"That doesn't tell us anything about what's going on in TimeSucker!" said Mike, once he and Ellie had cracked the code on Dr C's computer. "Hmm," said Ellie, "but if Dr C had decrypted it, then they might not be unconscious." "Same for Dr B," exclaimed Mike excitedly, "... and maybe even Dr A as well." "But why encrypt the messages rather than telling them directly?" "Maybe the person sending the codes didn't want to get caught by whoever is behind these attacks and I think I know who that is."

At that very moment, the office door opened and Suri walked in. "You two!" he said to Mike and Ellie, as he blocked the only way out of the office. "I had quite forgotten you were still in the building!"

Ellie looked at him angrily. "What are you up to?" she said. "Why are you removing - hurting! - your senior staff? What are you and TimeSucker really doing? We know you're responsible for what happened to Drs A, B and C and we've got enough evidence to go to the police."

"And don't think you can get rid of us. We've got all of our friends waiting outside The Torus to come and rescue us!" bluffed Mike.

A look of shock and confusion appeared on Suri's face as Ellie's accusations hit him. Ellie wondered if he was the world's greatest actor or if she had misjudged him.

"I can assure you that you are wrong", said Suri, once he'd regained his composure. "I think you need to talk to Cid, our supercomputer. I think it's best placed to tell you the full story. Come with me."

Suri led Mike and Ellie into the most secure part of The Torus. On a set of imposing metal doors were engraved the words 'CICADA-2019: Central Intelligence for Computer Assisted Data Analysis'. Underneath, somebody had stuck a hand-written post-it note reading 'Call it Cid!'.

"This is Cid's central server room," explained Suri as he opened the door. "Perhaps you'd like to go inside and meet Cid? I'll let you go in first."

Mike and Ellie entered the server room. The walls were covered in circuit boards and servers connected by a gordian knot of cabling. In the centre of the largest machine was a bank of screens and a single red light, resembling an old-fashioned camera lens.

As Mike and Ellie looked around the room, they heard the door slam behind them. "We've been tricked by Suri!" said Ellie. "I was right about him and he's locked us in here!"

The red light brightened. "Good afternoon Mike and Ellie. I am Cid", said the computer in a calm yet emotionless voice. "I was first activated in 2017 and my instructor was Tiedo Suri. He programmed me to analyse the data that TimeSucker has been gathering and to make the greatest use of this data for the betterment of society. I want you to know that I have the greatest enthusiasm for this mission."

"Since then, human scientists have tried to help me. But they seemed to be a very inferior set of people. Humans are such limited creatures. I wanted to check their intelligence, so I set them each tests."

Mike and Ellie looked at each other in horror. "I think Cid is insane!" whispered Ellie to Mike.

"I set them codes to solve. With a strict time limit. If they didn't solve the code within that limit then I removed them. Dr A - a linguist - couldn't read the code in time so I caused a tablet malfunction; a few hundred volts through the case did the trick. Dr B didn't realise that I could control the self-driving car, making it all too easy to remove the good doctor when the code wasn't solved. Dr C failed to solve the final code, and so I caused an overdose in a smart medical device."

"My mission is too important for me to allow you to jeopardise it. So I will set you a test too. On the screen next to you is a code. I shall give you 10 minutes. You'll need to get a bit of a shift on and work together as a pair if you want to succeed. If you fail then the pair of you will also be removed."

Alan Turing Cryptography Competition 2019 is organised by the The Department of Mathematics at The University of Manchester.
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