Patent ID: 7389530

Claim:
A method for secure unlocking of a door based on a shared secret key, comprising the steps of: providing a portable computing device, wherein the computing device is equipped with a memory, and the memory holds shared secret keys with matching door identifiers and a first certificate, wherein the computing device is adapted for performing operations with shared secret keys and certificates, and wherein the computing device is adapted for communicating with the door; the computing device communicating to the door a device identifier; the door makes a decision to issue a challenge to the computing device, wherein the challenge is issued only on randomly selected occasions; the computing device responding to the challenge by demonstrating possession of a private key of the first certificate; after a successful response to a challenge and after receipt of computing device identifier when a challenge decision is not made the door responding with a door identifier and with an encrypted message, wherein the message is encrypted with the shared secret key, and wherein using the shared secret key for encrypting the message resulted from recognizing the device identifier communicated by the computing device; the computing device responding with a signal attesting decryption of the message, wherein the message has been decrypted in the computing device by the shared secret key, and wherein using the shared secret key for decrypting the message resulted from recognizing the door identifier transmitted by the door; and the door unlocking upon recognizing validity of the signal attesting decryption of the message.