  Quantum key distribution (QKD) involving polarized photons could be
vulnerable to a jamming (or denial-of-service) attack, in which a third party
applies an external magnetic field to rotate the plane of polarization of
photons headed toward one of the two intended recipients. Sufficiently large
Faraday rotation of one of the polarized beams would prevent Alice and Bob from
establishing a secure quantum channel. We investigate requirements to induce
such rotation both for free-space transmission and for transmission via optical
fiber, and find reasonable ranges of parameters in which a jamming attack could
be successful against fiber-based QKD, even for systems that implement
automated recalibration for polarization-frame alignment. The jamming attack
could be applied selectively and indefinitely by an adversary without revealing
her presence, and could be further combined with various eavesdropping attacks
to yield unauthorized information.
