It is shown that from entanglemed states from which no pure entanglement can be obtained, secure key can be drawn. States that contain perfectly secret bit - "quantum p-bits" - are characterized. One of such states is presented, which has a peculiar feature: its entanglement cost can be reduced from 1/2 log d to zero by removing single qubit. This phenomenon of lockign entanglement by single qubit is discussed in terms of relations between entanglement loss versus information loss. Finally, thermodynamical cost of erasure of entanglement is defined and discussed.