Anonymously Publishing Liveness Signals with Plausible Deniability

Sometimes entities have to prove to others that they are still alive at a certain point in time, but with the added requirements of anonymity and plausible deniability; examples for this are whistleblowers or persons in dangerous situations. We propose a system to achieve this via hash chains and publishing liveness signals on Tor onion services. Even if one participant is discovered and (made to) cooperate, others still enjoy plausible deniability. To support arbitrary numbers of provers on a potentially limited list of online storage services, an additional "key" distinguishes multiple provers. This key should neither be static nor predictable to third parties, and provide forward secrecy. We propose both a derivation from user-memorable passwords and an initial pairing step to transfer unique key material between prover and verifier. In addition to describing the protocol, we provide an open source App implementation and evaluate its performance.