  There has recently been a flood of interest in potential new applications of
blockchains, as well as proposals for more generic designs called public
ledgers. Most of the novel proposals have been in the financial sector.
However, the public ledger is an abstraction that solves several of the
fundamental problems in the design of secure distributed systems: global time
in the form of a strict linear order of past events, globally consistent and
immutable view of the history, and enforcement of some application-specific
safety properties. This paper investigates the applications of public ledgers
to access control and, more specifically, to group management in distributed
systems where entities are represented by their public keys and authorization
is encoded into signed certificates. It is particularly difficult to handle
negative information, such as revocation of certificates or group membership,
in the distributed setting. The linear order of events and global consistency
simplify these problems, but the enforcement of internal constraints in the
ledger implementation often presents problems. We show that different types of
revocation require slightly different properties from the ledger. We compare
the requirements with Bitcoin, the best known blockchain, and describe an
efficient ledger design for membership revocation that combines ideas from
blockchains and from web-PKI monitoring. While we use certificate-based
group-membership management as the case study, the same ideas can be applied
more widely to rights revocation in distributed systems.
