Andrew Swanston - The King's Spy

‘This is an encrypted message, received three months ago. If you were the enemy, how long would it take you to decrypt it?’

Thomas looked at it. It was six lines long, each line consisting of about fifty seemingly random letters. ‘This is about the right length for a military report, rather than a battlefield order. It looks like a substitution cipher, probably with a keyword. A simple shift would be too easy, and a cipher alphabet would be written down somewhere, which would make it vulnerable to capture. So I’d assume a keyword. With only one encrypted text to work with, perhaps a morning.’

‘That’s what I thought. Not secure from the attentions of a good cryptographer.’ Abraham passed over another sheet. ‘What about this one?’

Again, Thomas studied the sheet for several minutes. ‘This message is shorter. It could be a military order. I guess that there are some coded words. Not knowing the context, I’d go about it as with the first message — frequencies and letter relationships — but if there are codes it would take longer.’

Abraham smiled. ‘Your instincts are as good as ever, Thomas. Now what about this one?’

Immediately Thomas said, ‘This is alpha-numeric.’ He imagined lines running through each number. ‘I think some of the numbers are nulls because there’s a pattern to them, but the others could be codes.’

‘Excellent. Now take these and practise your skills on them. Note that they may not all be what they seem.’ Abraham passed over the remaining papers. ‘Some of these are ours, others were intercepted. All incoming and outgoing messages will now be passed through me to you, for encryption and decryption. Next week you will need to send out the first half of a new keyword. Here is a list of all recipients. Please commit them to memory and destroy the list.’

‘And if I don’t, sir? Will I be confined to college?’

Abraham could hear the raised eyebrows and the mocking smile, even though he could not see them.

Armed with a handful of good duck-feather quills, a sharpening knife and a large pot of oak-apple ink supplied by Silas, Thomas set to work, starting with the simple task of memorizing the names of the twenty recipients of messages. He used a memory trick based on the vowels in each name which Abraham had taught him years ago, and most names surrendered without much of a fight. Within half an hour, he had them all.

Next he counted the papers. There were twenty of them. Deciding to tackle them in the order in which Abraham had put them, he took the first one from the top of the pile. Twenty lines of ten letters each, written in a neat hand in brown ink and with spaces between every four or six letters, covered one side of the paper. Thomas held it up to the light, looking for unusual marks or letter formations. There were none. He smiled. Kindly old Abraham had given him a simple substitution cipher to start with. On a blank sheet, he prepared a table with each letter of the alphabet across the top. Then he counted the number of times each letter occurred in the message, and wrote the number below it. He found that the letters C, F, and P appeared most often. They would probably represent E, A, and T, although not necessarily in that order. They were the three most commonly used letters in the alphabet, and if he could identify those he would be on the way to breaking the cipher. As C and P were preceded and followed by fifteen different letters, but F by only eight, F would represent T, which would appear between fewer letters than either E or A. Ignoring the spaces, which were merely intended to confuse, there were six instances of double Cs, but none of double Ps. C would represent E, leaving P as A.

Continuing with this strategy, Thomas quickly identified ten letters, which revealed the common words AND, A and THE, and parts of other words. With a little intuition and guesswork, he had found the keyword, ADVANCE, and decrypted the message within an hour. It revealed that Sir John Berkeley, with modest help from Sir Bevil Grenville and Sir Ralph Hopton, had defeated a strong Parliamentary force at Braddock Down in Cornwall, capturing cannon and muskets. Reading the plain text, Thomas wondered why Sir John had troubled to have the message encrypted. The gallant knight clearly wanted the king, his court and all his subjects to know of his valour and the great victory it had brought. No doubt there had been similar messages from the other two gentlemen.

Thomas moved on to the second paper on the pile. Roughly the same length as the first, this one was also a mixture of letters and spaces. As before, he checked it for hidden signs, found none, wrote out each letter of the alphabet across the top of a blank page, and put under each one the number of times it appeared. When this produced a more even distribution than he expected, Thomas suspected he was facing a more complex cipher. Twenty minutes later, he knew he was right. Two substitutions had been used alternately, and he had to find both, just as he had shown Simon in the inn. After another hour, Thomas had identified fifteen letters, which was enough for him to fill in the gaps and write out both substitution alphabets. The substitutions were random sequences of letters, which made them more difficult to decrypt, but both sender and recipient would have had copies, making them vulnerable to discovery.

A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

P L O M K I N J U B H Y V G T C F R X D E Z S W A Q

L A M Z P Q K S N X B C V J D F H G W I O R Y T EU

It was an intercepted message, revealing that in March Sir Thomas Fairfax had been concerned about his troops’ morale and had asked for them to be paid without further delay. As the message had failed to reach its intended destination, Thomas assumed that Sir Thomas’s troops had remained unpaid.

By the end of the first day, Thomas had decrypted seven complete documents. They were all alphabetic ciphers, using mixtures of single and double substitutions, and keywords. His eyes and back ached, his legs were stiff and he needed refreshment. He was about to go off in search of food and drink when there was a loud knock on the door. He opened it to find Tobias Rush outside, silver-topped cane in hand and, as at court, dressed all in black.

‘Master Hill, I find you hard at work no doubt. I trust I’m not disturbing you. I merely wondered if I could be of any assistance.’

Caught off balance, Thomas was less than articulate. ‘Master Rush. Good evening. No, no disturbance. I’ve just finished for the day, and was about to take some air and stretch my legs.’

‘In that case,’ replied Rush, smiling his thin smile, ‘perhaps I may accompany you. You can tell me how you’re progressing.’ Without waiting for an answer, he turned and strode out into the courtyard. Thomas locked his door and followed. ‘I do admire Pembroke,’ said Rush, as they picked their way through the debris towards the college gate. ‘A lovely building, and attractively small, although I see the officers here have paid scant regard to its care. Alas, it’s the same everywhere. Military mess and careless destruction.’

‘Yes,’ agreed Thomas, ‘there is something blinkered about the military mind. It seems able to ignore almost anything other than itself.’

Rush laughed. ‘Nicely put, Master Hill. Let us pray that this war is soon over, so that the university can resume its former life.’ They left the college and turned north up St Aldate’s. ‘And how does your work progress?’

‘It has not been arduous. So far, Abraham has given me quite simple tasks, although I expect them to get harder.’

‘Good. On Master Fletcher’s advice, the king has put his absolute trust in you. If we can anticipate the enemy’s movements while leaving him ignorant of ours, we shall gain a great advantage. You have a vital duty to perform.’