Tags: radare2 reversing 

Rating: 5.0

## s1mple (422 pts) (Reverse)

```
Can you find the right combination?

given: an x86-64 ELF executable
```

If we run the program, it asks for a password.

```
./simple
What is the password?
asdfasdfasdfa
Wrong!!
```

If we open up the file in radare2 (`r2 -Ad simple`) and disassemble the main function (`pdf@main`), we can see that it make a comparison between your input and some other string. If the two strings are equal, you win!

```
| 0x004006da bf08084000 mov edi, str.What_is_the_password ; 0x400808 ; "What is the password?" ; const char *s
| 0x004006df e8ecfdffff call sym.imp.puts ; int puts(const char *s)
| 0x004006e4 488d85f0feff. lea rax, [s1]
| 0x004006eb 4889c7 mov rdi, rax ; char *s
| 0x004006ee b800000000 mov eax, 0
| 0x004006f3 e818feffff call sym.imp.gets ; char *gets(char *s)
| 0x004006f8 488d95e0feff. lea rdx, [s2]
| 0x004006ff 488d85f0feff. lea rax, [s1]
| 0x00400706 4889d6 mov rsi, rdx ; const char *s2
| 0x00400709 4889c7 mov rdi, rax ; const char *s1
| 0x0040070c e8effdffff call sym.imp.strcmp ; int strcmp(const char *s1, const char *s2)
| 0x00400711 85c0 test eax, eax
| ,=< 0x00400713 750c jne 0x400721
| | 0x00400715 bf20084000 mov edi, str.flag_is_BTH_CTF_the_password_you_entered_to_reach_this_point ; 0x400820 ; "flag is BTH_CTF{the password you entered to reach this point}" ; const char *s
```

We can see that a pointer to the input string is stored in rax register, and a pointer to the other string is stored in the rdx register.
By setting a breakpoint at the comparison and examining the content at the address stored in rdx, we find the password.

```
[0x7f5504265c30]> db 0x0040070c
[0x7f5504265c30]> dc
What is the password?
adsg
hit breakpoint at: 40070c
[0x0040070c]> ps@rdx
assemblyisfun
```

`BTH_CTF{assemblyisfun}`