worst.
‘The problem is knowing the best way to put this to you …’
Put what, for Christ’s sake?
His smile was as unexpected as a self-peeling banana. ‘It’s quite an achievement thinking up LOPs without any help.’
I was as embarrassed as he was but returned to full alert when his expression hardened.
‘I must also say this – and let me emphasise it’s just my personal opinion, because you’ve rather sprung this on me … I don’t believe SOE needs letter one-time pads – WOKs are far better suited for agents’ traffic.’
I asked him to explain why.
His assessment of the codes’ relative merits was brusque, informative and apparently impartial. In his view, WOKs had one great advantage: they allowed two hundred messages to be passed on only two sheets of silk, whereas for two hundred to be passed in LOPs would require at least a dozen sheets of one-time pad.
As for WOKs’ security, provided that they were used once only every message would have to be attacked separately, which would place as great a strain on the enemy’s resources as it would on Bletchley’s. For all practical purposes, we need look no further than WOKs.
He paused for breath but I had none to offer him.
It was true (he continued) that LOP-users could send as few as ten letters, but how many agents could take advantage of this? Surely many of them had to transmit lengthy reports? In which case, wouldn’t their dozen sheets of pad be harder to camouflage and more dangerous to carry than two sheets of silk? Warning me by eye-glint not to interrupt, he compared the coding demands which each system made.
WOK-users had only to copy out one line of figures before starting the process of double-transposition. But LOP-users had to copy out at least fifty letters of their one-time pads, then write their messages beneath them and start using their substitution square. Although the system was simpler than double-transposition, the effort involved was just as great, and was likely to take longer.
I pointed out that it had taken him eighteen seconds to copy out his one-time pad groups and write his message beneath them, and another fifty seconds to encipher ‘long live tiltman’. Assuming that agents were twice as slow, they could still encode or decode the average message in under twenty minutes.
Straightening his cuffs, Dudley-Smith said he didn’t realise that I’d been testing him for a job in SOE or he’d have put up a better performance. He then made what was clearly his closing statement.
He appreciated that agents had to keep the messages as short as possible because of the efficiency of the Funk-Horchdienst (the German interception service). But it was equally important that they destroyed the used portions of their codes, and surely it would be easier for them to cut away one line of a WOK than a dozen lines of a one-time pad? Given the choice between WOKs and LOPs, he felt that most agents would opt for WOKs.
‘Which system would Bletchley opt for if it were faced with breaking it?’
‘You know damn well that letter pads are unbreakable on a depth of one’ (used once only).
‘Are they produced at Bletchley, Commander? And could you help us to get some?’
In the silence that followed I knew that I must leave nothing unsaid. I told him that I agreed with most of his reservations but the fact remained that 90 per cent of SOE’s messages could be far shorter than the minimum of a hundred letters required by WOKs, and that if Bletchley couldn’t help us we’d recruit a special team of girls and produce the pads by hand.
‘You realise how much work that would involve?’
‘Yes, Commander.’
‘And you’d be prepared to take it on?’
‘Yes, Commander.’
He took a deep breath. The ozone was comparatively clear as my cigar was unlit. ‘They’re produced at Oxford by Commander Hogg. He supplies SOE’s figure pads – Dansey knows the details.’
‘Thank you, Commander.’
‘You’ll have to make an appointment with him through the head of SOE – and Nick should have a word with him in advance.’
‘Thank you, Commander.’
He glanced at his watch. ‘God – I must go.’
To forewarn Hogg?
We both stood up. In his case it showed.
He thanked me for the sandwiches and reminded me of Tiltman’s invitation to visit Bletchley. He then held out his hand as if I were no longer a stowaway on a coding flagship. ‘I agree with you that the poem-code is shit and I want to help you to replace it if I possibly can! I hope you know that.’
‘Thank you, Commander. I do.’
And meant it …
And to help me continue meaning it if he were forced to be obstructive, I encoded a reminder to myself on a letter one-time pad: ‘vivat dudley-smith.’
