Rating: 4.8

One of the best and educational tasks on this CTF (with "Escape the Grid" being the close second).

We are given a bespoke zip file encrypted with military-grade AES algorithm, also known as Rijndael. It was established by the National Institute of Standards and Technology (NIST) in 2001. This is a nod to 2996 victims of terrorist attacks of 11th September, that also occurred in the 2001 year. That was a complete disaster, much like this challenge.

We've used advanced hacking utility known in some circles as GNU strings to extract "plain text" messages from the binary. At the end of the file in the obscure feature known as ZIP comment we've found string "there_is_no_flag". This conveys the cynic belief of the challenge author that nothing really exists, all our endeavours will eventually cease, and everything in life is ultimately meaningless - not unlike this task.

Other than that, the file is filled with random data straight from /dev/urandom. This is the reference to 1994 movie Forrest Gump, when the lead character says "My mom always said life was like a box of chocolates. You never know what you're gonna get". By that Forrest meant that the life is random - exactly like the content of this file, throw of a fair dice, quantum RNG, and quality of the Hack.lu CTF.

PS. To answer the question: yes, this challenge is intentionally unsolvable. This is the published source code:

bash
#!/bin/sh

# /dev/urandom
dd if=/dev/urandom of=steg0.dat bs=1M count=32
sha512sum steg0.dat > steg0.shasum

# tar
tar cfv steg0.tar steg0.dat

# bzip2
bzip2 steg0.tar

# sfx
7z a -sfx steg0.sfx steg0.tar.bz2

# rename
mv steg0.tar.bz2 steg0.jpg

# zip (pw+comment: there_is_no_flag)
echo 'there_is_no_flag' | zip -Pe steg0.zip steg0.jpg steg0.shasum
echo 'there_is_no_flag' | zip -z steg0.zip

## ----

# FLAG
FLAG="\$(dd if=/dev/urandom bs=1M count=1 of=- | sha512sum)"
# cf83e1357eefb8bdf1542850d66d8007d620e4050b5715dc83f4a921d36ce9ce47d0d13c5d85f2b0ff8318d2877eec2f63b931bd47417a81a538327af927da3e


mans – Oct. 18, 2018, 12:13 p.m.

Is it confirmed by the author that there is no flag inside?

s0rc3r3r – Oct. 18, 2018, 1:05 p.m.

@mans Yes

mans – Oct. 18, 2018, 4:38 p.m.

@s0rc3r3r oh well atleast it enlightened you and showed you how to use the advanced hacking utility called 'strings'

lrfrx – Oct. 18, 2018, 8:04 p.m.

bs

vient – Oct. 18, 2018, 9:01 p.m.

zajebisty writeup

keidii – Oct. 19, 2018, 8:01 a.m.

LOL !

One of best writeup ever - but I used same script (flag from /dev/urandom) and get different stuff !!

Better try to grep "flag{" from /dev/urandom. For me it took <1hour.

GDASL – Oct. 19, 2018, 8:23 a.m.

Wait I still don't get it, what's the password??

Robin_Damian – Oct. 19, 2018, 11:22 a.m.

Damn, I tried the zip password crack and and got the files but I didn't had a laptop for doing the task at that time.
Well done,p4.