Reputation

The first and oldest family of Sybil-resistance: trust built from accumulated history. Its digital lineage runs from PGP's web of trust (1991) and eBay feedback (1996) through Advogato's group trust metric (1999) to EigenTrust (2003) and its modern incarnation OpenRank. It provides earned trust but collapses when the underlying identity is cheap to discard (Friedman–Resnick), which is why it needs the identity family beneath it.