ecoin - hard
I think I'm getting crazy! I see things in things. Like this: PDF
First, I opened the PDF and did not notice anything out of the ordinary. This PDF contains an advertisement for Ecoin (wink to Mr.Robot) as well as a blank page "[This page unintentionally left blank]".
I then issued some basic forensics-related commands :
Syntax Error (1288034): Missing 'endstream' or incorrect stream length
That's interesting. Let's check it out:
I see a split ZIP file at the end of the PDF. Let's try to extract it.
Part II - Extract the ZIP file
First step, retrieving the pieces of the ZIP file in the PDF:
To do this I make use of the information previously retrieved using binwalk and the
and then I use the
zip -FF command to check the file and fix it if needed:
zip -FF ecoin-merged.zip --out ecoin.zip
and now I can extract it:
Ok, I need a password to extract hint.pdf.
I reread the binwalk output and notice something peculiar. In the PDF, I can only see one picture, but in the binwalk I see a PNG and a JPEG file. Weird.
pdfextract (origami) command allows me to extract both files. YEAH! The JPEG file seems very interesting :)
Let's try "Pure_Funk" as password for the ZIP.
It works. :-)
So now, we have two new files, flag.png and hint.pdf.
Part II - Recover the flag
I open hint.pdf and notice something strange at the top. I do a CTRL+A, CTRL+C and CTRL+V in SublimeText and obtain the following result:
AES IV: F01D86CDBB7E1CD88815BEB4106A558C
Very, very, very promising.
I summarize: we have a JPEG file containing the text "Pure_Funk", a flag.png file that is unusable in the current state and a hint.pdf file containing an AES IV as well as the text "AngeWouldLoveIt!".
I immediately think of a https://speakerdeck.com/ange/funky-file-formats-31c3 from 31C3 by Ange Albertini called "Funky File Formats".
In this presentation, there was a POC called "Angecryption". This POC showed that it is possible to retrieve a valid file from another valid file by encrypting it or decrypting it using a block cipher and a well-chosen IV.
Let's try this.
from Crypto.Cipher import AES IV="F01D86CDBB7E1CD88815BEB4106A558C".decode('hex') key = "AngeWouldLoveIt!" aes = AES.new(key, AES.MODE_CBC, IV) with open("flag.png", "rb") as f: d = f.read() d = aes.encrypt(d) with open("out", "wb") as f: f.write(d)
So, after encrypting flag.png using AES, I obtain a new PNG file.
The image is a white square containing transparent holes, I name it mask.png, and merge both images together to obtain the following result:
YES, that is the flag! :)