In Abingdon, after a day of late-summer showers, they found another inn with a room, this one busy and noisy. They sat quietly in front of the fire drying themselves, eating onion soup and rough bread and listening to the talk around them. It was about little other than the war. Once their tongues were loosened, the drinkers spoke freely, ignoring the two friars in the corner. In some parts of England feared and abominated, here it seemed priests were simply ignored. Perhaps it was the queen’s influence. After all, a country from which priests had been banished but which had a catholic queen with catholic courtiers made little sense. ‘King or Parliament — do I have to choose?’ asked one. ‘How can I? Which one will put food on my table and clothes on my back? I don’t know.’
‘Better stay out of it then,’ replied another. ‘Keep mending shoes. It’s safer.’
‘I’d rather support the king,’ offered a third, ‘only not this king. A lame Scot who cares nothing for us. It’s Queen Bess’s fault. No heirs. We should find a better king.’
‘Hush, William,’ hissed the first man, ‘you could lose your head for saying such things. And anyway, where do we find another king? We don’t want a Frenchman or a Dutchman, do we?’
For the first time, a large man, black-bearded and deep-voiced, spoke. ‘We should do what they’re doing in Cornwall. Organize ourselves to defend our homes and families from both sides. Arm ourselves with whatever we can find and frighten off any who approach the town. Kill them if we have to.’
‘Jeb, do we really need another army? Aren’t two enough?’ asked the first man.
‘Maybe that’s just what we need. Show them what we really care about. Our wives, children, land, homes. Food to eat and ale to drink. Not who sits on his arse in Parliament, nor who wipes the king’s. That makes no difference to us. Fight fire with fire, I say.’
‘And I say we should keep quiet and wait for peace,’ said the one called William.
‘And what good will peace be if our women have been raped and our homes torched?’ demanded Jeb. ‘Tell me that.’
Thomas and Simon sat and listened until the drinkers had left. Then Thomas said, ‘This is a strange war, don’t you think? Who wins or loses seems less important than how long it goes on and what happens afterwards.’
‘Strange indeed, Thomas.’ Simon poured his companion a glass of wine, and went on, ‘Now this seems an excellent oppor tunity for you to enlighten me on the matter of codes, a subject about which I know next to nothing.’
Thomas feigned surprise. ‘Oh, come now, a man who can tell a shaggy inkcap from a poisonous fungus has surely studied cryptography. And you are a Catholic friar.’
‘How is my faith relevant to codes, Thomas?’
‘Hmm. If you really want to understand the subject, I shall have to start with a history lesson.’
‘I am listening, Thomas. Please begin the lesson.’
‘Very well. The use of codes and ciphers is almost as old as warfare itself. The Persians and Greeks used them, and a very well-known cipher is named after Julius Caesar, who used it frequently when he was on campaign. Would you like me to show you how it works? It’s very simple.’
‘I would.’
‘Then find paper and ink, and a quill.’
Simon disappeared and came back within a few minutes clutching them. ‘The landlord was surprisingly obliging once I’d put a sovereign in his hand.’ He put the things on the table.
Thomas took the quill and wrote two lines on the paper.
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
S I M O N A B C D E F G H J K L P Q R T U V W X Y Z
He turned the paper around for Simon to see. ‘This is a cipher encrypted with a Caesar shift, using SIMON as the keyword. The cipher begins with the letters of your name, which do not then appear where they should in the alphabet. In a message encrypted using this keyword, for example, the letter C would appear as M. As I said, simple. But also simple to decrypt, using routine techniques.’
‘Why is it a cipher, not a code?’
‘When an individual letter is replaced by something else, it is a cipher. When a word or phrase is replaced, it is a code. You will have already realized that any number of variations on this cipher are possible.’
‘Will I?’
‘I imagine so. The alphabet could continue where the keyword leaves off. In that case, the sequence would be S I M O N P Q R T U V W X Y Z A B C, and so on.’
‘And what, pray, is the relevance of Catholicism?’
‘The Vatican has used ciphers for centuries, and has produced some of the most important cryptographers. About two hundred years ago, a certain Leon Alberti thought up the idea of using two cipher alphabets at once, and alternating between them.’ Again Thomas took the quill and wrote on the paper, adding a third line of letters.
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
S I M O N A B C D E F G H J K L P Q R T U V W X Y Z
R O M S E Y A B C D F G H I J K L N P Q T U V W X Z
‘Using this cipher, the word THE would be encrypted as TBN.’
‘Looks devilish, even if he was a Christian.’
‘Not so devilish. An experienced cryptanalyst will recognize a double or even a triple substitution without much difficulty. But Alberti’s work also led to the develop ment of the Vigenère cipher, which is so devilish that it has never been broken.’
‘How does the Vigenère cipher work?’
‘You’re not ready for that, Simon. Next time, perhaps.’
‘As you wish. What else should I know?’
‘You should know that messages are often encrypted using what are known as homophonic substitutions.’
‘Homophonic. Something that sounds the same as something else,’ said Simon with a grin.
‘Exactly. A number or a symbol which represents a letter. Numerical ciphers are common — each number representing a different letter. Encrypted texts may also include nulls, which are no more than meaningless symbols or letters designed to confuse. Deliberate variations in spelling are used for the same reason.’
‘Wouldn’t the recipient of the message also have to know about these?’
‘Well done, Simon, he would. If, for example, each letter in a text is replaced by a single number, no more than twenty-six numbers are needed. So if all the numbers between 0 and 49 appear, twenty-three of them will be nulls. However, those that are nulls must be memorized or written down, and that is also the weakness of codes, as opposed to ciphers. Codes are almost always written down somewhere. If they are easy enough to remember, they are also easy enough to decode, and therefore vulnerable.’
‘Keywords, double and triple alphabetic ciphers, homophonic ciphers, numerical substitutions, nulls, spelling mistakes, what else?’
‘Much else, Simon, including Monsieur Vigenère. Oh, and nomenclators, of course.’
‘What are they?’
‘A nomenclator is a mixture of code and cipher. The messages smuggled to and from the Queen of Scots when she was held prisoner were nomenclators. It was a priest who smuggled them in. His name was Gifford. Cunning fellows, priests. I can remember the numbers used.’ Again Thomas wrote on the paper.
2 = AND, 3 = FOR, 4 = WITH, 7 = X, 8 = Y, 9 = Z
‘And there were shapes and Greek letters for individual letters and words. It took an odd and very clever little man named Thomas Phelippes to decrypt it. If he hadn’t, who knows who would be on the throne now?’
‘How did he do it?’
‘It took him some time and much effort, but eventually he did it in the way most ciphers are decrypted. The Queen of Scots and her supporters did not know that their messages were being read, and with enough texts he was able to analyse how many times each symbol or number appeared and its position in relation to all the other symbols and numbers. In English, the most common letter, E, is about twice as common as S and six times as common as F. Each letter can be ranked according to the frequency of its usage. Despite the nulls and misspellings, Phelippes worked the cipher out, and proved that the Queen of Scots was plotting to murder Queen Elizabeth. It’s the way we cryptographers work. Much hard work, a little imagination and the occasional guess.’