checksum.cpp

#include <stdio.h>
#include <stdint.h>
#include <windows.h>

const char * ALPHABET32 = "ABCDEFGHJKLMNPQRSTUVWXYZ23456789\0";
int MAGIC = 91717;
int g_dwSeed = 0;

int8_t getCharIndex(char c)
{
    char *p = (char *)ALPHABET32;

    do
    {
        if ( *p == c )
            return p - ALPHABET32;
        ++p;
    }
    while ( *p );

    return -1;
}

void RandomizeSeed()
{
    if ( !g_dwSeed )
        g_dwSeed = 0x74D99E;

    g_dwSeed = (0x41A7 * g_dwSeed % 0x1F31D) - (0xB14 * g_dwSeed / 0x1F31D);

    if ( g_dwSeed <= 0 )
        g_dwSeed += 0x7FFFFFFFu;
    printf("seed: %.8X\n", g_dwSeed);
}

bool make_bits_array(char bits_array[12], const uint8_t *arKey)
{
    memset(bits_array, 0, 12);

    int64_t offset = 0;

    /* Make the bits array ==> array of 5 bits */
    for (int i = 0; i < 18; ++i )
    {
        uint8_t pos = offset & 7;
        uint8_t idx = getCharIndex(arKey[i]);
        if ( idx == 255 )
            return false;
        bits_array[offset / 8] |= idx << pos;
        bits_array[offset / 8 + 1] |= idx >> (8 - pos);
        offset += 5;
    }
    return true;
}

void print_bits_array(const char* title, char bits_array[12])
{
    puts(title);
    for(int i=0; i<12; i++)
        printf("%.2X ", (uint8_t)bits_array[i]);
    puts("\n");
}

const uint8_t get_expected_checksum(int seed)
{
    uint8_t expected_chk = 0;
    for (int i = 0; i < 8; ++i )
        expected_chk ^= seed >> 4 * i;
    expected_chk &= 0x0F;
    return expected_chk;
}

void brute(int seed)
{
    printf("possible bytes with seed %.8X:\n", seed);
    const uint8_t expected_checksum = get_expected_checksum(seed);
    for(int i=0,count=0; i<0x100; i++)
        for(int j=0; j<0x100; j++)
        {
            uint8_t computed_chk = (i >> 6) | ((j & 3) << 2);
            if(computed_chk == expected_checksum)
            {
                count++;
                if(!(count % 8))
                    puts("");
                printf("%.2X,%.2X=%.2X ", i, j, computed_chk);
            }
        }
    puts("\n");
}

bool KeyCheck(int seed, const uint8_t *arKey)
{
    char bits_array[12];
    if(!make_bits_array(bits_array, arKey))
    {
        puts("invalid characters in key!");
        return false;
    }

    print_bits_array("bits_array:", bits_array);

    g_dwSeed = seed + 0x2B8ECCB;
    for (int i = 0; i < 12; ++i )
    {
        RandomizeSeed();
        g_dwSeed ^= bits_array[i];
        bits_array[i] ^= g_dwSeed >> 16;
    }

    print_bits_array("\nrandomized:", bits_array);

    /* Compute the expected checksum */

    uint8_t expected_chk = get_expected_checksum(seed);
    printf("Expected Checksum=%x\n", expected_chk);

    uint8_t computed_chk = (bits_array[10] >> 6) | ((bits_array[11] & 3) << 2);
    printf("Computed Checksum=%x\n", computed_chk);

    return (computed_chk == expected_chk);
}

int main(int argc, const char *argv[])
{
    //brute(MAGIC);

    /* entered key: xxxxxx-xxxxxx-xxxxxx and then to upper string */
    const uint8_t *szKey = (const uint8_t*)"22222233333344444Z\0";

    if(!KeyCheck(MAGIC, szKey))
        printf("\n==> Invalid Key (checksum mismatch) !\n");
    else
        puts("\nyou win!");

    return 0;
}