  While it is usually known that the mean value of a single observable is
enough to detect entanglement or its distillability, the counterpart of such an
approach in the case of quatum privacy has been missing. Here we develop the
concept of a privacy witness, i.e. a single observable that may detect presence
of the secure key even in the case of bound entanglement. Then we develop the
notion of secret key estimation based on few observables and discuss the
witness decomposition into local measurements. The surprising property of the
witness is that with the help of a low number of product mesurements involved
it may still report the key values that are {\it strictly above} distillable
entanglement of the state. For an exemplary four-qubit state studied in a
recent experiment [K. Dobek {\em et al.}, Phys. Rev. Lett. {\bf 106}, 030501
(2011)] this means 6 Pauli operator product measurements versus 81 needed to
carry out the complete quantum state tomography. The present approach may be
viewed as a paradigm for the general program of experimentally friendly
detection and estimation of task-dedicated quantum entanglement.
