Building a certification authority that is both decentralized and fully reliable is impossible. However, the limitation thus imposed on scalability is unacceptable for many types of information systems, such as e-government services. We propose a solution to build an highly reliable certification authority, based on a distributed hash table and a dedicated protocol ensuring a very low probability of arbitrary failure. Thus, in practice, false positives should never occur. This talk briefly presents the protocol and shows its verification in two steps: (1) a formal model to assess that the protocol behaves as expected in an "ideal world" where communications are reliable, and, (2) a probabilistic analysis to evaluate the probability of failure of the certification.