If you don't mind to install a lot of perl modules, you could use Crypt::OpenPGP. I've been using it for a while, and it works quite well. You can find PGP software for pretty much any platform, so you could code/decode the files anywhere, even without Perl installed.

If installing the dependencies is too much work for you, most crypto modules for popular ciphers are safe and pretty 'standard' - try Crypt::Blowfish or Crypt::Rijndael, for instance. The former has shown it's use and security, the latter has been chosen as standard cypher for encryption of official data by e.g. the American government.