Agent 21: Codebreaker: Book 3 (3 page)

Zak nodded. He was feeling cross with his Guardian Angels, but he didn’t know why. It wasn’t like the bombing of Coventry more than seventy years ago was their fault, but he couldn’t help wondering whether they – or their handler Michael, whose office this was but who never seemed to appear unless he had a job for Zak – would consider
him
dispensable if it meant saving others. Michael viewed the world a bit differently to most people, after all.

Zak put the thought from his mind. He was being childish. Not for the first time, he reminded himself that he had chosen this life. Just like the other young agents, one of whom he had already met in the field, had. Nobody had forced any of them into it. Though, he mused, it was hard to leave once in. Apart from anything else, he was officially dead, buried in a cemetery in north London, where he had once lived.

‘With all that in mind, I’m now going to teach you a perfect cipher,’ Gabs said. ‘Easy to use, impossible to crack, even with infinite computing power.’

Zak gave Gabs a sceptical look. They had already spent the previous day working on codes – everything from Morse codes to written substitution codes or number codes. Why would this be any different?

‘That doesn’t sound very likely to me, Gabs,’ he said as he retook his place at the table. He knew something about computers too; he had faith in their abilities.

There was a piece of blank paper on the table, and a pencil. Gabs started to write with a twinkle in her eye. ‘Trust me,’ she said. ‘This method is called the one-time pad. It was used by the Special Operations Executive during the Second World War and any sensitive military unit will be trained in some form of this. It’s been mathematically proven to be unbreakable so long as you use it correctly.’ She continued scribbling for a minute, then passed the sheet of paper over to Zak. Along the top of the paper, Gabs had written the alphabet, with a number below each letter.

Below this, she had written what looked like random groups of letters:

‘Each number represents a letter, OK?’

Zak nodded.

‘The random letters below – the line beginning with a “J” – are the key, and it has to be as long as, or longer than, the message you want to send. The person writing the code and the person deciphering it need this key. Let’s say we want to send an order to advance. First of all, you write the message directly underneath the code.’

‘Then you convert the letters to numbers and add them up. If the number you get is more than twenty-five, you count from zero again. See? The third letter is seven plus twenty-one, which equals twenty-eight: that’s twenty-five plus the zero plus two. So the decoded number to turn into a letter is a two.’

‘Finally, convert these final numbers back into letters.’ Gabs did this and wrote them down.

‘So, using the random key JEHFYJD, the word ADVANCE becomes JHCFLLH. To decipher it, you simply need to do the same thing in reverse – convert the coded message and the cipher to numbers and
subtract
the cipher number from the message number. Don’t forget to include the zero if the top number needs to be increased by twenty-five. Look at the third number again. Two minus seven. Add the twenty-five and the zero – that’s twenty-six numbers in total – to the two and you get twenty-eight minus seven. Twenty-one. V. Got it?’

Zak stared at the numbers and letters on the page. It seemed too straightforward to be unbreakable.

It was almost like Gabs was reading his mind. ‘Most letter-based ciphers can be broken because we know how often certain letters appear in the English language. The beauty of the one-time pad is that the same letter in the original message can become different letters when it’s enciphered. Look – the word ADVANCE has two As in it – the first one has become a J, the second one has become an F.’ Gabs’s eyes twinkled. ‘I once told a boy I’d go out with him if he cracked a code I’d written with a one-time pad. He thought he was a bit of a brainbox, see. Bit unfair, really, with it being impossible and all, but I didn’t really fancy him anyway.’ She winked at Zak.

‘So,’ she continued, ‘the killer feature of the one-time pad is that any letter can be turned into any other, depending on the key, which makes it impossible for the enemy to guess what you’re saying. Try deciphering our encoded message of the word ADVANCE – JHCFLLH – using this random key instead.’ Gabs handed him a piece of paper with the following letters: SDJOHLO

Zak carefully deciphered the original code using this new key. To his surprise, it revealed a very different message: RETREAT.

Gabs smiled at his obvious astonishment. ‘It’s not perfect, of course,’ she said. ‘You can only use the cipher key once, and because it has to be a long, random string of letters, it’s practically impossible to remember so both people using it have to write it down. But as long as the key doesn’t fall into enemy hands, the one-time pad is completely secure.’

