Collective Authorities: Transparency and Decentralized Trust at Scale

Keynote talk by Bryan Ford, FCS 2016

Online infrastructure depends on many security-critical authorities
such as logging, time, directory, and software update services. These
authorities represent high-value attack targets to hackers, criminals,
and spy agencies, who can secretly compromise many hosts by stealing
keys from or coercing only one such "weakest-link" authority.  We
claim there is an urgent need to develop systems that mitigate these
weaknesses by decentralizing authorities into scalable, diverse,
"strongest-link" collective authorities, which we term "cothorities."
A cothority efficiently splits trust among tens, hundreds, or
thousands of independent parties, collectively enforcing agreed-upon
policies, and remaining globally secure unless many compromised
participants collude.

As a first step in this long-term program we introduce CoSi, a
cothority architecture for decentralized witness cosigning, which
increases the transparency and security of traditional centralized
authorities while remaining backward-compatible with and incrementally
deployable alongside their existing logic.  By increasing the
scalability of existing multisignature techniques, CoSi efficiently
ensures that every authoritative statement is validated and publicly
logged by a diverse group of witnesses before any client will accept
it, forcing secrecy-minded attackers to risk that any compromise will
be detected quickly.  As a second step, we adapt CoSi's collective
signing techniques to create ByzCoin, a blockchain architecture that
enhances Bitcoin with strong consistency, Byzantine fault tolerance,
higher throughput, and lower transaction latencies.  CoSi and ByzCoin
have been demonstrated to scale efficiently to support over 8,000
globally-distributed participants, while keeping collective signing
and transaction latencies to within a few seconds.