  We develop a framework that allows a description of measurements in Hilbert
spaces that are smaller than their natural representation. This description,
which we call a "squashing model", consists of a squashing map that maps the
input states of the measurement from the original Hilbert space to the smaller
one, followed by a targeted prescribed measurement on the smaller Hilbert
space. This framework has applications in quantum key distribution, but also in
other cryptographic tasks, as it greatly simplifies the theoretical analysis
under adversarial conditions.
