Hello everyone,
Recently we’ve participated in TFC CTF 2022 hosted by The Few Chosen. The event was really great. I really thank the organizers for hosting such a quality event.
There was a forensic challenge named as Adding In Parts and now I’m gonna demonstrate how I solved that problem.
In this challenge, we were provided with a zip file which contains 22 more zip files inside of it.
So, I thought of unzipping all of them at once. So, I wrote the following bash one-liner and executed it.
for i in {0..21}; do unzip $i.zip; done
And that gave me errors. But the zip files were unzipped as well. Which gave us 22 text files. Each file contains a single character.
So, I tried to concatenate all the characters from those extracted text files to see whether it gives us flag or any potential hint.
And yes, we got a hint.
There is something wrong with the compression method.
So, I looked at the errors that I got while unzipping the files. And all of them were referring to the same issue, which is Bad CRC.
Then I started to google about that issue and in a while I found that script.
So, I downloaded the script and tried to unzip a zip file again using that script and I was returning the following message.
So, now I started to unzip all the files with that script with the following one-liner.
for i in {0..21}; do python crack.py $i.zip 2>/dev/null | grep "$i :" >> output.txt; done && cat output.txt
And that gave us the flag.
Flag: TFCCTF{ch3cksum2_g0od}
So, now you know how I solved the challenge. Hope you found that interesting.
Thanks for reading.