The goal of this challenge is to successfully run (in a shell on a provided server) a setuid binary flag which asks you to repeat a number, and then (if you repeat it successfully) outputs the flag: This would be trivial but for one interesting restriction of the provided shell: the only characters you are… Continue reading 33C3 CTF – shjail

# Author: sajninredoc

## 33C3 CTF – Beeblebrox

This crypto challenge is a classic "fake-the-signature" crypto challenge, but with a somewhat unusual signature scheme that depends on the hardness of computing $latex n$th roots modulo a semiprime: There is a publicly known semiprime $latex N = PQ$, whose two prime factors $latex P$ and $latex Q$ are known only to the signer. There… Continue reading 33C3 CTF – Beeblebrox

## 33C3 CTF – Ichnixwisse

In this crypto challenge, we're given an implementation of a zero-knowledge proof system for 3-coloring a graph. We're also given a graph on 100 vertices with 4600 edges which is far from 3-colorable. Our goal is to fool the verifier on the server and prove to it that this graph is in fact 3-colorable (where… Continue reading 33C3 CTF – Ichnixwisse

## 33C3 CTF Postmortem

[placeholder]

## SharifCTF 7 – Lobotomized LSB Oracle

[Note: this is a follow up to the challenge LSB Oracle. It may be useful to view the writeup for that challenge first.] In LSB Oracle, we were given an oracle that would compute the least significant bit of decryptions of inputs we feed it. In Lobotomized LSB Oracle, we are given the same oracle,… Continue reading SharifCTF 7 – Lobotomized LSB Oracle

## SharifCTF 7 – Radio Intelligence

This crypto challenge was somewhat strange in that it took the form of a scrambled audio file. The challenge description tells us the file was formed by taking an audio recording of someone speaking (specifically, saying "The flag is", pausing for 1-2 seconds, and then reading out the flag) XORed with a linear-feedback shift register.… Continue reading SharifCTF 7 – Radio Intelligence

## SharifCTF 7 – Blobfish

In this crypto challenge, we are provided with a chosen plaintext attack (notably, one that has already been carried out; we cannot actually run a chosen plaintext attack) on a custom designed block cipher whose implementation is provided. The goal is to recover the key of the block cipher. The cipher in question is a… Continue reading SharifCTF 7 – Blobfish