It seems that the papers I am reading do not talk much about this point. That is how the attributes of a user are authenticated in ABE system.
To my understanding, there is an attribute authority (AA) who authenticates the attribute of a user and there is a PKG who generates the private key based on the authenticated attributes. So, are AA and PKG two separated parties or a single party?
Also, I am reading some papers about the trust issue in AA and PKG. To my understanding, we can only reduce the trust in PKG but not in AA. Because, the AA can authenticate anyone (including itself) as an authenticated user possessing some attributes. Then after going through the normal process, that user can always obtain the valid private key. Thus, it seems that we still requires a fully trusted AA. I am not sure whether my understanding is right or not. Any comments on this point would be helpful.