攻防世界逆向第二題【果然還是最拿手的C擅長一點】

題目名稱：open-source

題目地址：open-source

解題思路

首先

下載附件=-=
發現是.c文件？？
陷入沉思hhh
這不就是源碼嗎？
DEV-C++打開

#include <stdio.h>
#include <string.h>

int main(int argc, char *argv[]) {
    if (argc != 4) {
    	printf("what?\n");
    	exit(1);
    }

    unsigned int first = atoi(argv[1]);
    if (first != 0xcafe) {
    	printf("you are wrong, sorry.\n");
    	exit(2);
    }

    unsigned int second = atoi(argv[2]);
    if (second % 5 == 3 || second % 17 != 8) {
    	printf("ha, you won't get it!\n");
    	exit(3);
    }

    if (strcmp("h4cky0u", argv[3])) {
    	printf("so close, dude!\n");
    	exit(4);
    }

    printf("Brr wrrr grr\n");

    unsigned int hash = first * 31337 + (second % 17) * 11 + strlen(argv[3]) - 1615810207;

    printf("Get your key: ");
    printf("%x\n", hash);
    return 0;
}

其次

分析一下
輸出hash即可
嘗試刪除四個if發現不行
因爲second沒有值了
細看一次代碼
更改如下

#include <stdio.h>
#include <string.h>

int main(int argc, char *argv[]) {
    /*if (argc != 4) {
    	printf("what?\n");
    	exit(1);
    } */
    //好像沒用上 

    /*unsigned int first = atoi(argv[1]);
    if (first != 0xcafe) {
    	printf("you are wrong, sorry.\n");
    	exit(2);
    }*/ 
    //跳出語句就令first爲該值 
    /*unsigned int second = atoi(argv[2]);
    if (second % 5 == 3 || second % 17 != 8) {
    	printf("ha, you won't get it!\n");
    	exit(3);
    }*/
	//讓second的值等於8就跳出去了
    /*if (strcmp("h4cky0u", argv[3])) {
    	printf("so close, dude!\n");
    	exit(4);
    }
    printf("Brr wrrr grr\n");*/ 
    //讓aygv[3]爲 "h4cky0u"跳出 
    //unsigned int hash = first * 31337 + (second % 17) * 11 + strlen(argv[3]) - 1615810207;
      unsigned int hash = 0xcafe * 31337 + (8 % 17) * 11 + strlen("h4cky0u") - 1615810207;
    printf("Get your key: ");
    printf("%x\n", hash);
    return 0;
}

最後

運行源碼，得到flag：c0ffee

提交完成