  Theoretical quantum key distribution (QKD) protocols commonly rely on the use
of qubits (quantum bits). In reality, however, due to practical limitations,
the legitimate users are forced to employ a larger quantum (Hilbert) space, say
a quhexit (quantum six-dimensional) space, or even a much larger quantum
Hilbert space. Various specific attacks exploit of these limitations. Although
security can still be proved in some very special cases, a general framework
that considers such realistic QKD protocols, as well as} attacks on such
protocols, is still missing.
  We describe a general method of attacking realistic QKD protocols, which we
call the `quantum-space attack'. The description is based on assessing the
enlarged quantum space actually used by a protocol, the `quantum space of the
protocol'. We demonstrate these new methods by classifying various (known)
recent attacks against several QKD schemes, and by analyzing a novel attack on
interferometry-based QKD.
