  We present a protocol for quantum cryptography in which the data obtained for
mismatched bases are used in full for the purpose of quantum state tomography.
Eavesdropping on the quantum channel is seriously impeded by requiring that the
outcome of the tomography is consistent with unbiased noise in the channel. We
study the incoherent eavesdropping attacks that are still permissible and
establish under which conditions a secure cryptographic key can be generated.
The whole analysis is carried out for channels that transmit quantum systems of
any finite dimension.
