pdfcrack can find the numeric password of my bank pdf file credit-card bill in less than 1 second

Every month my bank sends me a PDF file with all my credit card expenses. Then normally I save these files. Today I tried to open an old file (more than 3 years), but I cannot recall the 4 numeric digits to open it.

Then I decided to use the pdfcrack tool to help me on this.

First, this file password is only 4 digits based on the credit card number:

xxxx xxxx XXXX xxxx

I can to instruct pdfcrack to only use numeric characters to test the password, passing “-c 0123456789”. Also because the password is 4 digit fixed I need to instruct pdfcrack to avoid testing password lesser than 4 chars using “-n 4”.

So that is it:

$ pdfcrack -f filepass.pdf -c 0123456789 -n 4

PDF version 1.4
Security Handler: Standard
V: 2
R: 3
P: -4
Length: 128
Encrypted Metadata: True
FileID: ddcb8e715b3c7347822d9548344a5964
U: ae718555d827f01765d0da553e9675e228bf4e5e4e758a4164004e56fffa0108
O: f86379060fbc201da762108dcdfe7846d1625dae9b9422e8d8fa7687eee97d05
found user-password: '7867'

I just pressed <ENTER> and it returned the password! Incredible!!!

Reference: http://www.cyberciti.biz/tips/linux-howto-crack-recover-pdf-file-password.html