becojo

REG EX

Writeup

This is a writeup of the reversing challenge 'REG EX' from PlaidCTF 2015.

The challenge provided two files: a python server program and a crazy-huge regex (see regex.txt). The python server is very basic: it simply listens for a connection, prompts for a 'key', and then checks whether the key matches the regex. It will then send the flag, but only if the provided key did NOT match the regex.

After the initial shock of looking at many screenfulls of regex, it actually breaks down into simple chunks. The regex consists of a single group which must match the entire string (^(...)$), containing a number of alternatives. Because we need to find a string which does not match the regex, we need to ensure that none of the alternatives match our string. The first alternative (.*[^plaidctf].*) will match any string which contains any character not in 'plaidctf', so we know our solution will contain only those characters. The second two alternatives (.{,170}|.{172,}) will match any string

	[
	"1k3rnelpan1c",
	"1nfinit3",
	"antiquark",
	"apus-apus",
	"aseal",
	"azolotov_eh9xx",
	"b3ach",
	"b67f994ced21e2f1d7c5434",
	"bassguitar",

	require "rubocop"

	# Useful for debugging and seeing how the nodes deconstruct
	def deep_deconstruct(node)
	return node unless node.respond_to?(:deconstruct)

	node.deconstruct.map { deep_deconstruct(_1) }
	end

	def block_to_shorthand?(a, b)

	FROM golang:alpine AS builder

	RUN apk update && \
	apk add git make

	WORKDIR /go/src

	ARG DIVE_VERSION=${DIVE_VERSION:-v0.10.0}
	RUN git clone https://github.com/wagoodman/dive.git dive && \
	git -C /go/src/dive checkout ${DIVE_VERSION}

	#!/usr/bin/env python
	import sys

	class Memoize:
	def __init__(self, fn):
	self.fn = fn
	self.memo = {}
	def __call__(self, arg):
	if arg not in self.memo:
	self.memo[arg] = self.fn(arg)

	{-# LANGUAGE ConstraintKinds #-}
	{-# LANGUAGE GeneralizedNewtypeDeriving #-}

	module ParserCombinators where

	{-

	We'll build a set of parser combinators from scratch demonstrating how
	they arise as a monad transformer stack. Actually, how they arise as a
	choice between two different monad transformer stacks!

	-- Given a list of values, returns a list of the differences between
	-- neighbouring pairs of values.
	nextLevelDiffs (a:b:rest) = (b - a):(nextLevelDiffs (b:rest))
	nextLevelDiffs _ = []

	-- Given a list of values, returns a list of the differences such that the
	-- first element is the 0-level differences, the second the 1-level and so
	-- on.
	nLevelDiffs [] = []
	nLevelDiffs elms = elms:(nLevelDiffs (nextLevelDiffs elms))

	// by dave @ beesandbombs.tumblr.com >:)

	void setup() {
	setup_();
	result = new int[width*height][3];
	result_ = new int[width*height][3];
	}

	int[][] result, result_;
	float time;

	@import MobileCoreServices;

	static CFStringRef UTTypeForImageData(NSData *data) {
	const unsigned char * bytes = [data bytes];

	if (data.length >= 8) {
	if (bytes[0] == 0x89 && bytes[1] == 0x50 && bytes[2] == 0x4E && bytes[3] == 0x47 && bytes[4] == 0x0D && bytes[5] == 0x0A && bytes[6] == 0x1A && bytes[7] == 0x0A) {
	return kUTTypePNG;
	}
	}