'One of…'
'Which is why I took your puzzle to him. Joan, Saladin, this is Roger Bacon, born in Ilchester, trained in Oxford, and now lecturing in Westminster.'
'Don't forget Paris,' Bacon put in.
'I have been aware of his career since his student days – oh, a decade back now. You studied the classics at Oxford, did you not, Roger?'
'And geometry, arithmetic, music, astronomy. I worked under Robert Grosseteste.'
'The bishop of Lincoln-'
'Who has led the reintroduction of the works of the Greeks into England.'
'Roger lectured in Paris-'
'I earned my master of arts degree there. I saw Alexander of Hales there, and twice saw William of Auvergne dispute…'
This fast-paced duologue was hugely confusing to Saladin, who had heard of none of these scholars.
'I see myself now as a dominus experimentorum,' Bacon said.
Joan glanced at Thomas. 'What does that mean?'
'One who studies the physical world,' Thomas said, 'and tinkers with it, in the hope of learning more about the truths of God.'
'I have always 'tinkered',' Bacon said. 'I once set up a candle and a mirror in a darkened room, and peered into the eye of a cat. Have you ever tried such a thing, brother Saladin?'
'I can't say I have.'
'You see a carpet of dusky red vessels overlaid by a golden tracery. Quite beautiful, quite mysterious. My study in optics began with those first observations. And if you could look into the head of a man, what would you find? But I have never been able to persuade anybody to sit still long enough to let me see. Ah, well.'
'I thought all truth was to be found in the Bible.'
'Of course, and in the authorities of antiquity. I myself am one of Europe's leading scholars on Aristotle,' Bacon said without a shred of modesty. 'But there are many routes to the same destination, which is God's truth. The role of the natural philosopher is to understand how phenomena reveal that truth. Saint Augustine himself instructed us not to embarrass ourselves by quoting the word of God to contradict some fact of nature, because that would only reveal that we understood neither the word nor the nature. Experimentation: that is the way to that deeper truth, that final reconciliation. Or so I am coming to believe. Perhaps you have heard of the work of Master Peter de Maricourt, a Picard who once took the Cross, and subsequently-'
'Yes, yes, Roger,' Thomas cut in. 'But perhaps we should get to the point?'
Bacon smiled, utterly in control. 'Quite right, Father, quite right. You!' He jabbed a finger at the novice, who jumped. 'More wine for our guests. And bring a lamp over here.' He sat before the low table, opened his leather wallet and extracted papers that he proceeded to spread out. 'We have a deep mystery to unravel.'
Saladin murmured to Thomas, 'He's quite a showman, isn't he?'
'And he knows it. But it's not necessarily good for him. Ah, Roger, Roger, how your busy head distracts your pious heart!'
But they sat before Roger Bacon, wide-eyed, as he began to reveal the truth of the Incendium Dei cipher.
XXI
'We begin with your fragment of coded text, as Thomas presented it to me,' Bacon said. He spread out a parchment on the table: BMQVK XESEF EBZKM BMHSM BGNSD DYEED OSMEM HPTVZ HESZS ZHVH
'I was intrigued by the puzzle…'
'I knew he would be,' Thomas whispered to Joan. 'Very useful thing about scholars, that curiosity. He didn't even ask for a fee.'
Bacon glanced at Saladin. 'You. Tell me what you see.'
'I'm no scholar-'
'Just answer.'
'I see ten words,' Saladin said. 'Latin letters, not Arabic. I recognise none of the words, though.'
'And nor should you, for they aren't words at all. Even these groupings are a decoy, I quickly realised. This is no sentence. Look at them! What sentence has words of such regular lengths?'
'It is written in a cipher,' said Joan. 'That much is obvious.'
'Yes! But what cipher? What do we known about ciphers? You, Thomas?'
'Just get on with it, Roger.'
'Oh, very well. The first cipher was used by the Spartans, long before the birth of Christ. They had a device called a scytale. You would wrap a strip of leather around a baton, and write out your message; once unwrapped the letters are scrambled, you see, illegible to anyone who doesn't have a baton of the same dimensions. Tacitus wrote of codes and ciphers, as did the Greek Polybius. Julius Caesar used a substitution cipher, which depended on a simple cyclic displacement of the alphabet. Caesar used a displacement of three positions, while Augustus later used one.'
'I'll be the one to ask,' Joan said heavily. 'What is a 'simple cyclic displacement'?'
Bacon reached for a bit of chalk and scribbled on the tabletop. 'You write out your alphabet. A, B, C, D. And you write it out again with the letters shifted through three spaces, say. D, E, F, G. You have the word you wish to encode, say 'CAESAR'. And you exchange the true letters for the shifted ones. So C becomes F, A is D, E is H…'
'I understand,' said Joan.
'Now, history tells us there have been ciphers a good deal more sophisticated than that. Polybius himself described a bilateral substitution system, which means… never mind! Happily for your weary brains, I soon concluded we aren't dealing with anything much more complex than Caesar's substitutions.'
'Why do you believe that?' Saladin asked.
'This is a message in the Latin alphabet, not Arabic or Persian or Greek. So it is surely a Latin message. The Moors of Spain are developing extremely advanced cryptographic systems, I'm told. But a thousand years after the Caesars, we Latin scholars still lag behind the rest in our ciphers as in everything else. One point on which I kept an open mind was which alphabet we are using here.'
'The Latin one,' said Saladin.
'Ah, but which Latin? Caesar used twenty-three letters. We use twenty-five, for we have added U and J. I thought it most likely the classic alphabet was the one employed.
'So I began my analysis. A common technique in breaking ciphers is to study the distribution of letters. The most common symbol is likely to correspond to a common letter in plain language – E perhaps, or S, or T. But this fragment is too short to enable such a count. I experimented with scytales of various dimensions, to no avail. And I tried all the possible cyclic permutations, with no luck either. With all the permutations exhausted, I racked my brains for a new way forward.'
Joan murmured, 'And in the end, after much heroic struggle, you found a way, did you?'
Bacon blithely ignored her sarcasm. 'A simple variant on cyclic substitution is to use a key.'
'A key?' Saladin asked.
'Caesar, for instance, could have used his own name.' He wrote it out: CAESAR. 'We must eliminate repetitions.' He crossed out the second A. 'Now we use this five-letter key as the foundation of our cipher.' He wrote out a twenty-three Latin letter alphabet with a code beneath it:
'You see? A substitution with the shift depending on the key word, and with those letters removed. So the word CAESAR now encodes as-' He wrote it out: ECRQCP
'It's a poor sort of code,' Saladin observed. 'The last few letters are transcribed without change.'
'You're a practical man, I can see that,' Bacon said. 'That's true. But there are easy variants. The simplest is to put the key word at the end of the alphabet, not the beginning, and to proceed backwards.' He scribbled
