> For the complete documentation index, see [llms.txt](https://ret2basic.gitbook.io/ctfwriteup/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://ret2basic.gitbook.io/ctfwriteup/web2-ctf/pwnable.kr/collision.md).

# collision

## Challenge

Daddy told me about cool MD5 hash collision today. I wanna do something like that too!

ssh <col@pwnable.kr> -p2222 (pw:guest)

## Code Review

```c
#include <stdio.h>
#include <string.h>
unsigned long hashcode = 0x21DD09EC;
unsigned long check_password(const char* p){
        int* ip = (int*)p;
        int i;
        int res=0;
        for(i=0; i<5; i++){
                res += ip[i];
        }
        return res;
}

int main(int argc, char* argv[]){
        if(argc<2){
                printf("usage : %s [passcode]\n", argv[0]);
                return 0;
        }
        if(strlen(argv[1]) != 20){
                printf("passcode length should be 20 bytes\n");
                return 0;
        }

        if(hashcode == check_password( argv[1] )){
                system("/bin/cat flag");
                return 0;
        }
        else
                printf("wrong passcode.\n");
        return 0;
}
```

The program takes 5 4-byte hex numbers as `argv[1]` and it calls `check_password()`. If the sum of these 5 hex numbers equals `0x21DD09EC` then the program prints out the flag.

### Solution

We need to find 5 integers in hex that sum to `0x21dd09ec`. The naive idea is to divide `0x21dd09ec` by 5. It won't work because `0x21dd09ec` isn't divisible by 5. But, following from this logic, we could find a solution using the formula:

```
0x21dd09ec = (0x21dd09ec // 5) * 4 + <residue>
```

Here we have `0x21dd09ec // 5 = 0x6c5cec8`, so `residue = 0x21dd09ec - 0x6c5cec8 * 4 = 0x6c5cecc`. Since the convertion is done by `int* ip = (int*)p` (pointer, not string), we should use **little-endian** in `argv[1]`.

### Exploit

```python
#!/usr/bin/env python3
# -*- coding: utf-8 -*-
# This exploit template was generated via:
# $ pwn template --host pwnable.kr --port 2222 --user col --password guest --path /home/col/col
from pwn import *

# Set up pwntools for the correct architecture
exe = context.binary = ELF('col')

# Many built-in settings can be controlled on the command-line and show up
# in "args".  For example, to dump all data sent/received, and disable ASLR
# for all created processes...
# ./exploit.py DEBUG NOASLR
# ./exploit.py GDB HOST=example.com PORT=4141
host = args.HOST or 'pwnable.kr'
port = int(args.PORT or 2222)
user = args.USER or 'col'
password = args.PASSWORD or 'guest'
remote_path = '/home/col/col'

# Connect to the remote SSH server
shell = None
if not args.LOCAL:
    shell = ssh(user, host, port, password)
    shell.set_working_directory(symlink=True)

def start_local(argv=[], *a, **kw):
    '''Execute the target binary locally'''
    if args.GDB:
        return gdb.debug([exe.path] + argv, gdbscript=gdbscript, *a, **kw)
    else:
        return process([exe.path] + argv, *a, **kw)

def start_remote(argv=[], *a, **kw):
    '''Execute the target binary on the remote host'''
    if args.GDB:
        return gdb.debug([remote_path] + argv, gdbscript=gdbscript, ssh=shell, *a, **kw)
    else:
        return shell.process([remote_path] + argv, *a, **kw)

def start(argv=[], *a, **kw):
    '''Start the exploit against the target.'''
    if args.LOCAL:
        return start_local(argv, *a, **kw)
    else:
        return start_remote(argv, *a, **kw)

# Specify your GDB script here for debugging
# GDB will be launched if the exploit is run via e.g.
# ./exploit.py GDB
gdbscript = '''
tbreak main
continue
'''.format(**locals())

#===========================================================
#                    EXPLOIT GOES HERE
#===========================================================
# Arch:     i386-32-little
# RELRO:    Partial RELRO
# Stack:    Canary found
# NX:       NX enabled
# PIE:      No PIE (0x8048000)

argv = [p32(0x6c5cec8) * 4 + p32(0x6c5cecc)]
print(f"{argv = }")
io = start(argv)
success(io.readall().decode())
```