make it ask a key from the user, then store it in a dot file in the homedirectory (Linux) or eventually in the registry (Win32). Have the script check for the value before it asks the user for it.

I have only one remark of a linguistic nature: I see that you're from Germany, and hence probably not a native English speaker. I'm not, either: I'm from Italy. Here we have the adverb "eventualmente" which may seem to map 1-to-1 to the English "eventually". However the former means "possibly", which I assume is what you really meant, and the latter "in the end" or "finally". I suspect that something similar happens with German...

There are other cases, like "attualmente" and "pretendere" that have quite different meanings than "actually" and "to pretend" respectively.

I would have rather /msg'd this, but it wouldn't fit - an maybe it's not so bad to post it for the benefit and hopefully the interest of other readers.