Hi Caolan!

I do not know exactly how Word handles the 0x200 bytes wrap, but I have
knowledge how
Excel does (it has 0x400 page size). I believe that both programs use almost
the same encryption Algo:

// from wvDecryptTest.c

/* ... skipped ... */
int verifypwd (u_char pwarray[64], u_char docid[16], u_char salt[64], u_char
hashedsalt[16])
{
/* ... skipped ... */
    pwarray[56] = 0x80;
    pwarray[57] = 0x0A;
    MD5Transform (keytab2, (u_long *) pwarray);

    /* Generate 40-bit RC4 key from 128-bit hashed password */

    MD5Init (keytab3);
    memset(pwarray, 0, 64);
    memcpy(pwarray, keytab2, 5);
    pwarray[9] = 0x80;
    pwarray[56] = 0x48;

    /* !!! Before this MD5Transform the fifth byte of keytab3 is set to the
page number !!! */
    MD5Transform (keytab3, (u_long *) pwarray);

    prepare_key ((u_char *) &keytab3, 16, &key);

Hope this will help you with your converter. If you have any other questions
regarding
MS Word encryption, please feel free to contact me.

===
Dmitry Sumin, Passware,