A trusted device comprising: authentication circuitry configured to authenticate, with an authentication operation, a user having physical possession of said trusted device and thereby switch said trusted device to an authenticated state; retention monitoring circuitry configured to monitor physical possession of said trusted device by said user following said authentication operation and to switch said trusted device out of said authenticated state if said trusted device is not in physical possession of said user, wherein the retention monitoring circuitry comprises a plurality of detection circuits, the plurality of detection circuits comprising at least a photo-detector shielded from light when said trusted device is in physical possession of the user and a clasp configured to hold said trusted device attached to said user when said clasp is in a closed state, and wherein the retention monitoring circuitry is configured to: combine detection results from the plurality of detection circuits to determine a retention probability that said trusted device has remained in physical possession of said user since a previous authentication operation and to switch said trusted device out of said authenticated state if said retention probability falls below a threshold probability level, and reduce a retention probability that said trusted device is in physical possession of said user as a time since said authentication operation increases; communication triggering circuitry configured to detect a request to establish communication received from a target device that is one of a plurality of different target devices; and communication circuitry configured to communicate with said target device if said trusted device is in said authenticated state; wherein said authentication circuitry is configured to accept an authentication operation with an accepted probability of a false positive in identifying said user, wherein said accepted probability decreases as said retention probability increases. authentication circuitry configured to authenticate, with an authentication operation, a user having physical possession of said trusted device and thereby switch said trusted device to an authenticated state; retention monitoring circuitry configured to monitor physical possession of said trusted device by said user following said authentication operation and to switch said trusted device out of said authenticated state if said trusted device is not in physical possession of said user, wherein the retention monitoring circuitry comprises a plurality of detection circuits, the plurality of detection circuits comprising at least a photo-detector shielded from light when said trusted device is in physical possession of the user and a clasp configured to hold said trusted device attached to said user when said clasp is in a closed state, and wherein the retention monitoring circuitry is configured to: combine detection results from the plurality of detection circuits to determine a retention probability that said trusted device has remained in physical possession of said user since a previous authentication operation and to switch said trusted device out of said authenticated state if said retention probability falls below a threshold probability level, and reduce a retention probability that said trusted device is in physical possession of said user as a time since said authentication operation increases; combine detection results from the plurality of detection circuits to determine a retention probability that said trusted device has remained in physical possession of said user since a previous authentication operation and to switch said trusted device out of said authenticated state if said retention probability falls below a threshold probability level, and reduce a retention probability that said trusted device is in physical possession of said user as a time since said authentication operation increases; communication triggering circuitry configured to detect a request to establish communication received from a target device that is one of a plurality of different target devices; and communication circuitry configured to communicate with said target device if said trusted device is in said authenticated state; wherein said authentication circuitry is configured to accept an authentication operation with an accepted probability of a false positive in identifying said user, wherein said accepted probability decreases as said retention probability increases.