little client + server app : has this been done?

[Python Baby]

I'm about to try a little test project in Python, but want to make sure
I'm on the right foot.

Any advice appreciated before I start this.  MOST important : I want
to make sure I'm not just re-inventing the wheel, if there's a library
out there that does this exact thing already.


SERVER:
Has some protected files, not in a webroot, that I want to let the client
download only if they pass me a verified key in the server's "OK" database.

CLIENT:
Knows which files it wants (by file id# not full path).  Knows its key to
get them.  Passes some kind of message to the remote server saying, "Here
is my key.  Here is the file I want."  Server replies by passing the file,
and a little metadata about the file.


Sound familiar?
Should I do it from scratch or is this just some well-known library I
don't know about?   (I searched around, of course, but couldn't find one.)

Thanks!