APSS, a proactive secret sharing (PSS) protocol for asynchronous systems, is derived and proved correct. A PSS protocol enables a set of secret shares to be periodically refreshed with a new, independent set, thereby thwarting so-called mobile adversary attacks. APSS tolerates certain attacks that PSS protocols for synchronous systems cannot, because protocols for asynchronous systems are inherently less vulnerable to denial of service attacks, which slow processor execution or impede message delivery and thus violate the defining assumptions of a synchronous system.