This was my favorite crypto challenge from this year's CSAW Quals (and probably my favorite crypto challenge that I've done in a while). On the surface, it's relatively standard: we have a flag encrypted with some cryptosystem (along with a Python implementation of this cryptosystem) and we have to decrypt it. The cryptosystem itself is… Continue reading CSAW Quals 2018 – holywater

# Author: sajninredoc

## Tokyo Westerns CTF 2017 – Palindromes Pairs (Challenge Phase)

This challenge was a follow-up to an earlier challenge, titled "Palindromes Pairs - Coding Phase". In that earlier warmup ppc challenge, the goal was to write an algorithm to solve the following problem: given up to 1000 strings $latex s_i$, each of length at most 1000, count the number of pairs $latex (i, j)$ such… Continue reading Tokyo Westerns CTF 2017 – Palindromes Pairs (Challenge Phase)

## Tokyo Westerns CTF 2017 – Liar’s Trap

On the surface, this challenge appears to be about secret sharing. The setup is simple: the flag is divided into $latex N=100$ pieces using Shamir's secret sharing scheme so that it can be recovered given any $latex K=25$ of these pieces (but is impossible to recover with any fewer). The server provides you with all… Continue reading Tokyo Westerns CTF 2017 – Liar’s Trap

## 33C3 CTF – shjail

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

## 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

## 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