Collision

#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] )){
                setregid(getegid(), getegid());
                system("/bin/cat flag");
                return 0;
        }
        else
                printf("wrong passcode.\n");
        return 0;
}

这是源代码，这题简单，难点在于如何计算。只需结果等于hashcode即可

hashcode转换为十进制是：568134124

所以可以构造：113626825*4+113626824=568134124

再转换为16进制

printf '\xC9\xCE\xC5\x06\xC9\xCE\xC5\x06\xC9\xCE\xC5\x06\xC9\xCE\xC5\x06\xC8\xCE\xC5\x06' | ./col $(cat)

Collision

#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[])
{
    ifargc<2){
        printf("Usage: %s [passcode]\n", argv[0]);
        return 0;
    }
    if strlen(argv[1]) != 20){
        printf("The passcode must be 20 bytes long.\n");
        return 0;
    }

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

This is the source code. The task is simple; the challenge lies in figuring out how to calculate the correct passcode. The passcode just needs to match the value of hashcode.

Converting hashcode to decimal gives: 568134124.

Therefore, the passcode that satisfies the condition is: 113626825 * 4 + 113626824 = 568134124.

To represent this passcode in hexadecimal format:

printf '\xC9\xCE\xC5\x06\xC9\xCE\xC5\x06\xC9\xCE\xC5\x06\xC9\xCE\xC5\x06\xC8\xCE\xC5\x06' | ./col $(cat)

collision

collision

Collision

Collision