NotBoundAuthentication

How is the shared secret "holder" in an NotBoundAuthentication intended to be
identified?

Apart from altering the schema (adding a "Name" attribute) the only
reasonable option seems to be, to combine these two pieces of
information and include their base64 encoding in the Value attribute.

For example, a protocol defined out of scope to XKMS and identified by the URI
urn:example-protocol:username-password specifies that the Value
attribute carries
a username/password pair separated by a ':'  would take the form of
the following
instance fragment

<NotBoundAuthentication 
    Protocol="urn:example-protocol:username-password"
    Value="YWxpY2U6c2VjcmV0"/>

Regards
Tommy