Quote - Do I understand it correctly when assuming I need only one OS version when encoding (for example Windows) or do I need one for each OS?  It is not stated clearly on the site.

That's mostly correct.

The problem is that sometimes the file used as a key will have different line endings on Windows and Macs, which will mess up cross platform decoding (this can only be cured by new code).
Otherwise the only solutions to this is to use a binary file (instead of a text file) as the key, or provide a seperate utility to preprocess the key file to standardise line endings before encoding or decoding.