Generally speaking, the simplest idea to deal with revocation problem is to maintain a revocation list (or a ``black list''). The signer reveals a personal value and the verifier is then able to say, by matching the received value against each entry of the ``black list'', if the person is revoked or not. Unfortunately, in the context of group signatures, it is not possible to reveal a personal value since it would compromise the anonymity of the signer. Using a smart card allows to give a simple solution to this problem. Figure 4 shows the general principle of this approach.
In a few words, each member owning a personal value (an identifier), the smart card will get the revocation list from the group manager database (or any database where the ``black list'' stands, e.g. the verifier device) and will check if one value of the list and its personal value match. If the card reaches the end of the list, it will accept to sign as a group member; and if its personal value lies in the list, then the card will refuse to sign and make itself out of order.
