Automatically transferring (syncing) files between multiple computers is easy these days. Dropbox, owncloud or bitpocket to name a few. You can imagine I use the latter (if you want a recommendation)1.
In any case you want to encrypt what you send to be stored in "the cloud" even if it is just for a short time. There are many options how to encrypt the "in flight" data. Symmetric ciphers are probably the safest and most widely researched cryptography these days and easier to use than asymmetric key pairs in this context as well.
Encryption is notoriously hard to implement correctly and worthless when the implementation is flawed. So I looked at gpg, a well known reference implementation, and was amazed that it can neither use a proper keyfile for symmetric encryption (you can just supply a passphrase via
--passphrase-file) nor does it handle multiple files on the command line consistently.
You can use
--multifile (wondering...why does a command need that at all?) with
--encrypt (asymmetric public/private key pair encryption) but not with
--symmetric (symmetric shared key encryption). Duh!
With a bit of scripting around the gpg shortcomings, you end up with
crypt_gpg that can nicely encrypt or decrypt multiple files (symmetric cipher) in one go.
Dropbox is closed source so it cannot be assessed for its security. Owncloud needs a thorough code review before I would dare to run it on my systems. ↩