A Tor-format key is cryptographic key material used by Tor onion services to establish identity and publish service data. It is not just a login secret; it is part of the material that ties a .onion address to a specific service and helps produce or sign the descriptors clients rely on to find it.
This matters because onion-service trust is built on the key, not on DNS names or certificates in the usual web sense. If an attacker steals, derives, or abuses this key, they may be able to impersonate the service, publish fraudulent descriptors, or take over the public identity seen by users. Defenders should store Tor-format keys as high-value secrets, limit which tools can access them, monitor the publication workflow, and rotate or replace identity material if compromise is suspected.