Command line encryption tool for both Linux and Windoze?

[Søren Neigaard]

I could use a small simple command line tool to encrypt/decrypt the
files on my USB pendrive, and it could be cool if it was small enough to
be on my pendrive also. And it has to work in both Linux and Windows
(probably to versions of same tool, or two tools that can
encrypt/decrypt the same files).

Does any of you guys know of such a tool, or have any ideas oh how to do
this?

Best regards
Søren