It is best to choose an encryption key value that cannot be easily guessed. The key can be of arbitrary length, but generally the longer the key, the better because a shorter key is easier to guess than a longer one. As well, including a combination of numbers, letters, and special characters decreases the chances of someone guessing the key.
Encryption keys are always case sensitive, and they cannot contain leading or trailing spaces or semicolons.
You must supply this key each time you want to start the database. Lost or forgotten keys result in completely inaccessible databases.
You can choose whether the encryption key is entered at a command prompt (the default) or into a prompt box. Choosing to enter the key in a prompt box provides an extra measure of security because the key is never visible in plain sight. Clients are required to specify the key each time they start the database. If the database administrator starts the database, clients never need to have access to the key.