The following writeup is about a R3D5, a small Information Security Club, meeting.
We decided that for our first activity, we would look for a website that already have Information Security challenges to decrease the amount of setup time needed. We found https://backdoor.sdslabs.co website which had a list of challenges, so we decided to go with the first one https://backdoor.sdslabs.co/challenges/2013-BIN-50. The rest of this writeup will be how we chose to work through this exercise.
The challenge provides a file, and its details are as follows:
So the first thing we did was download the only file provided “binaary50”. We immediately noticed that the file did not have an extension and attempted to open it with a text editor. It was clear that the file was not a text file as plenty of random ASCII character were littered throughout the file. To determine the file type of “binaary50” we ran
$ file binaary50
binaary50: ELF 64-bit LSB executable, x86-64, version 1 (SYSV), dynamically linked (uses shared libs), for GNU/Linux 2.6.24, BuildID[sha1]=745172a5d855872a241d62ddf8a40bcad4bb07bc, not stripped
Showing us that this file is executable. Running it:
yields the following output “Please provide the password”, so we guessed correctly. This is an executable program, but that begs the question of “what kind of executable”? So we can try to figure that out using the following program
Now that we know that this file is an executable that prompts us for the password, we wanted to see if we could extract any information from it. Remember, we saw a lot of gibberish and some human readable strings when we opened the file up in a text editor.
We can run the following command
$ strings binaary50
and filter the unrelated results by hand to get
- Password is Advicemallard
- Password is Butter
- Password is Hoobastank
- Password is Darth
- Password is Jedimaster
- Password is Masternamer
- Password is Morpheus
- Password is Neutron
- Password is Coyote
- Password is Tweety
- Nothing to see here.
- Please provide the password
So now we have some potential passwords. We than ran the program with each of the passwords as input on at a time like so:
$ ./binaary50 Advicemallard
Each of which ended execution with “Nothing to see here.”, except for “Masternamer” which returned
3cd50c6be9bbede06e51741928d88b7e, an MD-5.
Looking back to the original problem
The flag is SHA-256 of the MD-5 hash obtained.
We obtained the MD-5 , so we simply need to take the SHA-256 of this MD-5 and we have our key. Running:
echo -n 3cd50c6be9bbede06e51741928d88b7e | sha256sum
returns us our key