Crypto for Kids: Messenger's Story

A portal suddenly opened on the starboard ejecting a fleet of imperial pursuit vessels. The propulsion system of my ship got hit before the shield activated. I’ve tried to switch on the backup drive but before it charged to as much as 5% I was already dangling off a dozen tractor beams.

It wasn’t much of a fight. They’ve just came and picked me up as one would pick up a box of frozen strawberries in a supermarket.

I must have passed out because of pressure loss. The next thing I remember is being in a plain white room with my hands cuffed behind my back.

There was a sound of door opening and a person walked into my field of vision.

It took me few seconds to realize who the man was. He was wearing an old-fashioned black suit and a bowler hat. No sign of the baggy trousers seen on his official portraits. But then he smiled and showed the glistening golden teeth on the left side and his own healthy camel-like teeth on the right and the realization hit me.

It was him. Beylerbey Qgdzzxoglu in person.

“Peace be upon you,” he said. He sat down at the other side of the coffee table, made himself comfortable and put his umbrella on the floor.

“We have a little matter to discuss, you and I,” he said.

He took a paper out of his pocket and put in on the coffee table, spinning it so that I can read it.

“Attack the Phlesmus Pashalik,” said one line.

“Attack the Iconium Cluster,” said the line below it.

The rest of the sheet was empty except for holographic seal of the High Command of Proximian Insurgency.

"Comandante Ribeira is no idiot," he said, "And this scrap of paper is not going to convince me that he's going to split his forces and attack both those places at the same time."

He looked at me and continued: "Our strategic machines are vastly more powerful than Proximian ones. They've been running hot for the past week and our lab rats tell us that there's no way to win that way."

The mention of the lab rats was weird but I guessed he had just meant the scientists.

"You are right, O leader of men," I said.

I knew that this kind of empty flattery was used at the Sublime Porte but I was not sure whether it wasn't reserved for the sultan alone. Anyway, it was worth a try.

Qgdzzxoglu smiled snarkily but haven't said anything. Maybe I was going to live in the end.

"I have no loyalty for the Proximian cause. Before I was conscripted at a gunpoint I have lived happily and had no thoughts of betrayal. So now, hoping for your mercy, I am going to cast away all deception and disclose the true meaning of this message to you."

"It is a code, O you, whose slipper weights heavily upon the neck of nations," I said.

Qgdzzxoglu raised his eyebrows a little. I took that as an invitation to continue.

"The recipient is supposed to ignore the first command and only follow the second one."

I hoped I haven't overdone it. Being honest with the enemy is hard.

He took the paper from the table and looked at it intently:

"So you are trying to convince me that de Ribeira is going to attack Iconium."

He gave me a sharp look, apparently trying to determine whether I was lying or not.

"You know what?' he said: "It's not like we are completely blind. We have spies in the Insurgency and we've got our reports. And our reports say that rebels will try to trick us into moving all our forces to Iconium and then attack the pashalik while it's undefended. And if that's what you are trying to do bad things are going to happen to your proboscis."

Yikes. If he believes that I am here to plant false information this is not going to be fun. But, on the other hand, he may be bluffing, trying to confuse me and make me give some hints about whether I was lying.

I've lowered my head: "I have no intention to suffer to win Ribeira's war. But the truth is truth and your sources are wrong, O mighty. The Most Holy Cross, the John XXIII and Our Lady of Africa have already got a command to move to Iconium cluster. And you should expect at least comparable fire power coming from elsewhere, even if this particular message is never delivered."

"That's exactly what you'd say if you wanted me to leave Phlesmus undefended." He tapped his cheek: "I wonder whether I should torture you a little. Just in case."

The honesty, apparently, wasn't working. I had to try a new strategy. This one was going to be risky. But who cared. I was in a pretty deep trouble either way.

"Wait, your excellence!" I exclaimed: "You were right! The code is to follow on the first command and ignore the second one! Ribeira is going to attack Phlesmus!"

Qgdzzxoglu looked surprised: "You were lying before?"

"Yes! Please don't torture me! It's going to be Phlesmus pashalik!"

"Coward," beylerbey said with disgust.

He called on the guards to escort me, presumably to the torture chamber.

"No! I was telling truth all along! It's Iconium cluster that's going to be attacked!"

Qgdzzxoglu picked up his umbrella and stood up, frowning. This was my last chance.

"As you can see," I said and he, being already on his leave, turned back, surprised by sudden calmness in my voice: "I am willing to admit anything. I am happy to say that it's Phlesmus that's going to get attacked. I am all right saying that it's Iconium. If I was here to nudge you to a particular course of action I wouldn't do that. Would I?"

That made him pause for a second.

"What is the message saying then?" he asked.

"They are going to attack Iconium cluster, as I've told you in the begining."

"You may have said all that you did just to escape the torture."

"You've already noticed that I am not the blabbering idiot I pretended to be to get your attention. If I was scared to death by the threat of torture I would still be trying to come up with an answer that would satisfy you, no matter whether it would be true or false."

"Hmm," he said: "But you may be also specially trained in psychological manipulation and the entire show was staged to convince me that you are honest. Let me think about it for a moment."

He sat down again, closed his eyes and dived into meditation. There was nothing more I could do. I kept silent, leaving him alone with his thoughts.

He awakened from the stupor after a minute or so.

"Today," he said, maybe a little bit bitterly: "The God, the Compassionate, the Merciful have taught me that there are secrets that cannot be given away. You cannot give them away to save yourself from torture. You cannot give them away to save your kids from being sold to slavery. You cannot give them away to prevent the end of the world. You cannot give them away and whether you want to or not matters little."

He summoned the guards and I was dragged away towards my uncertain fate.


I've been trying to write a book about cryptography for kids for some time now. The idea was not to focus too much on the maths (there are enough such books already) but rather on the kind of twisted thinking you need when dealing with cryptography. The story, for example, illustrates Claude Shannon's concept of perfect secrecy. If you look closely you can even see a simple 1-bit one-time-pad.

However, I've been slacking it for quite a while. By publishing individual chapters on this blog I am trying to incentivize myself to put more work into it. Blog has more visibility than a repository with the text. It's also more interactive. Please do comment, extol, criticize. Even better, propose ideas of your own to contribute to the topic.

Martin Sústrik, December 28th, 2017

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