The SSL Visibility user store is encrypted with a chain of AES 256-bit keys using tree data structure terminology:
the “leaf” of the chain encrypts the user store
each key in the chain is encrypted with its “parent”
the “root” of the chain is encrypted either with either
a password stored on the SSLV internal hard disk OR
a key stored on a remove USB memory stick.
The chain of keys is generated during the appliance bootstrap sequence at first power-up. At this time, the user chooses whether to protect the chain with an internal password, or a removable USB memory stick.