  Measurements with photodetectors necessarily need to be described in the
infinite dimensional Fock space of one or several modes. For some measurements
a model has been postulated which describes the full mode measurement as a
composition of a mapping (squashing) of the signal into a small dimensional
Hilbert space followed by a specified target measurement. We present a
formalism to investigate whether a given measurement pair of mode and target
measurements can be connected by a squashing model. We show that the
measurements used in the BB84 protocol do allow a squashing description,
although the six-state protocol does not. As a result, security proofs for the
BB84 protocol can be based on the assumption that the eavesdropper forwards at
most one photon, while the same does not hold for the six-state protocol.
