Christmas CTF 2019-welcome rev

처음에는 CRC를 역연산 해보려고 삽질하다. 그냥 부르트 포싱했다

#include <inttypes.h>
#include <stdio.h>
#include <string.h>
#include<iostream>
#include<string>
using namespace std;


uint32_t rc_crc32(uint32_t crc, const char *buf, size_t len)
{
	static uint32_t table[256];
	static int have_table = 0;
	uint32_t rem;
	uint8_t octet;
	int i, j;
	const char *p, *q;

	/* This check is not thread safe; there is no mutex. */
	if (have_table == 0) {
		/* Calculate CRC table. */
		for (i = 0; i < 256; i++) {
			rem = i;  /* remainder from polynomial division */
			for (j = 0; j < 8; j++) {
				if (rem & 1) {
					rem >>= 1;
					rem ^= 0xedb88320;
				}
				else
					rem >>= 1;
			}
			table[i] = rem;
		}
		have_table = 1;
	}
	crc = -1;
	q = buf + len;
	for (p = buf; p < q; p++) {
		octet = *p;  /* Cast to unsigned octet. */
		crc = (crc >> 8) ^ table[(crc & 0xff) ^ octet];
	}
	
	return crc;
}

int main()
{
	uint32_t num = 0;
	char s[5];
	char brute[]=
		"abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789{}_";
	uint32_t nam[100] = { 0xb3406737,0x94789c6e,0x93574866,0x0ebfe856,0xc039f1d5 };
	for (int i = 0; i < strlen(brute); i++) {
		for (int j = 0; j < strlen(brute); j++) {
			for (int k = 0; k < strlen(brute); k++) {
				for (int z = 0; z < strlen(brute); z++) {
					s[0] = brute[i];
					s[1] = brute[j]; 
					s[2] = brute[k]; 
					s[3]=brute[z];
					s[4] = NULL;
					num = rc_crc32(0, s, 4);
					for (int q = 0; q < 5; q++) {
						if (num == nam[q]) {
							cout << s << "::::";
							printf("%" PRIX32 "\n", num);//B3406737
						}
					}
				}
			}
		}
	}
	
	//printf("%" PRIX32 "\n", de_crc32(nam[0], s, 4));
	return 0;
}

돌리면 2초안으로 결과가 나온다

HEX값을 보고 맞추면

XMAS{veryy_easy_crc}