Nasm Crack

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

Challenge Description

Author: BitFriends Language: Assembler Upload: 7:05 PM 04/25/2020 Platform: Unix/linux Difficulty: 1.0 Quality: 4.0 Architecture: x86-64 Challenge link: https://crackmes.one/crackme/5ea48a1433c5d47611746436

Solution

Looking at the info of the file we see that its a 64 bit ELF file that is not stripped which is useful because we get to see most of the original code.

Unlike other high-level programming languages like C/C++, this program was written in Nasm. I used pwndbg to look at the assembly code. Looking at the info functions we see that there are two correct_func and _start. The latter is promising, looking at its code we see this:

The line at +79 looks interesting. Let’s set a break point and run the program and see the values at the register at that point.

Now we can see that our input is saved in the rsi register, while the value being compared is kept in the rdi register.

Passing the value at the rdi register we can confirm that it is the correct password.