  Quorum systems are a key abstraction in distributed fault-tolerant computing
for capturing trust assumptions. They can be found at the core of many
algorithms for implementing reliable broadcasts, shared memory, consensus and
other problems. This paper introduces asymmetric Byzantine quorum systems that
model subjective trust. Every process is free to choose which combinations of
other processes it trusts and which ones it considers faulty. Asymmetric quorum
systems strictly generalize standard Byzantine quorum systems, which have only
one global trust assumption for all processes. This work also presents
protocols that implement abstractions of shared memory, broadcast primitives,
and a consensus protocol among processes prone to Byzantine faults and
asymmetric trust. The model and protocols pave the way for realizing more
elaborate algorithms with asymmetric trust.
