value proposition with privacy protection, as these groups are used to implement price and value discrimination of tickets. Note that the PCA is free in the choice of methods to implement group certificates. This could be done by simply using the same key pair for the group or by the existing, sophisticated group signature schemes [@Chaum91].

If a TA wants to acquire a rating ticket from group $g$, he first generates an AIK using the `TPM_MakeIdentity` command. Next, TA requests from the PCA a credential for this AIK, belonging to group $g$, by sending AIK, group identifier and supplementary data as required by TCG protocols, to the PCA. The PCA now knows the identity of the TA. This can be used to perform a charging for the ticket, either by contacting CP or by the PCA itself (how charging actually works is not in the scope of this paper). It is important that, at this stage, an authorisation decision on the ticket generation can be made by the PCA, for instance to blacklist misbehaving participants. If the authorisation succeeds (and not earlier, to save bandwidth and resources), the PCA performs a handshake operation with the TA to ensure that the 