Configuration of CPBM 1.4 - Cloud Portal Encryption Key
Yes, the details on the use of the key is not documented. A sort note is described in the System overview as follows:
"Encryption is applied to all sensitive data in the CloudPortal Business Manager databases such as Email addresses and passwords. These values are encrypted using Advanced Encryption Standard standard AES-128."
CPBM uses this key to encrypt all sensitive data in cloud portal databases.
This key can be different than the key you use for CloudPlatform.