Just about anything will do:

1. Just add all the content of the bytes you send.
2. If you send a lot of 0x00, add the number of bytes sent to the checksum.
3. For simple 'security', assign each customer or application an 8 bit ID.
XOR the results of 1 or 2 with this ID.

With serial, there shouldn't be much data loss, so something simple like
this is fine.
Ed Todd <edtodd@sni.net>
http://www.sni.net/cedardell