攻防世界-parallel-comparator-200
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源代码:
#include <stdlib.h>
#include <stdio.h>
#include <pthread.h>
#define FLAG_LEN 20
void * checking(void *arg) {
char *result = malloc(sizeof(char));
char *argument = (char *)arg;
*result = (argument[0]+argument[1]) ^ argument[2];
return result;
}
int highly_optimized_parallel_comparsion(char *user_string)
{
int initialization_number;
int i;
char generated_string[FLAG_LEN + 1];
generated_string[FLAG_LEN] = ‘ ‘;
while ((initialization_number = rand()) >= 64);
int first_letter;
first_letter = (initialization_number % 26) + 97; //经过调试发现是固定值 112
pthread_t thread[FLAG_LEN];
char differences[FLAG_LEN] = {0, 9, -9, -1, 13, -13, -4, -11, -9, -1, -7, 6, -13, 13, 3, 9, -13, -11, 6, -7};
char *arguments[20];
for (i = 0; i < FLAG_LEN; i++) {
arguments[i] = (char *)malloc(3*sizeof(char));
arguments[i][0] = first_letter;
arguments[i][1] = differences[i];
arguments[i][2] = user_string[i];
pthread_create((pthread_t*)(thread+i), NULL, checking, arguments[i]);
}
void *result;
int just_a_string[FLAG_LEN] = {115, 116, 114, 97, 110, 103, 101, 95, 115, 116, 114, 105, 110, 103, 95, 105, 116, 95, 105, 115};
for (i = 0; i < FLAG_LEN; i++) {
pthread_join(*(thread+i), &result);
generated_string[i] = *(char *)result + just_a_string[i];
free(result);
free(arguments[i]);
}
int is_ok = 1;
for (i = 0; i < FLAG_LEN; i++) {
if (generated_string[i] != just_a_string[i])
return 0;
}
return 1;
}
int main()
{
char *user_string = (char *)calloc(FLAG_LEN+1, sizeof(char));
fgets(user_string, FLAG_LEN+1, stdin); //输入20个字符
int is_ok = highly_optimized_parallel_comparsion(user_string);
if (is_ok)
printf("You win!
");
else
printf("Wrong!
");
return 0;
}
粗略看一遍,发现 pthread_t
以及pthread_join
pthread_create
函数看不懂,于是网上查资料,得知
pthread_t
typedef unsigned __int64 uintptr_t
typedef uintptr_t pthread_t;
pthread_create :作用为创建新线程,如果创建成功则返回0
,否则返回error number
int pthread_create(pthread_t *thread, const pthread_attr_t *attr,void *(*start_routine)(void*), void *arg);
-
pthread_t *thread
是线程的标识符 -
pthread_attr_t *attr
用来设置线程属性(如果为NULL,则为默认属性} -
void *(*start_routine)(void*)
是运行函数的起始地址 -
void *arg
运行函数的参数
prhread_join : 作用为等待一个线程的结束,如果成功则返回0
,否则返回error number
int pthread_join(pthread_t thread, void **value_ptr);
-
pthread_t thread
线程标识符 -
void **value_ptr
存储被等待线程的返回值
有了以上知识,可以很容易可以知道user_string[20] (输入数组)的每个元素加上first_letter(经调试为固定值112),然后再与 differences[20]的元素异或,最后与
just_a_string[20]的元素进行比较,用代码描述就是:
strcmp( ( user_string[i] + first_letter ) ^ differences[i] , just_a_string[i] )
脚本:
a = [0, 9, -9, -1, 13, -13, -4, -11, -9, -1, -7, 6, -13, 13, 3, 9, -13, -11, 6, -7]
flag = ‘‘
for i in range(len(a)):
for j in range(127):
if ((a[i]+112) ^ (j+1)) == 0:
flag += chr(j+1)
break
print(flag)
结果当我满怀欣喜地提交flag后,却得知flag是错的,后来实在没有思路就看了别人的wp才知道真相:
first_letter
在windows平台上是112,在linux上是108
wtf?
参考文章:
https://www.cnblogs.com/mq0036/p/3710475.html
https://pubs.opengroup.org/onlinepubs/7908799/
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