Renzi nodded.
The gendarme smiled unexpectedly. "Bonne chance, mon brave," he said, stepping back and folding his arms in dismissal.
In good time Renzi was sitting on the bench as instructed. At four there was no one, and at a quarter past the hour a young mother insisted on occupying the other end while she dandled her baby on her knee in the sunshine, cooing and clucking, inviting him to admire the now squalling infant.
It was a clever ruse and, within a few minutes, Renzi had been invited to impart the essence of the difficulty. In return he received a businesslike solution. A vase of flowers would later appear in his room. If he placed it in the window it would indicate that a message for Fulton was concealed under its base. Likewise, Fulton's message for Renzi might be found under the base should the vase appear again in the window. At the other end there would be different arrangements. How it was done was not his affair.
That night he wrote a short message, in anonymous block capitals, which simply explained how Fulton's new friend might be contacted and hoped that he would hear from him soon.
He placed the vase in the window and went to bed. In the morning when he woke it was still there, but when he returned from another morning of stupefying boredom at the prisoner-of-war negotiating tables, he found a different paper in the hollow base. Feverishly he tore it open.
"The matter is not impossible," it read, in a beautifully neat and characterful hand. "What can you offer?"
Exultant, Renzi paced up and down while he considered. In his body-belt he held eight hundred pounds in gold, intended for travel expenses and the like. Would this be enough to tempt Fulton to leave immediately, the form of the contract to be discussed in England?
The thought of quick acceptance followed by rapid departure from this fearful world of danger and deceit was intoxicating. Quickly he penned a reply, emphasising immediate payment and rosily reviewing the prospects he had mentioned earlier.
For the rest of the day he was forced to attend a legal hearing and did not arrive back until late—but there was a reply. Renzi scanned it rapidly, and his heart sank. In lordly tones Fulton was demanding no less than ten thousand pounds to leave France. Carefully he composed a reply. It would not be possible to raise such an amount at short notice but the eight hundred would be more than sufficient to ensure a swift passage to England where all things would be possible. His overriding objective was to ensure his freedom to negotiate at the highest level he chose.
When the response came it was long-winded, hectoring, and demanded, as a condition to Fulton's considering any proposal, an undertaking that the British government form a plenary committee within three weeks of his arrival to examine the scope not only of his submarine craft but of other inventions. In return he would be able to offer the plans for an improved Nautilus and his torpedoes to the Admiralty for a hundred thousand pounds. Further, written proof of the offer from the British at cabinet level would be necessary before he would contemplate acceptance or leaving France.
It was an impossibility. The annual salary of a senior clerk for a thousand years? The man must be mad—or was he? Whoever stalked the undersea realm would surely command the seas, and it was plain that those who stood to lose the most were the English.
Renzi slumped. His first impulse was to promise anything at all, as long as Fulton left for England. He was living on borrowed time—and the stakes could not have been higher. But he knew he could not compromise his principles still further.
He sighed deeply and reached for his pen. With the utmost regrets he admitted he was not in a position to bind the British government to the amount indicated. However, to keep faith with Fulton he would, with all dispatch, open secret communications with Whitehall to establish a basis for negotiation.
There was little more he could do, now that he was passing the responsibility to a higher authority—and, wearily, he realised that this presented a grave problem in itself. How the devil would he get messages of explosive content safely to England when he had no means to secure them? Trusting the agents to perform some kind of coding was asking too much—and, apart from that, they would then be privy to state secrets of the highest importance.
He had no cipher materials: possession of such in any context was prima facie evidence of espionage. Yet if the communications were not enciphered he could not risk divulging vital and necessary details. In any case, to meet Fulton's demands he had to obtain a written undertaking at the highest level, which must be secure. He was going round in circles. There must be a way.
Renzi was by no means ignorant of secret codes after his experiences in Jersey: could he find a method from first principles to encrypt the message? The gravest difficulty of all was that in virtually every case the key had to be known beforehand at the receiving end or it must be sent in clear by other means—with catastrophic consequences if compromised.
Despite everything, Renzi found himself drawn into the logical challenges of the dilemma. After the intense boredom of the prisoner-of-war negotiations, the danger and frustration of dealing with Fulton, this bracing intellectual exercise was congenial, and he bent his mind to the task.
Any cipher whose key could be discovered was by definition unusable. Classical ciphers, such as the famous Caesar Shift, with no key but letter substitution, were unsafe—code-breaking had moved on in modern times. The same applied to the transposition types and, without prior arrangements, more complex techniques would require a key or method-type to be sent on before in clear.
A book cipher? This had the advantage that the key was already in the possession of the receiver—the text of a pre-agreed book held by both. A word in the message was specified as a precise location of that same word in the book. The disadvantage was that not only was it essential for each to have precisely the same edition but it was laborious, and the resulting encipherment could be large in size. The Bible had been used many times, with its exact chapter and verse convenience, but of course it would be the first that code-breakers tried.
There was another method: the running-key cipher. This used a source book too, but at individual character level. From a given point the ongoing text was used as a continuous key-stream to yield coded values against the message contents. This was better—and if the book's title was protected the resulting encipherment was near unbreakable.
So, what volume was to be used, known precisely by both parties? The Admiralty's own King's Regulations? Or the Articles of War? But without them to hand he could not swear to accuracy. And if it was to be some other book, its name and edition had to be divulged first. He was back where he had started.
He lay down and closed his eyes. It was the separate transmission in clear of a key or decoding method that was the sticking point. If he could only—
He sat bolt upright. That was it! The method, the key-text—and a cast-iron secure way of transmitting the key!
Galvanised, he set to work. He would not be disturbed—he had uncovered some time ago that Haslip's concern to be left alone was on account of a certain woman, and the French could not trespass on diplomatic territory.
Snatching up paper and a pencil he began to set up his tabula recta, the encoding matrix. Not needing to consult a book, he was able to work swiftly, and at a little after midnight he had the result. Carefully he burned his workings, folded the papers as small as he could and sealed them tightly together.
He hesitated over the forwarding instructions but eventually settled on "Foreign Office, per Smith, Paris." It would find the right handler easily enough. Underneath, in smaller lettering, was the more important entry: "Refer Cdr Thomas Kydd, HMS Teazer." It was done.