The King's Spy (Thomas Hill Trilogy 1) by Swanston Andrew

Author:Swanston, Andrew [Swanston, Andrew]
Language: eng
Format: epub, mobi
ISBN: 9781446487273
Publisher: Transworld
Published: 2012-08-02T00:00:00+00:00

CHAPTER 9

ABRAHAM WAS BLUNT to the point of rudeness. ‘You will not give up, Thomas, until I say you can. And I am unlikely ever to do so. I have told the king that you are the most accomplished cryptographer in England, and you are not to embarrass me by quitting.’

‘Abraham, I’ve tried everything. The cipher is unbreakable. I cannot do it.’

‘You can, Thomas, and you will. There will be a way. Find it.’

Thomas sighed in resignation. ‘Oh very well, Abraham, for you I’ll keep trying. I wouldn’t do it for anyone else.’

‘Good. Go now and begin again. I wish to be informed the moment you have made progress.’

‘Rest assured that you will be the first to know if there is even a hint of it.’

For two days, he laboured on the message and his square, trying all manner of approaches – some logical, others absurd, all futile. None worked. On the third morning, he sat at his table and tried again.

Can there really be such a thing as an unbreakable cipher? he asked himself for the hundredth time. Monsieur Vigenère thought so, and so far he was right. Twenty-six possible substitutions for each letter, determined by an agreed keyword, from whose own letters the encryption row was taken. If the keyword were THOMAS, a single letter in the text might have been encrypted according to any of the rows starting with T, H, O, M, A or S. It would not respond to analysis of frequencies. Yet the more he thought about it, the more Thomas’s logical mind could not accept the concept of an unbreakable cipher. That the square had not yet been broken did not mean it never would be. Abraham was right. There would be a way.

Putting himself in the shoes of an encrypter, he wrote down five repetitions of his keyword THOMAS, and, underneath them, his ‘message’ in plain text.

T H OM A S T H O M A S T H O M A S T H OM A S T H OMA S

O N E E Y E I S B R O W N Y E T T H E O T H E R I S B L UE

Then, using the square he had already constructed, he en coded each letter of his message according to the letter above it, which indicated which row of the square to use. This gave him a third, enciphered line:

H U S Q Y W B Z P D O O G F S F T Z X V H T E J B Z P X UW

The recipient of this message would decrypt it by reversing the process, as long as he knew the keyword, or the sequence of rows to be used, which could, less securely, be communicated by numbers. But as the numbers had proved time-consuming and unhelpful, Thomas had decided that they were codes and that he would return to the letters. Two things stuck out from his encryption. Double letters

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