/*
The following is the simple algorithm used by Half-Life to know if a CD-Key is
valid or not.

by Luigi Auriemma
e-mail: aluigi@autistici.org
web:    aluigi.org

*/


    unsigned char   cdkey[],    /* buffer that contains the Half-life CD-KEY */
                                /* the key must be 13 bytes long and all the */
                                /* '-' chars must be removed                 */
                    *ptr;       // a pointer for faster operations
    unsigned long   eax,
                    edx,
                    edi,
                    i;          // counter


    eax = 3;
    ptr = cdkey;
    for(i = 0; i < 12; i++, ptr++) {
        edi = eax << 1;
        edx = *ptr - 0x30;      // char '0'
        edx ^= edi;
        eax += edx;
    }
    eax %= 10;
    eax -= *ptr;
    if(eax == 0xFFFFFFD0) fputs("CD-Key is OK!\n", stdout);
        else fputs("CD-Key is invalid\n", stdout);