When you first connect a CTERA Agent to the CTERA Portal, as part of the initial configuration, you can specify a passphrase to protect your backups. For example, when first connecting an agent running on a Windows platform to a CTERA Portal:

You can use an automatically generated encryption key or set your own passphrase.

You can only set the passphrase when setting up the cloud backup for the first time.
If a passphrase is set, then when trying to access the backups for that device, you are prompted for the passphrase:

Remember to keep the passphrase in a safe place. If lost, it cannot be recovered.
You can change a passphrase, as described in Changing a Device Backup's Passphrase.