A management site (10) generates an encrypted message by a public-key symmetrical encryption algorithm, the algorithm and the key being selected by the management site among a memorized list. The message (DKE), which includes an identifier of the encryption algorithm and key used, is transmitted to a portable device (16), which stores it. For the use, the message is transmitted to a secured module (20) that decrypts it, checks its compliance with an internal reference, and generates a digital accreditation controlling the unlocking of a lock device (18). The decryption is operated with the algorithm and the key recognized based on the identifiers of the message, selected among a list memorized by the secured module.