We analyze the problem of efficient distributed storage of information in a message-passing environment where both less than one third of the servers, as well as an arbitrary number of clients, might exhibit Byzantine behavior, and where clients might access data concurrently. In particular, we provide a simulation of a multiple-writer multiple-reader atomic read/write register in this setting which uses erasure-coding for storage-efficiency and achieves optimal resilience. Additionally, we give the first implementation of non-skipping timestamps which provides optimal resilience and withstands Byzantine clients; it is based on threshold cryptography.