  We study the role of discrete rotational symmetry in quantum key distribution
by generalizing the well-known Bennett-Brassard 1984 (BB84) and
Scarani-Acin-Ribordy-Gisin 2004 (SARG04) protocols. We observe that discrete
rotational symmetry results in the protocol's invariance to continuous
rotations, thus leading to a simplified relation between bit and phase error
rates and consequently a straightforward security proof.
