pwnable.kr col之write up

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

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

  先看源代码：

#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;
}

函数大体意思是，输入20个字节的数，在check_password里把char转换为int类型，，char为1字节，Int为4字节，这样5次循环正好是20个字节，且这20个字节之和为0x21DD09EC,

将0x21dd09ec分解为5个16进制数组

于是我们输入正确的密码，得到flag!