© The British Computer Society 2017. All rights reserved. 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, which require an infrastructure able to support heavy computation loads while remaining highly reliable. Our scalable approach opts for the next best thing to full reliability: a certification authority with a probability of arbitrary failure so low that, in practice, false positives should never occur.