  Exploring the symmetries underlying a previously proposed encryption scheme
which relies on single-qubit rotations, we derive an improved upper bound on
the maximum information that an eavesdropper might extract from all the
available copies of the public key. Subsequently, the robustness of the scheme
is investigated in the context of attacks that address each public-key qubit
independently. The attacks under consideration make use of projective
measurements on single qubits and their efficiency is compared to attacks that
address many qubits collectively and require complicated quantum operations.
