  In a recent Letter (Phys. Rev. Lett. 95 (2005) 010503) Barrett, Hardy and
Kent (BHK) considered a very interesting question which of the fundamental laws
of physics ensure security of quantum cryptographic protocols. In particular,
they presented quantum key distribution protocol and claimed its security in a
very general framework, which allows - what they call - postquantum
eavesdropping strategies i.e. which "allows for eavesdroppers who can break the
laws of quantum mechanics, as long as nothing they can do implies the
possibility of superluminal signaling". In this Comment, however, we show that
it is still possible to construct a strategy, which although consistent with
all asumptions explicitely stated in BHK paper, enables perfect eavesdropping
in their protocol.
