Abstract: Method for obtaining authenticity identification devices for using services in general wherein authenticity is guaranteed without resorting to the manufacturer of the device to ensure its validity; the method has the particularity that it consists in preparing an identification device with a read-only area and at least one writable area; the manufacturer of the device applies a first permanent and always different code to the read-only area. The service provider applies to the writable area a second code obtained by means of one-way functions that have a secret encryption key by computing the first permanent code. The identification device validated with the second code can be verified by means of a decryption key which may optionally be public.