Abstract: A key holder includes a ferrule, a multimode light guide at least partly embedded inside the ferrule, an optical key which has a light scattering material, and a mechanical mount which mounts each of the ferrule, the multimode light guide, and the optical key. The multimode light guide has a front facet and a back facet which are arranged at opposite ends. The back facet of the multimode light guide contacts the optical key. Light can enter into the multimode light guide via the front facet, propagate through the multimode light guide, be scattered by the optical key, and propagate back through the multimode light guide and exit via the front facet. The mechanical mount is detachably connected to a mechanical mount terminator. The front facet of the multimode light guide is oriented in a direction of the mechanical mount terminator.

Abstract: A method for a PUK authenticated communication includes creating an optical challenge in a time-frequency domain, providing the optical challenge to a tPUK which includes a spatial input channel and a plurality of spatial output channels, and detecting in which of the plurality of spatial output channels a short temporally focused pulse is created. The tPUK provides a complex challenge-response behavior in the time-frequency domain. The optical challenge is created so that the tPUK creates the response having a short temporally focused pulse in only one of the plurality of spatial output channels of the tPUK.