  We study the problem of information masking through nonzero linear operators
that distribute information encoded in single qubits to the correlations
between two qubits. It is shown that a nonzero linear operator cannot mask any
nonzero measure set of qubit states. We prove that the maximal maskable set of
states on the Bloch sphere with respect to any masker is the ones on a
spherical circle. Any states on a spherical circle on the Bloch sphere are
maskable, which also proves the conjecture on maskable qubit states given by
Modi et al. [Phys. Rev. Lett. 120, 230501 (2018)]. we provide explicitly
operational unitary maskers for all maskable sets. As applications, different
protocols for secret sharing are introduced.
