The Moses Stone

do know about. The good news is that we're dealing with an object that dates back around two millennia. And that's good because although there are very few known examples of codes and ciphers from that period of history, those that we do know about are very, very simple. The best known was probably the Caesar Cipher, which was allegedly used by Julius Caesar in the first century BC to communicate with his generals. It's a really basic monoalphabetic substitution cipher.'

Bronson sighed. He knew that Angela had done some research on cryptology as part of a project at the museum. 'Don't forget I'm just a simple copper. You're the one with the brains.'

Angela laughed. 'Now why don't I quite believe you?' She took a deep breath. 'To use a Caesar Cipher, you write out the message as a plaintext, apply whatever shift you've selected to the alphabet, and then transcribe the enciphered message.'

Bronson still looked blank, so Angela moved her empty plate to one side and took a piece of paper and ballpoint from her handbag.

'Let me give you an example. Say your message is 'move forward',' she said, writing the words in capital letters on the paper, 'and the shift is left three. You write out the alphabet, then write it out again underneath, but this time you move each letter three places to the left, a socalled left rotation of three. So you'd find 'A' directly above 'D', 'B' above 'E' and so on. In this case, the enciphered message 'move forward' would read 'pryh iruzdug'. The obvious problem with this method is that every time a particular letter appears in the plaintext, the same enciphered letter will be in the coded message. So in this example, which is only two words in length, two letters – the 'R' and 'U' – are repeated, and somebody trying to decrypt the message can use frequency analysis to crack it.'

She looked hopefully at Bronson, who shook his head. 'Sorry, you'll need to explain that as well.'

'Right,' Angela said. 'Frequency analysis is a simple method of cracking a basic code. The twelve most common letters in the English language, in order, are 'E', 'T', 'A', 'O', 'I', 'N', 'S', 'H', 'R', 'D', 'L' and 'U'. I remember it as two words – 'ETAOIN SHRDLU'. And you probably already know the most famous example of a Caesar Cipher.'

'I do?' Bronson looked blank and shook his head. 'Help me out here.'

'2001,' Angela said, and sat back in her chair. '2001 – A Space Odyssey. The sci-fi film,' she added.

Bronson frowned, then his expression cleared. 'Got you,' he said. 'The film-makers didn't want to use the acronym 'IBM' for the computer on the spaceship, so they came up with the name 'HAL', which, if I've understood you correctly, is a Caesar Cipher with a right rotation of one.'

'Exactly. There's another slightly bizarre example,' Angela said. 'The French 'oui' becomes the English 'yes' if you apply a left rotation of ten.'

'Do you think anything like that is probable in this case?'

'No,' Angela replied, 'and for one very simple reason: we can read the Aramaic words on the tablet. One of the obvious problems with a Caesar Cipher is that every word of the enciphered text is invariably gibberish, which is the biggest clue that the message is encrypted. That definitely isn't the case here.'

'What about other kinds of ciphers?' Bronson asked.

'You've got the same problem with all of them. If the individual words are encrypted, they cease to be recognizable as words and end up as collections of letters. The Aramaic words on this tablet' – she tapped the paper in front of Bronson – 'aren't encrypted. But that doesn't mean there isn't some kind of message hidden in the text.'

'You need to explain that,' he said, 'but wait until we're back on the road.'

'Just wait in here a second,' Bronson said as they reached the door of the restaurant. 'I want to check there's nobody waiting for us out there. Then I'll bring the car over.'

Angela watched him walking around the handful of cars parked outside, glancing into each of them, then stepped through the door as Bronson pulled the hire car to a halt just outside.

'So if the words aren't encrypted, how can there be a message in the text?' he asked, as he pulled out on to the main road.

'Instead of an alphabetic substitution, you can use word substitution. You choose particular words to mean something completely different. The Islamic terrorist groups have been doing this for quite some time. Instead of saying something like 'We will plant the bomb at three this afternoon' they say 'We will deliver the fruit at three this afternoon'.'

'So the sentence still makes sense, but the apparent meaning is entirely different to its real meaning,' Bronson said.

'Exactly. Shortly before the attack on the World Trade Center, the lead terrorist, Mohammed Atta, contacted his controller and passed him a message that made no sense to the American security forces at the time. He used a sentence that included a phrase something like 'plate with one stick down, two sticks'. With a bit of imagination, you can see that he meant the numbers '9' and '11'. He was actually telling his al-Qaeda contact the exact date when the attacks on America were going to take place.'

'And on this tablet?'

Angela shook her head in the darkness of the car, the headlamps boring a tunnel of light down the almost empty road in front of them. 'I don't think there's anything like that incorporated in the text, simply because the sentences don't make sense.'

She paused as she looked through the side window at the clear night sky. Casablanca was now several miles behind them, and away from the light pollution of the city, the stars looked brighter and closer, and far more numerous, than she'd ever seen them. She glanced back at Bronson, glimpsing his strong profile in the faint light cast by the jade-green illumination of the dashboard instruments.

'But there's one possibility that we haven't even considered,' she said.

25

Izzat Zebari waited until after one in the morning, when the lights in the house had been switched off for well over an hour, before he walked across to the double steel gates and lobbed two large raw steaks into the compound beyond. Melting back into the darkness, he heard a low growl and the swift patter of clawed feet as the two guard dogs ran out of their kennels to investigate the intrusion.

'How long before it works?' Hammad asked, as Zebari slid down into the passenger seat of the car they'd parked in a deserted side-street about a hundred metres away.

Hammad would deal with any burglar alarms or other electronic devices they encountered at the property. On the floor beside him was a small fabric case that held specialist tools and other equipment. Zebari knew that because Hammad had opened and checked the contents at least six times since they'd returned to the car. They'd cautiously walked down from the hillside just after darkness fell, and had been waiting in the vehicle ever since.

'Half an hour should do it,' Zebari replied. 'We just have to wait for the drugs to do their work. My chemist friend calculated the dose very carefully.'

Zebari waited another forty-five minutes before he gave the order to move. They climbed out of the car, easing the doors closed as carefully and quietly as they could, then opened the boot to remove the rest of their equipment. The biggest single item was a collapsible ladder long enough to reach the top of the boundary wall of the property.

Minutes later, they were crouched beside the wall, their all-black clothing making them almost invisible in the darkness. Swiftly Hammad and Zebari assembled the ladder, quietly slotting the sections together, then rested the base on the ground. The other end of the ladder was padded with cloth, and made no sound as Zebari leant it against the top of the wall.

'OK, up you go,' Zebari whispered.

Hammad climbed silently almost to the top, where he carefully examined the wall. He shone a pencil torch in both directions along it, the narrow beam barely visible. Next, he removed a spray can from his fabric bag and depressed the nozzle, aiming the jet at the area directly above the top of the wall, at the point where they would have to climb over it. Then he descended again.