A transaction authentication system authenticates a transaction by determining whether a mobile device and POS device involved in the transaction are at the same location. A POS registry stores location data for POS devices. A PAN registry stores mobile device IDs corresponding to account numbers. A mobile device ID can be provided from the PAN registry in response to receiving an account number from a POS device. The mobile device ID can then be used to retrieve location information from a home location register maintained by a mobile service provider. The retrieved location data for a POS device and the retrieved location data for a mobile device are compared.