This is a writeup for the Emoji voting challenge, part of the Hack the box's Cyberapocalypse CTF 2021, category Web.

Prompt​

A place to vote your favourite and least favourite puny human emojis!

This is a writeup for the Soulcrabber challenge, part of the Hack the box's Cyberapocalypse CTF 2021, category Crypto.

Propmpt​

Aliens heard of this cool newer language called Rust, and hoped the safety it offers could be used to improve their stream cipher.

This is a writeup for the Wild goose hunt challenge, part of the Hack the box's Cyberapocalypse CTF 2021, category Web.

Prompt​

Outdated Alien technology has been found by the human resistance. The system might contain sensitive information that could be of use to us. Our experts are trying to find a way into the system. Can you help?

This is a writeup for the challenge Phasestream 1, part of the Hack the box's Cyberapocalypse CTF 2021, category Crypto.

Phasestream 1​

In the first challenge, we're given a hex code, and the knowledge that it's been encoded using XOR with a 5-byte long key.

This is a writeup for the challenge Phasestream 2, part of the Hack the box's Cyberapocalypse CTF 2021, category Crypto.

Prompt​

The aliens have learned of a new concept called "security by obscurity". Fortunately for us they think it is a great idea and not a description of a common mistake. We've intercepted some alien comms and think they are XORing flags with a single-byte key and hiding the result inside 9999 lines of random data, Can you find the flag?

1. Quantum Programming by abstracting ourselves from Quantum Mechanics: Abstraction level 0
2. Quantum Programming - Abstraction level 1: Logic Gates
3. Quantum Programming - Abstraction level 2: State Machine and Algorithms (this)

We have been on quite the adventure so far. Let's recap our progress.

• We've shown that by encoding information onto n qubits in superposition, we can manipulate the whole system of 2n states by only changing the state of one qubit.
• We represented the state of each qubit as a simple 2D vector.
• We've established that we can change the state of a single qubit (or two) by applying different logic gates to it, similarly to a classical logic gate.
• We can now visualise the superposed state of the qubit (indeed of the whole system) on the Bloch sphere.
• We know how to entangle two qubits, so that their states are correlated.

Now the time has come to start putting it all together. Wouldn't it be nice to actually string some gates together, and see what happens?

Well, in order to see what happens, we might want to be able to visualise the flow of our qubits' states, so we can see how each gate manipulates each qubit at each step.

Unit circle State machine​

1. Quantum Programming by abstracting ourselves from Quantum Mechanics: Abstraction level 0
2. Quantum Programming - Abstraction level 1: Logic Gates (this)
3. Quantum Programming - Abstraction level 2: State Machine and Algorithms

We have previously worked out that a quantum computer operates logically on qubits, which are the quantum counterparts of classical bits. We've learned that a qubit can stay in a superposition while we operate on it, and then we can collapse it into a definite state upon measurement. So, for the majority of our time spent programming on the quantum computer, we will be thinking of the qubits in their state of superposition.

What would be extremely useful for our intuition now, would be to have a way to visualise this superposition. Enter the Bloch Sphere. This tool allows us to represent the whole state-space of a single qubit in one (very simple) geometric shape - the unit sphere. Think of it as the unit circle in trigonometry.

1. Quantum Programming by abstracting ourselves from Quantum Mechanics: Abstraction level 0 (this)
2. Quantum Programming - Abstraction level 1: Logic Gates
3. Quantum Programming - Abstraction level 2: State Machine and Algorithms

The invention of the personal computer, and the further development of the hundreds of programming languages which utilise it has allowed us programmers to completely abstract ourselves from the world of electrons flowing through solid matter, and further from the low level programming of turning gates and transistors on and off to produce binary data.

Nowadays you would be hard-pressed to amaze anyone with the knowledge that stars are these ever-moving objects in the sky, at wildly different distances from us and each-other. Not only that, but many of the objects in the night sky we recognise as stars are often whole galaxies containing billions of stars, or giant clouds of gas glowing with the intensity of stars in our eyes, or god knows what else.

Rational numbers are easy. They include the integers, with which you can count up and down the number line, as well as fractions - written as the division of two integers or as a finitely-long decimal expansion.

But there aren't that many rational numbers on the real number line, as there are irrationals. These beasts of infinitely-long decimal expansions take the vast majority of the infinity of real numbers. They can't be written as a fraction, and they can't be written exactly as a decimal expansion (it would be infinitely long).