Reverse engineering

 · 5 mins


I have written a simple C program that can check 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 in order to reveal its designs, architecture, or to extract knowledge from the object, like scientific research.

Let’s test this program with a few numbers:

[email protected] ~/Downloads$ ./key 9
Result: false
[email protected] ~/Downloads$ ./key 4
Result: false
[email protected] ~/Downloads$ ./key 8
Result: false
[email protected] ~/Downloads$

Hmm… I’m not very lucky

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

Assembly code

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

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 also often 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 into 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 looks like this if we use the command di -n main in lldb:

[email protected] ~/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 side that will give us a hint about what this program is doing.

Do you see the is_prime function called at 0x100000f11?

This means the key has to do with prime numbers, so let’s check it 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 debuggers like Hopper that will help us convert Assembly to C code more easily: Hopper reconstruction for the main function main function represented

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

Real program

Like I’ve said earlier, I’ve 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 fairly close to it. __You now know that reverse engineering is, in fact, about reconstruction. __