  Quantum key distribution, which allows two distant parties to share an
unconditionally secure cryptographic key, promises to play an important role in
the future of communication. For this reason such technique has attracted many
theoretical and experimental efforts, thus becoming one of the most prominent
quantum technologies of the last decades. The security of the key relies on
quantum mechanics and therefore requires the users to be capable of performing
quantum operations, such as state preparation or measurements in multiple
bases. A natural question is whether and to what extent these requirements can
be relaxed and the quantum capabilities of the users reduced. Here we
demonstrate a novel quantum key distribution scheme, where users are fully
classical. In our protocol, the quantum operations are performed by an
untrusted third party acting as a server, which gives the users access to a
superimposed single photon, and the key exchange is achieved via
interaction-free measurements on the shared state. We also provide a full
security proof of the protocol by computing the secret key rate in the
realistic scenario of finite-resources, as well as practical experimental
conditions of imperfect photon source and detectors. Our approach deepens the
understanding of the fundamental principles underlying quantum key distribution
and, at the same time, opens up new interesting possibilities for quantum
cryptography networks
