Fortress: Fork Resistant Transparency logs Relying on Social graph Structure

Transparency systems -- such as Key Transparency (KT), Certificate Transparency, and binary transparency -- require that all honest participants see the same view of an untrusted server's directory, or, failing that, that the misbehaviour is publicly detected. Every existing design closes this gap with an external trust anchor: a publicly available bulletin board (BB), a blockchain, or an out-of-band (OOB) gossip network among auditors. We observe that transparency systems already have a built-in resource that has so far been overlooked: the social graph induced by the users themselves. We ask whether it can replace the bulletin board or the external gossip channel entirely, and answer in the affirmative (if occasional out-of-band interactions are allowed). We formalise a new primitive, Social-Graph based Forking-Resilient Transparency (sg-FORT), which captures the security guarantees of a BB-free transparency system and we construct Fortress Fork Resistant Transparency logs Relying on Social graph Structure, to our knowledge the first such protocol. In every epoch either all users output the same post, or at least one honest user produces a publicly verifiable Proof of Misbehaviour. Our guarantees hold against an arbitrary malicious server, including a snapshot adversary that may compromise a user and learn its secret state, as long as the server does not compromise close users. In honest executions, OOB interactions occur only when a user resets their secrets, so the total OOB cost approaches the natural lower bound of one call per reset -- dramatically below the full-gossip alternative. We prove security under standard cryptographic assumptions on signatures and dictionary commitment. Under simplifying assumptions (synchronous epochs, always-online users), we view this as a first feasibility result that opens up the design space of social-graph-based transparency systems.