create password with VolumeSerialNumber

Hi to all,
I want to implement on my project a license file where inside
there's a number code.
I would use the VolumeSerialNumber for this purpose but I don't know how...
e.g.
the customer give me the serialnumber and I have to "modify" it, to obtain
a valid code number that is inside my program.

so -->VolumeSerialNumber-->Modify number -->inside program "If code is == xxxx than --> OK".
Someone have some suggestions...

Many Tanks :-)
mkbs