1. A method of unlocking a locked structure comprising the steps of: (a) creating a cryptogram at an encryption device located remote of said locked structure, said encryption device having a first clock, by encrypting a time representation based upon said first clock with an encrypting cryptographic key for said locked structure using an encryption algorithm; (b) transporting said cryptogram from said encryption device to said locked structure; (c) inputting said cryptogram into a decryption device at said locked structure, said decryption device having: (i) a decryption algorithm corresponding to said encryption algorithm, (ii) a decrypting cryptographic key corresponding to said encrypting cryptographic key at said encryption device, and (iii) a second clock at the encryption device operating at a fixed time relationship to said first clock; (d) decrypting said cryptogram using said decryption algorithm and said decrypting cryptographic key at said decryption device to produce a decrypted time representation; (e) comparing said decrypted time representation with a time representation based upon said second clock; and (f) unlocking said lock at said structure if said decrypted time representation meets a time-based criteria relative to said time representation based upon said second clock.