What is a viewing key?
A viewing key is a Zcash credential that lets its holder grant someone else read-only access to their shielded address activity. A compliance team, auditor, or investigator who receives a viewing key can see the relevant incoming (and, with an extended key, outgoing) transactions, but cannot move funds. Spending control stays entirely with the spending key holder.
Why it exists
Shielded pools hide sender, receiver, and amount behind zk-SNARK proofs. That creates a legitimate problem: how does a user prove their own financial history to a counterparty without abandoning privacy entirely? The viewing key is Zcash’s answer. Privacy becomes selectively disclosable — the holder chooses what to reveal, to whom, and can revoke future visibility by rotating addresses.
How investigators use it
Viewing keys change the shape of investigations involving shielded pools. Public tracing breaks inside the pool, but a suspect, an exchange customer, or a court order can produce a viewing key that turns opaque history into readable evidence without touching the funds. This is the sharpest contrast with Monero, which has no equivalent mechanism and therefore no way for a user to cryptographically self-disclose.
Spotting the fake version
Some products advertise “encrypted” or “private” identity features that are really just interface choices. A quick tell: genuine optional disclosure always involves a cryptographic act the holder performs (issuing a viewing key, signing a message, presenting a proof). If a product claims privacy features but never asks anyone to perform a verifiable act, nothing in it is actually proven. See Tornado Cash for the mixer-side pattern of provable-without-revealing withdrawals.