  The ideal Bennett-Brassard 1984 (BB84) quantum key distribution protocol is
based on the preparation and measurement of qubits in two alternative bases
differing by an angle of pi/2. Any real implementation of the protocol, though,
will inevitably introduce misalignments in the preparation of the states and in
the alignment of the measurement bases with respect to this ideal situation.
Various security proofs take into account (at least partially) such errors,
i.e., show how Alice and Bob can still distil a secure key in the presence of
these imperfections. Here, we consider the complementary problem: how can Eve
exploit misalignments to obtain more information about the key than would be
possible in an ideal implementation? Specifically, we investigate the effects
of misalignment errors on the security of the BB84 protocol in the case of
individual attacks, where necessary and sufficient conditions for security are
known. Though the effects of these errors are small for expected deviations
from the perfect situation, our results nevertheless show that Alice and Bob
can incorrectly conclude that they have established a secure key if the
inevitable experimental errors in the state preparation and in the alignment of
the measurements are not taken into account. This gives further weight to the
idea that the formulation and security analysis of any quantum cryptography
protocol should be based on realistic assumptions about the properties of the
apparatus used. Additionally, we note that BB84 seems more robust against
alignment imperfections if both the x and z bases are used to generate the key.
