Get unique identifier for security key

[mkalioby]

This is just a question

How to get a unique identifier for the key so that the user can't register it again?

[cpiper]

The key handle that is listed for each key is a unique identifier. If you go to the advanced options and check "Prevent Reregistration" it will send down the key handle as part of the excludeCredentials list and it won't allow it to be registered again.