  Quantum data locking is a protocol that allows for a small secret key to
(un)lock an exponentially larger amount of information, hence yielding the
strongest violation of the classical one-time pad encryption in the quantum
setting. This violation mirrors a large gap existing between two security
criteria for quantum cryptography quantified by two entropic quantities: the
Holevo information and the accessible information. We show that the latter
becomes a sensible security criterion if an upper bound on the coherence time
of the eavesdropper's quantum memory is known. Under this condition we
introduce a protocol for secret key generation through a memoryless qudit
channel. For channels with enough symmetry, such as the d-dimensional erasure
and depolarizing channels, this protocol allows secret key generation at an
asymptotic rate as high as the classical capacity minus one bit.
