Guess The Password

This is a solution submitted to the crackmes.one challenge by b4te

Challenge Description

Author: b4te Language: C/C++ Upload: 2:27 PM 05/12/2024 Platform: Unix/linux Difficulty: 1.1 Quality: 3.8 Architecture: x86 Challenge link: https://crackmes.one/crackme/6640d1d26b8bd8ddfe33c80a

Solution

Let’s take a look at some basic information about the file

file guess_the_password 

guess_the_password: ELF 64-bit LSB pie executable, x86-64, version 1 (SYSV), dynamically linked, interpreter /lib64/ld-linux-x86-64.so.2, BuildID[sha1]=4b5316bbef627d72efaa8d38723b2e9725065ec2, for GNU/Linux 3.2.0, not stripped

It’s a 64 bit ELF file. It’s not stripped so we can see most of the original logic.

Looking at the main function we can see that it’s using the check function to validate the input.

Going forward, ghidra would be a useful tool to quickly identify the code.

Looking at the check function in ghidra we get this:

Looking at the condition. It checks for three things

1. If the length of the input is 10
2. If the first character in the input is 1
3. if the 5th index is 9

With this we can try the input 1999999999

We get the correct password.