Reaching agreement in the presence of Byzantine (arbitrary) faults is a fundamental problem in distributed systems. It has been shown that message authentication is a useful tool in designing protocols with high fault tolerance, but it imposes the additional problem of key distribution. In the past, agreement protocols using message authentication required complete agreement on all public keys. Because this pre-agreement h a s t o rely on techniques outside the system (e.g., trusted servers

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... h n e v er fail), it is useful to consider lower levels of key distribution which n e e d as few assumptions as possible. In this paper, we identify several levels of key distribution and describe their properties with regard to the achievable fault tolerance in two agreement problems.