Risk of personal identity theft, especially in connection with on-line commerce, is mitigated by maintaining private data in a secure database maintained by a trusted third party verification service. To authenticate the identity of a user or customer, in one embodiment, a knowledge-based challenge is issued to the user, and the response is compared to stored data by the verification service. The verification service reports to the vendor, to authenticate the user identity, capability and or authorization for the proposed transaction without disclosing private data to the vendor.