distinguishing identifier x and its public key publickey x and token represents a token field defined as follows token tb r a u2225 r b pub a u2225 pub b u2225 ss tp r a u2225 r b u2225 pub a u2225 pub b u2225 text3 token tb 1 r b u2225 pub a u2225 text5 u2225 ss tp r b u2225 pub a u2225 text5 token tb 2 r a u2225 pub b u2225 text6 u2225 ss tp r a u2225 pub b u2225 text6 step s 21 the entity a transmits to the entity b a message 1 including a time variant parameter r a an identity id a the token tokenab and an optional text field text 1 step s 22 upon reception of the message 1 the entity b transmits to the ttp a message 2 including time variant parameters r a and r b entity identities id a and id b and an optional text field text 2 step s 23 upon reception of the message 2 the ttp verifies the entities a and b particularly if the entity identities of the entities a and b in the message 2 are certificates then the validity of the certificates of the entities a and b are checked and if they are invalid then the message 2 is discarded directly or a message 3 is returned if they are valid then the message 3 is returned if the entity identities of the entities a and b in the message 2 are entity distinguishing identifiers then respective public keys of the entities a and b are retrieved and verified and if no public keys are found or are invalid then the message 2 is discarded directly or the message 3 is returned if the.