Lense

randomware

SECCON 2016 quals

description

300 points
My PC suddenly got broken. Could you help me to recover it please?
NOTE: The disk can be virus-infected. DO NOT RUN any programs extracted from the disk outside of sandbox.
disk.qcow2.zip
Challenge files is huge, please download it first. Password will release after 60min.
password: h9nn4c2955kik9qti9xphuxti

Lense

First thing I did was figure out that it was a git repo, search Google for how to extract the code, and run git archive master | tar x -C code. It asks for 2 prime factors of a number which doesn't have exactly 2 prime factors, from which it can be assumed that something was wrong with the code.

Running git fsck (after more Google searching) showed 3 hashes not matching up, which means that the hash of the file contents (plus header metadata) didn't match the filename. I then read https://git-scm.com/book/en/v2/Git-Internals-Git-Objects (side-note: the Pro Git book is a fantastic introduction to and reference for git). 3 Files are corrupted: sharp.cpp in each of the 3 commits (see git log).

Then I tool a leap of faith and assumed that a single byte was wrong (corrupted, hence the SATA controller comment) in each blob, and wrote a script to brute force it. It worked for first one, and then doom had to remind me to propagate the changes: the first

Lense

diff --git a/x.c b/x.c
index 5828a3b..ace044f 100644
--- a/x.c
+++ b/x.c
@@ -116,8 +116,6 @@ typedef struct {
 	int width;
 	int ascent;
 	int descent;
-	int badslant;
-	int badweight;

Lense

#!/usr/bin/python3
import json

from CTFd import create_app
from CTFd.models import Users, Challenges


scoreboard = dict()
scoreboard["standings"] = list()