Described are various embodiments of a cryptographic process for portable devices, and user presence and/or access authorization systems and methods employing such protocols. In one embodiment, a digital user authentication system is described to comprise a wireless digital user authentication device (UAD) operable to authenticate the user and wirelessly communicate an authenticated identity thereof; and a network application operatively associated with a wireless access point and operable to authenticate the user presence. Upon the network application authenticating the user presence based, at least in part, on the authenticated identity, the UAD and the network application securely establish a short-term symmetric advertising (STSA) key. During a prescribed advertising lifetime of the STSA, the UAD periodically computes and advertises authentication codes encompassing the STSA key so to securely advertise the authenticated user presence.