逆向入门练习-功防世界

1、re1

ida反编译看到：

 所以flag就在程序中隐藏着，用strings工具进行关键字查找，找到flag：

strings工具下载地址：https://docs.microsoft.com/zh-cn/sysinternals/downloads/strings

2、game

手动通关：1 2 3 4 5 6 7 8 | 8 7 6 5 4 3 2 1

ida中查找字符串：


 查看这个字符串所在函数的源码：

 根据这个函数和这些数字解出flag：
 

b = [123,32,18,98,119,108,65,41,124,80,125,38,124,111,74,49,83,108,94,108,84,6,96,83,44,121,104,110,32,95,117,101,99,123,127,119,96,48,107,71,92,29,81,107,90,85,64,12,43,76,86,13,114,1,117,126]
a = [18,64,98,5,2,4,6,3,6,48,49,65,32,12,48,65,31,78,62,32,49,32,1,57,96,3,21,9,4,62,3,5,4,1,2,3,44,65,78,32,16,97,54,16,44,52,32,64,89,45,32,65,15,34,18,16]
c = ''
for i in range(len(a)):
    a[i] ^= b[i]
    a[i] ^= 19
    c += chr(a[i])
print(c)

3、Hello,CTF

题目告诉我们，不一定是要明文比对。
平时做逆向的时候主要在于反汇编的代码的观察练习，这个题16进制解密这串字符串就行：

解密结果：CrackMeJustForFun

4、open-source

给了源码：
 

#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;
}

分析一下，要能跳过前面三个if语句，才能把hash输出，根据条件我们可以算出argv[1]=51966,argv[2]=25,argv[3]=h4cky0u

然后把hash输出就行了：
 

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

int main() {

    unsigned int hash = 51966 * 31337 + (25 % 17) * 11 + strlen("h4cky0u") - 1615810207;

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

Get your key: c0ffee