We have to reverse the binary. Decompiled code:

The program basically XOR our input with number 3 and then compare the result with 'flag_matrix' which is:

The XOR has the commutative property:

input ^ 3 = flag_matrix

flag_matrix ^ 3 = input

Let's write a program wich XOR the flag with 3:

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

void main(){
    char v7[104] = "[.NBPx67m47\\26\\a7g\\74\\o2`0m60\\`k0`h2m5~" ;
    int i;
    char s1;

    for (i = 0; v7[i]; ++i){
        s1 = v7[i]^3;
        printf("%c", s1);
    }
    return;
}

FLAG

X-MAS{54n74_15_b4d_47_l1c3n53_ch3ck1n6}

Original writeup (https://github.com/Internaut401/CTF_Writeup/blob/master/2019/X-MAS%20CTF%202019/Santa's%20crackme.md).