Encrypting with gpg
Encrypting and decrypting is easy with gpg.
To encrypt the text "ABC" and write it to a file called "abc.enc":
$ echo "ABC" | gpg --symmetric --force-mdc > abc.enc
To decrypt the "abc.enc" file:
$ gpg -d < abc.enc
gpg: CAST5 encrypted data
gpg: gpg-agent is not available in this session
gpg: encrypted with 1 passphrase
ABC
Each step asks for a passphrase. Symmetric encryption is encryption that can be encrypted and decrypted with the same information, in this case that information is the passphrase.
--force-mdc enables the modification detection code, so that gpg will be able to know if the encrypted text has been modified or not.