# Ecoin
## Challenge description
> ecoin - hard>> I think I'm getting crazy! I see things in things. Like this: PDF
The PDF file : [https://github.com/Inshallhack/Write-ups/raw/master/34C3-2017/ecoin/ecoin_vuln_notes.pdf](https://github.com/Inshallhack/Write-ups/raw/master/34C3-2017/ecoin/ecoin_vuln_notes.pdf)
## First glanceFirst, 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 :
```bashpdfinfo ecoin_vuln_notes.pdf```![](https://github.com/Inshallhack/Write-ups/raw/master/34C3-2017/ecoin/screenshots/1.png)```Syntax Error (1288034): Missing 'endstream' or incorrect stream length```
That's interesting. Let's check it out:
```bashbinwalk ecoin_vuln_notes.pdf```![](https://github.com/Inshallhack/Write-ups/raw/master/34C3-2017/ecoin/screenshots/2.png)
I see a split ZIP file at the end of the PDF.Let's try to extract it.## Part II - Extract the ZIP fileFirst 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 `dd` command,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```![](https://github.com/Inshallhack/Write-ups/raw/master/34C3-2017/ecoin/screenshots/3.1.png)
and now I can extract it:
![](https://github.com/Inshallhack/Write-ups/raw/master/34C3-2017/ecoin/screenshots/3.2.png)
F.CK
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.
The `pdfextract` (*origami*) command allows me to extract both files. YEAH! The JPEG file seems very interesting :)
![](https://github.com/Inshallhack/Write-ups/raw/master/34C3-2017/ecoin/extracted.jpg) Let's try *"Pure_Funk"* as password for the ZIP. **It works. :-)**
So now, we have two new files, **flag.png** and **hint.pdf**.
flag.png:![](https://github.com/Inshallhack/Write-ups/raw/master/34C3-2017/ecoin/flag.png)
hint.pdf:![](https://github.com/Inshallhack/Write-ups/raw/master/34C3-2017/ecoin/hint.pdf.png)
## 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](presentation) 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.
```pythonfrom 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)
```
![](https://github.com/Inshallhack/Write-ups/raw/master/34C3-2017/ecoin/screenshots/arf.png)
Arff… :(
![](https://github.com/Inshallhack/Write-ups/raw/master/34C3-2017/ecoin/screenshots/whew.png)
Whew! :)
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:
![](https://github.com/Inshallhack/Write-ups/raw/master/34C3-2017/ecoin/flagged.png)
YES, that is the flag! :)
**Flag**: 34c3_F1L3_FORMA7S_AR3_COMMUN17Y_CONNEC7ORS