Windows 95 Key generator

win95_key_gen.c

#include <stdio.h>
#include <stdint.h>
#include <stdlib.h>
#include <stddef.h>
#include <string.h>
#include <stdbool.h>
#include <time.h>

#define random_between(min, max) (rand() % (max - min) + min)

bool keygen_first_3_letters_are_invalid(const char *string) {
    for (char c = '3'; c <= '9'; c++) {
        if (string[0] == string[1] == string[2] == c) return true;
    }
    return false;
}

bool keygen_last_7_letters_is_divisble(const char *string) {
    int n = 0;
    for (int i = 0; i < 7; i++) {
        n += string[i] - '0';
    }
    return !(n % 7);
}

char keygen_return_random_digit(void) {
    return random_between('0', '9');
}

void keygen_generate_non_oem_key(char *key, size_t length) {
    if (length < 12) return;

    memset(key, '0', 11);
    key[12] = 0;

    key[3] = '-';

    do {
        for (int i = 0; i < 3; i++) {
            key[i] = keygen_return_random_digit();
        }
    } while (keygen_first_3_letters_are_invalid(key));

    do {
        for (int i = 0; i < 7; i++) {
            key[i + 4] = keygen_return_random_digit();
        }
    } while(!keygen_last_7_letters_is_divisble(&key[4]));
}

void keygen_generate_oem_key(char *key, size_t length) {
    if (length < 24) return;

    memset(key, '0', 23);
    key[23] = 0;

    key[5] = '-';
    key[6] = 'O';
    key[7] = 'E';
    key[8] = 'M';
    key[9] = '-';

    key[17] = '-';

    int day = random_between(1, 366);
    int year = (random_between(1995, 2002)) % 100;

    key[0] = ((day / 100) % 10) + '0';
    key[1] = ((day / 10) % 10) + '0';
    key[2] = ((day) / 100) + '0';

    key[3] = (year / 10) + '0';
    key[4] = (year % 10) + '0';

     do {
        for (int i = 0; i < 6; i++) {
            key[i + 11] = keygen_return_random_digit();
        }
    } while(!keygen_last_7_letters_is_divisble(&key[10]));

    for (int i = 0; i < 5; i++) {
        key[i + 18] = keygen_return_random_digit();
    }
}

int main(void) {
    char key[24] = {0};

    srand(time(0));

    keygen_generate_non_oem_key(key, 24);
    printf("Key: %s\n", key);

    keygen_generate_oem_key(key, 24);
    printf("OEM Key: %s\n", key);

    return 0;
}