It took me two minutes and twenty seconds.
Note
* Even in 1998, when so much has been written about SOE that only its secrets remain, I am still credited with inventing the letter one-time pad. WOKs, yes – but with LOPs I was pre-empted, and I wish I knew by whom! Whoever you are, and wherever you may be, my apologies and thanks. L.M.
THIRTY-ONE
Accidents Will Happen
Oxford had a variety of irresistible attractions. It was 65 miles from Baker Street, every one of them a continent. It had yet to be troubled by air raids (I was one of the few sandbags in sight), and it had managed to retain so much of its awesome presence that the GI visitors who swarmed across the cobblestones allowed the spires to do the strutting. Above all, it was the home of the Bodleian Library, which was not only the shire’s soul, and one of the main reasons for resisting invasion, but was allowed six months’ credit by 84.
Yet the feature of Oxford which transformed it into an English Lourdes was unknown to all but a privileged few.
It produced letter one-time pads.
Commander Hogg’s office was on the first floor of a large country house, which had been taken over for official purposes with no obvious signs of desecration.
There were no obvious signs of a welcome either.
The commander rose from his desk, took my pass from me and examined it as if it were written in code. He was middle-aged, greyhaired, with none of Dudley-Smith’s elegance but all of his authority. The quality of his product was reflected in his face, which was impossible to read on a depth of one.
Something about my pass seemed to be troubling him and he put on his glasses for a second reading.
It was an uncomfortable start to a critical meeting for which I was inadequately briefed. I knew nothing about this inscrutable man except that he was a purveyor of one-time pads to the nation of which SOE believed itself a part.
The first words he addressed to me weren’t so much clipped as stapled together.
‘Is it Marks with an “x”?’
Dear God, one of those. ‘Not according to my birth certificate or my great-grandfather’s! May I ask if it’s Hogg with one “g”!?’
There was a short pause during which the room was filled with the stimulating throb of mutual antipathy. He then invited me to sit down and brusquely informed me that he’d been asked by the head of SOE to supply us with letter one-time pads and to discuss the details in the course of our meeting.
He looked at me with a ‘torpedoes away’ expression. ‘I’m still not sure what you people do.’
‘The Germans, sir. In every way we can.’
I was absolutely certain that Captain Bligh didn’t need a briefing from Fletcher un-Christian about SOE
or any other wartime anomaly. But I needed one about him. There must be some reason for his hostility other than good taste.
‘Who recommended letter pads to SOE?’
I admitted responsibility.
‘What experience have you had with them?’
‘None, sir.’
He said ‘Good God’ so softly that unless the Almighty were in the room with us, of which there was increasingly little sign, he couldn’t expect much feedback from the supreme crow’s nest.
‘Can I assume you know how they work?’
I said that I believed I did; and he pounced at once: ‘Who explained them to you?’
I daren’t admit that until very recently I thought I’d invented them. ‘Dansey, sir. He used to be in charge of agents’ codes.’
His expression said, ‘It’s a pity he still isn’t.’
‘Do you usually recommend coding systems you know nothing about?’
‘No, Commander. But I rarely hear of any as good as LOPs.’
He winced at the word. And continued wincing while I explained why letter pads were ideally suited for agents’ traffic.
He waited until I’d finished the litany, then looked at me as if my bilges were leaking. (One of them was.) ‘Tell me, Marks, what other codes have you recommended to SOE?’
I gave him a thorough WOK-ing, and he suddenly brightened. ‘That idea sounds secure and practical. Who devised it?’
I admitted paternity, which seemed to surprise him.
‘Commander, may I give you two examples of why letter pads would be better?’
Without waiting for permission, I told him about the Norwegians who reported the movements of German battleships to London while surrounded by the enemy and about the agents who attacked submarines with limpets, who also reported to London. The information the Admiralty was waiting for could have been transmitted in less than fifty letters, had the codes been safe enough.