Zak took a few moments to absorb all of that, then he looked up at Gabs and grinned. ‘Nice,’ he said.

‘The one-time pad is unbreakable, but awkward to use. There are other similar methods we’ll teach you that sacrifice a little security for a lot of convenience, but the important thing to remember is this: don’t get so blinded by technology that you forget the simpler ways of doing things. Sometimes they’re the best.’

‘And it’s sobering to think,’ Raf added, ‘that if the Germans had used one-time pads instead of Enigma, it might not just have been Coventry that fell, but Britain too.’

‘Sobering indeed,’ a new voice announced.

Zak looked around sharply. The new voice had no body, and it seemed to echo slightly around the room.

‘I’m glad to see that Gabriella and Raphael are not neglecting your history tuition. The past can tell us a great deal about the future, Zak. We’re in a privileged position, if we can learn from other people’s mistakes. That’s a charming hairpin, if I may say so, Gabriella.’

Zak realized that the new voice came from circular speakers embedded in the ceiling, and he recognized it as belonging to Michael, the mysterious old man who had recruited him in the first place and who controlled the missions he was sent on. He noticed that Raf’s and Gabs’s faces instantly grew sharp. Gabs stood up, walked towards the door and flicked a switch on the wall. A hatch in the ceiling that Zak had never noticed before opened up, and a screen descended silently along the oak-panelled wall opposite the windows. Michael was on-camera, sitting behind an empty glass desk in front of a bare beige wall, with no clue as to where he was in reality. Zak had no idea where his handler went when he wasn’t here on St Peter’s Crag; he was pretty sure that Michael intended to keep it that way.

‘Your studies are going well?’ Michael asked.

Zak nodded.

If Michael noticed that he was quieter than normal, he didn’t mention it. He just smiled again before continuing to speak. ‘I wonder if I might ask you to switch on the television?’ he said. It was framed as a polite question, but of course it was anything but.

The TV was in the corner of the room. It was almost never switched on, but now Raf strode over, pressed a button on the side and it flickered into life. And for the next couple of minutes, the sound of the television filled the room, and silenced its occupants.

‘
You join us at the scene of the devastating blast at Pimlico underground station
.’ The young news reporter was slightly wild in the eyes, and breathless. ‘
Rescue workers are still trying to gain access to the platform where it is thought that an explosive device was triggered by the arrival of the first train, a little before six o’clock this morning. As yet, no contact has been made with the driver or any of the passengers. It appears unlikely, however, that there are any survivors
.’

The camera panned round to a scene of absolute confusion. The whole area was cordoned off and there were countless emergency vehicles. Zak caught sight of grim-faced men in yellow hard hats disappearing down the underpass that led into the station, and a crowd of anxious onlookers had congregated nearby, some of them weeping.

‘My information is a little more up-to-date than our delightful correspondent’s,’ Michael said. ‘I think we’ve seen enough, Raf?’

Raf switched the off TV, and they all looked back up to the screen that showed Michael’s face.

‘My sources tell me that the arrival of the first Victoria Line train into Pimlico triggered the detonation of approximately twenty-five kilograms of C4 plastic explosive.’

Gabs gave a low whistle.

‘My thoughts exactly, Gabriella. If there are any survivors, it will be nothing short of a miracle.’

The screen flickered. Michael disappeared and was replaced by some grainy camera footage. It took a moment for Zak to realize that he was looking at CCTV footage of Pimlico Station. The time code at the bottom of the screen told him that the footage was taken at 0231hrs. The platform was empty.

The screen juddered. Zak immediately noticed that the timecode had changed to 0145hrs.

‘Hey, what just happened?’

‘Oldest trick in the book,’ Gabs murmured. ‘Record some innocent CCTV footage, then superimpose it over the live feed when you want to camouflage yourself. Chances of someone noticing are tiny.’ She raised her voice and addressed Michael. ‘I take it, then, that the device was planted between 0145 hours and 0230 hours? Forty-five minutes isn’t long for a job like that. Whoever it was
really
knew what they were doing.’