How NOT to solve the IEEE Day Badge Challenge

Recently, we were taking the IEEE Day Badge Challenge in We wanted to give another go on solving the clues, so instead of following the clues to open the encrypted and password-protected PDFs, we got the clue that the password is composed only of numeric digits and we used pdfcrack to open the files!

We installed pdfcrack using the following command:

sudo apt-get install pdfcrack;
$ sudo apt-get install pdfcrack
[sudo] password for bob: 
Reading package lists... Done
Building dependency tree       
Reading state information... Done
The following NEW packages will be installed:
0 upgraded, 1 newly installed, 0 to remove and 28 not upgraded.
Need to get 31,0 kB of archives.
After this operation, 90,1 kB of additional disk space will be used.
Get:1 focal/universe amd64 pdfcrack amd64 0.18-2 [31,0 kB]
Fetched 31,0 kB in 1s (40,1 kB/s)
Selecting previously unselected package pdfcrack.
(Reading database ... 452721 files and directories currently installed.)
Preparing to unpack .../pdfcrack_0.18-2_amd64.deb ...
Unpacking pdfcrack (0.18-2) ...
Setting up pdfcrack (0.18-2) ...
Processing triggers for man-db (2.9.1-1) ...

To crack the files, we used the following commands that limited the input to the numeric digits and got the password back in seconds on a normal CPU:

pdfcrack -f IEEE+Day+2021+Clue++3.pdf -c 0123456789;
[email protected]:~$ pdfcrack -f IEEE+Day+2021+Clue++3.pdf -c 0123456789
PDF version 1.7
Security Handler: Standard
V: 2
R: 3
P: -1060
Length: 128
Encrypted Metadata: True
FileID: 79c15a021438224ba4df58b0e7fa9a20
U: 4990feee0d63f411cf4eba3c1346ff2100000000000000000000000000000000
O: cc5e6a95577573cac6f6683d4c7f02d6605fe42e5622feb6dc36636263ba838e
found user-password: '490000'

[email protected]:~$ pdfcrack -f IEEE+Day+2021+Clue++5.pdf -c 0123456789
PDF version 1.7
Security Handler: Standard
V: 2
R: 3
P: -1060
Length: 128
Encrypted Metadata: True
FileID: cf72bd9b3fb24145a6d2b578fa52c0e4
U: 8cd5ea45b59168ca10674bdd81f06f5800000000000000000000000000000000
O: 70301a6ff93ac7a91c28895180e8ad57a41388d2b7f3a813b83f4b3fd5274945
Average Speed: 49297.7 w/s. Current Word: '348478'
found user-password: '1470000'

Information on the version we used is below:

$ apt info pdfcrack
Package: pdfcrack
Version: 0.18-2
Priority: optional
Section: universe/utils
Origin: Ubuntu
Maintainer: Ubuntu Developers <[email protected]>
Original-Maintainer: Joao Eriberto Mota Filho <[email protected]>
Installed-Size: 90,1 kB
Depends: libc6 (>= 2.14)
Suggests: pdf-viewer
Download-Size: 31,0 kB
APT-Manual-Installed: yes
APT-Sources: focal/universe amd64 Packages
Description: PDF files password cracker
 PDFCrack is a simple tool for recovering passwords from pdf-documents.
 It should be able to handle all pdfs that uses the standard security handler
 but the pdf-parsing routines are a bit of a quick hack so you might stumble
 across some pdfs where the parser needs to be fixed to handle.
 The main PDFCrack features are:
   - Supports the standard security handler (revision 2, 3 and 4) on all known
   - Supports cracking both owner and userpasswords.
   - Both wordlists and bruteforcing the password are supported.
   - Simple permutations (currently only trying first character as Upper Case).
   - Save and load a running job.
   - Simple benchmarking.
   - Optimised search for owner-password when user-password is known.
 This program can be used in forensics investigations or similar activities,
 to legal password crack.