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=;
     for(i=; i<; i++){
         res += ip[i];
     }
     return res;
 }

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

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

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!