OpenSSL is on every system and can encrypt files with aes-256-gcm if for whatever reasons libgcrypt can't be used http://stackoverflow.com/questions/12153009/openssl-c-exampl...
Tarsnap you can copy keys to any platform that will run Tarsnap http://www.tarsnap.com/man-tarsnap-keygen.1.html http://jamesoff.net/site/2009/09/10/tarsnap-under-cygwin/