gistfile1.c

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

int main(int argc, const char **argv) {
    uint32_t check = 0xBADBEEF;
    uint32_t i;
    uint8_t digit;
    const char *key;
    char c;

    if (argc < 2) {
        printf("USAGE: prosum <cdkey>\n");
        return 1;
    }

    key = argv[1];

    if (strlen(key) != 8) {
        printf("ERROR: cdkey must be 8 chars\n");
        return 1;
    }

    for (i=0; i < 8; i++) {
        c = key[i];

        if (c < 'A' && c > 'F' && c < '0' && c > '9') {
            printf("ERROR: cdkey must contain uppercase letters and numbers\n");
            return 1;
        }

        digit = (uint8_t)c;
        check += (0x1337B00B * digit);
    }

    digit = ((check & 0xFF) / 2) * 2;

    if (digit != 42) {
        printf("ERROR: cdkey is invalid\n");
        return 1;
    }

    printf("Pro version unlocked\n");
    return 0;
}