  The paper analyzes security aspects of practical entanglement-based quantum
key distribution (QKD), namely, BBM92 or entanglement-based BB84 protocol.
Similar to prepare-and-measure QKD protocols, practical implementations of the
entanglement-based QKD have to rely upon non-ideal photon sources. A typical
solution for entanglement generation is the spontaneous parametric
down-conversion. However, this process creates not only single photon pairs,
but also quantum states with more than two photons, which potentially may lead
to security deterioration. We show that this effect does not impair the
security of entanglement-based QKD systems. We also review the available
security proofs and show that properties of the entanglement source have
nothing to do with security degradation.
