‘It is nothing but a game we play,’ I protested.

‘Aye, well, for Sir Francis it is a game with high stakes.’

We had stopped before Walsingham’s town house and I knew suddenly that I did not want to enter it. I shook my arm free of him.

‘I have decided that I do not want this work,’ I said, though my stomach constricted when I saw a flash of something dangerous in his eyes.

‘Listen to me, you pup,’ he said, pushing his face close to mine. ‘You say that you are not an English man. Right on both counts. I have been watching you, young Master Alvarez. That time you glimpsed me not far from here was not the first time I have seen you coming away from those secret Jewish practices. Who knows what treason you and your like may be hatching? Those practices are illegal, treasonous, and heretical, and you could be executed merely for attending. Jews have been banned from England for three hundred years.’

I stood very still, my heart hammering in my throat, and felt the cold sweat break out on my back.

‘That is one count of heresy against you. There is another. I have known it since the night you came to the Marshalsea. A heresy so foul it too could bring you to execution by burning. You flaunt yourself in the streets as a youth, and defy God’s laws, for I know that you are a girl.’

It had come at last. God had sprung the trap.

Chapter Three

Poley led me round the side of Walsingham’s house, through a small door and up a back staircase. The servants we passed made no attempt to bar his entrance, nor did they acknowledge him, but turned away, almost as if they chose to look through him, like some ghost or other visitant. I followed silently, for cold panic had frozen my tongue. What harm did he mean to do me, this man with his glib smile and his cruel eyes?

On the first floor we followed a wide corridor with polished floorboards covered in fine Turkey carpets and with walls of carved panelling, above which a number of portraits observed us gloomily. I was suddenly conscious that my hand was sticky with fig juice, and wiped it surreptitiously on the seat of my breeches. Poley’s knock on a massive oak door was answered with a brisk, ‘Enter!’

I was thrust ahead of him into a spacious room where a man sat behind a great table covered with papers, with his back to a leaded window of clear glass. He was dressed entirely in black, except for a ruff so small and tightly pleated that it seemed hardly more than a collar. The long face, melancholy and haggard, with its perpetual worried frown and the eyes baggy – shadowed, it seemed, by late nights and long hours pouring over documents in poor candlelight – betrayed him at once as Sir Francis Walsingham, the Queen’s spymaster. I had never seen him close before, but once or twice my father had been called to attend him and he had pointed out the spymaster to me in the street. He was said to be just, patient, and ruthless, with a brilliant mind and an implacable will. With his colouring he might have come from Iberia like me. It was said the Queen called him, not altogether kindly, ‘my dark Moor’.

Walsingham fixed me with a stern and penetrating gaze.

‘You are Christoval Alvarez, son to Dr Baltasar Alvarez?’

‘I am, Sir Francis.’ My voice sounded thin and hoarse in my own ears.

‘I am told you know something of the breaking and devising of codes.’

‘A little, sir. It has never been more than an amusement for me.’

‘An amusement?’ He looked as though he did not know the meaning of the word. ‘Tell me what you know of the kinds and varieties of codes.’

‘Well,’ I searched my memory for what Harriot had taught me and what I had discovered for myself in books of the Arabs, which I had read laboriously with my father’s help, for I had never found that the Arab tongue came easily to me.

‘Probably the oldest code we know of was used by the Greeks,’ I began hesitantly. He nodded encouragingly. ‘They would wind a strip of cloth or parchment around a staff, then write the message across the strips. When the cloth was unwound the letters seemed a meaningless jumble, unless you had a staff of the same diameter. This kind of code is called a skutale, but it is really only a simple form of displacement code. If you write the letters in one long line, the message will consist of, perhaps, the first letter, then the fifth, ninth and so on, till you reach the end of the sequence. After that, it continues with the second letter, the sixth, the tenth. The displacement depends on the circumference of the staff.’

I glanced from Walsingham to Poley, to see if this was what they wanted of me. They were both listening carefully.

‘A skutale isn’t difficult to solve, you just need to try different displacements. Of course,’ I said slowly, for the thought had not occurred to me before, ‘if you were to use a tapered staff, it would be much more difficult, but then it would be difficult to have two staffs with exactly the same taper.’

I caught a sudden flash of movement from the corner. There was another man in the room, sitting withdrawn into the shadows, and the movement I had seen was a stray beam of light catching his spectacles as he leaned forward, resting his arms on his knees. He was small and slender, perhaps a little past thirty, with fair hair and a beard pale as cream. His face bore the traces of smallpox, but otherwise he was not ill-looking, despite his insignificant presence. I wondered who he could be, and why he watched me even more intently than the other men.

‘Another method of encoding,’ I said, as they seemed to expect me to continue, ‘is to use a grid of numbers.’

I stepped towards Walsingham’s desk. I can always think better on paper when it is a case of mathematics or puzzles or codes.

‘May I?’ I pointed to paper and quill, and Walsingham nodded.

‘Like this.’ I wrote the numbers one to five down the left hand side of the paper and again across the top.

‘If we take the letter I to stand for both I and J, then there are twenty-five letters in the English alphabet, and we can write them into the grid like this.’

I quickly wrote the letters from A to E down the first column, F to K down the second, and so on until I had filled the grid. Below it I wrote:

25 11 13 34 42 33 22 32 11 23

The third man had risen and peered over my shoulder.

‘It says Walsingham,’ he said.

‘Yes. Of course, if you run all the numbers together it makes it more confusing. And to make it more difficult to decipher you can start the alphabet in the second column, or go from left to right or bottom to top.’

‘Good,’ he said. ‘Do you know any more?’

‘If you cannot guess the pattern used to make the code, you can look at the frequency of the code letter or number in a message. For example, the most common letter in English is E, then S. But every language is different, so if the language is unknown, this is not such a useful method. It is better to try to find the key.’

‘Well, young Master Christoval,’ said Walsingham, ‘you seem to have some knowledge of codes.’

‘Oh, but there are many more!’ I was carried away by my own eagerness, and did not recognise my own impudence in interrupting.

‘See, if you draw these . . .’

I drew two vertical lines crossed by two horizontal lines, then a large X, then the pattern of lines again, then another X. I put a dot in each space of the last two drawings, then filled in all the spaces with the letters of the alphabet. This time there were twenty-six spaces, so that J was included.