collision

Posted by rk700 on November 15, 2014

代码如下

#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;
}

从题意来看,输入5个int共20bytes,其和要求为0x21DD09EC。于是可以让前4个数为0xffffffff=-1,最后一个数为0x21dd09ec+4=0x21dd09f0

具体在输入时,用的是perl。一开始没有用双引号,由于0x09\t,被shell认为是参数分隔了。于是用双引号括起来:

$ ./col "$(perl -e 'print "\xff"x16 . "\xf0\x09\xdd\x21"')"