Language: C

Source code:

#include <stdio.h> int main() { char input[256]; char serial[] = "f2hwldozg|:wbq"; int i; printf("Enter flag to check: "); scanf("%s", input); for (i = 0; i < strlen(serial); i++) { if (input[i] + i != serial[i]) { printf("Wrong position %d!\n", i); return 0; } } printf("Yes! Correct flag is %s\n", input); return 0; }

Compiled Linux binary: mc5
Compiled Windows binary: mc5.exe