Never share the link to an encrypted file and the encryption key using the same transmission mechanism.
For example, email the link to a file, and then send the encryption key by text message, never send the file and encryption key by email.
Never re-use encryption keys
You can generate and store encryption keys in the Mac application Keychain Access.
Open the Applications folder, browse to Utilities .
Select Local Items in the sidebar labelled Keychains , select the Keys category.
Select the key icon, and use the Password Assistant to generate a new passphrase with the following settings;