  We propose a scheme for authentication of physical keys that are materialized
by optical multiple-scattering media. The authentication relies on the optical
response of the key when probed by randomly selected coherent states of light,
and the use of standard wavefront-shaping techniques that direct the scattered
photons coherently to a specific target mode at the output. The quadratures of
the electromagnetic field of the scattered light at the target mode are
analysed using a homodyne detection scheme, and the acceptance or rejection of
the key is decided upon the outcomes of the measurements. The proposed scheme
can be implemented with current technology and offers collision resistance and
robustness against key cloning.
