Private set intersection (PSI)

PSI answers “which entries do our sets share?” without either side learning the non-matching entries. The classic construction blinds each side’s entries with a commutative hash/ECDH transformation, so only items present in both sets produce comparable outputs.

Confium’s confium-privacy ships a PSI surface (the CLI demo uses salted hashing; the ECDH variant is the production path). Typical uses: contact discovery, fraud-list checks, ad conversion measurement. When the cardinality is enough, prefer cardinality-only mode — it leaks less.