In MacOS or Linux, you can use openssl on the CLI easily.
$ echo -n "private-message" | openssl enc -e -aes-256-cbc -a -salt enter aes-256-cbc encryption password: Verifying - enter aes-256-cbc encryption password:
Then to decrypt echo 'U2Fsxxxxxxxxxx' | openssl aes-256-cbc -a -d -salt