Reverse engineering

 · 5 mins


I’ve written a simple C program that checks if a given int is a valid key.

We want to crack this program, without knowing the code.

This way of hacking is called Reverse Engineering because we’re trying to deconstruct a program to reveal its designs, architecture, or to extract knowledge from the object; like scientific research, the only difference being that scientific research is about a natural phenomenon.

Let’s test this program with a few numbers:

user@Server ~/Downloads$ ./key 9
Result: false
user@Server ~/Downloads$ ./key 4
Result: false
user@Server ~/Downloads$ ./key 8
Result: false
user@Server ~/Downloads$

Hmm… I’m not very lucky

Ok, so now, let’s reverse engineer it!


First, I need to tell you about Assembly code. An assembly (or assembler) language, often abbreviated asm, is any low-level programming language in which there is a very strong correspondence between the program’s statements and the architecture’s machine code instructions. Assembly code

Each assembly language is specific to a computer architecture and operating system. In contrast, most high-level programming languages are generally portable across multiple architectures but require interpreting or compiling. Assembly language may also be called symbolic machine code.

Assembly language usually has one statement per machine instruction, but assembler directives, macros and symbolic labels of program and memory locations are often also supported.

Assembly code is converted into executable machine code by a utility program referred to as an assembler. The conversion process is referred to as assembly or assembling the source code.

It’s basically a way to visualise operations executed by the processor.


A debugger is a tool to help you debug your code at runtime or by disassembling it. Disassembling mean to convert compiled program to Assembly code.

Famous debuggers are gdb and lldb. We’ll use lldb, but the process is roughly the same on gdb

In our case the main function, it looks like this if we use the command di -n main in lldb:

user@Server ~/Downloads$ lldb key
(lldb) target create "key"
Current executable set to 'key' (x86_64).
(lldb) di -n main
key[0x100000ec0] <+0>:   pushq  %rbp
key[0x100000ec1] <+1>:   movq   %rsp, %rbp
key[0x100000ec4] <+4>:   subq   $0x30, %rsp
key[0x100000ec8] <+8>:   movl   $0x0, -0x4(%rbp)
key[0x100000ecf] <+15>:  movl   %edi, -0x8(%rbp)
key[0x100000ed2] <+18>:  movq   %rsi, -0x10(%rbp)
key[0x100000ed6] <+22>:  cmpl   $0x1, -0x8(%rbp)
key[0x100000eda] <+26>:  jne    0x100000ef6               ; <+54>
key[0x100000ee0] <+32>:  leaq   0x9d(%rip), %rdi          ; "Please give a key to verify."
key[0x100000ee7] <+39>:  movb   $0x0, %al
key[0x100000ee9] <+41>:  callq  0x100000f58               ; symbol stub for: printf
key[0x100000eee] <+46>:  movl   %eax, -0x2c(%rbp)
key[0x100000ef1] <+49>:  jmp    0x100000f4a               ; <+138>
key[0x100000ef6] <+54>:  movq   -0x10(%rbp), %rax
key[0x100000efa] <+58>:  movq   0x8(%rax), %rax
key[0x100000efe] <+62>:  movq   %rax, -0x18(%rbp)
key[0x100000f02] <+66>:  movq   -0x18(%rbp), %rdi
key[0x100000f06] <+70>:  callq  0x100000f52               ; symbol stub for: atoi
key[0x100000f0b] <+75>:  movl   %eax, -0x1c(%rbp)
key[0x100000f0e] <+78>:  movl   -0x1c(%rbp), %edi
key[0x100000f11] <+81>:  callq  0x100000e10               ; is_prime
key[0x100000f16] <+86>:  leaq   0x8f(%rip), %rdi          ; "Result: %s\n"
key[0x100000f1d] <+93>:  leaq   0x82(%rip), %rcx          ; "false"
key[0x100000f24] <+100>: leaq   0x76(%rip), %rdx          ; "true"
key[0x100000f2b] <+107>: movl   %eax, -0x20(%rbp)
key[0x100000f2e] <+110>: movl   -0x20(%rbp), %eax
key[0x100000f31] <+113>: cmpl   $0x1, %eax
key[0x100000f34] <+116>: cmoveq %rdx, %rcx
key[0x100000f38] <+120>: movq   %rcx, -0x28(%rbp)
key[0x100000f3c] <+124>: movq   -0x28(%rbp), %rsi
key[0x100000f40] <+128>: movb   $0x0, %al
key[0x100000f42] <+130>: callq  0x100000f58               ; symbol stub for: printf
key[0x100000f47] <+135>: movl   %eax, -0x30(%rbp)
key[0x100000f4a] <+138>: xorl   %eax, %eax
key[0x100000f4c] <+140>: addq   $0x30, %rsp
key[0x100000f50] <+144>: popq   %rbp
key[0x100000f51] <+145>: retq

Pretty ugly right?

Hopefully, there are some comments on the right that will give us a hint about what this program does.

Do you see the is_prime function called at 0x100000f11?

This mean the key has to do with prime numbers, so let’s check with 23!

(lldb) run 23
Process 7577 launched: '/Users/user/Downloads/key' (x86_64)
Result: true
Process 7577 exited with status = 0 (0x00000000)

And it works!

GUI Disassembler

Ok, so it was a little bit complex, even for a very simple program. Hopefully, there are some GUI debugger like Hopper that will help us convert Assembly to C code: Hopper reconstruction for the main function main function represented

As we can see, it’s pretty readble and looks like the original main function. We can do the same with the is_prime function, so we could theorically rebuild the program.

Real program

As I said earlier, I built that program, so here it is:

#include <stdlib.h>

int is_prime(int num) {
     if (num <= 1) return 0;
     if (num % 2 == 0 && num > 2) return 0;
     for(int i = 3; i < num / 2; i+= 2) {
         if (num % i == 0)
             return 0;
     return 1;

int main(int argc, char *argv[]) {
	typedef enum { false, true } bool;
	if (argc == 1) {
		printf("Please give a key to verify.");
	} else {
		char *key = argv[1];
		int k = atoi(key);
		int p = is_prime(k);
		char *r = p == 1 ? "true" : "false";
		printf("Result: %s\n", r);
	return 0;

The disassembler was close to it. __Now, you know reverse engineering is about: reconstruction. __

Written by Arthur Guiot