pwnable.kr第二题collision

col@prowl:~$ ls -al
total 36
drwxr-x---   5 root    col     4096 Oct 23  2016 .
drwxr-xr-x 114 root    root    4096 May 19 15:59 ..
d---------   2 root    root    4096 Jun 12  2014 .bash_history
-r-sr-x---   1 col_pwn col     7341 Jun 11  2014 col
-rw-r--r--   1 root    root     555 Jun 12  2014 col.c
-r--r-----   1 col_pwn col_pwn   52 Jun 11  2014 flag
dr-xr-xr-x   2 root    root    4096 Aug 20  2014 .irssi
drwxr-xr-x   2 root    root    4096 Oct 23  2016 .pwntools-cache
col@prowl:~$ cat col.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]
", argv[0]);
                return 0;
        }
        if(strlen(argv[1]) != 20){
                printf("passcode length should be 20 bytes
");
                return 0;
        }

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

参数构造20位，check_password函数内用int读char，即分成5个数

字符串以‘x00'结尾，所以用'x01'来填充，

0x21DD09EC-0x01010101*4=0x1DD905E8

col@prowl:~$ ./col $(python -c "print 'x01' * 16 + 'xE8x05xD9x1D'")
daddy! I just managed to create a hash collision :)