Enforcing proof-of-possession

Recall that the TokenCanceller interface has a method "setVerifyProofOfPossession" which defines whether proof-of-possession is required or not to cancel a security token. The default value for the SCTCanceller is "true".

This means that for the client to successfully cancel a SecurityContextToken it must prove to the STS that it knows the secret key associated with that SecurityContextToken. The client must do this by signing some portion of the request with the same secret key that the SCTCanceller retrieves from the security token stored in the cache.