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Operating under the assumption that entangled states with zero distillable key do not exist, we define the key cost of a quantum state, and device. We study its properties through the lens of a quantity that we call the key of formation. The main result of our paper is that the regularized key of formation is an upper bound on the key cost of a quantum state. The core protocol underlying this result is privacy dilution, which converts states containing ideal privacy into ones with diluted privacy. Next, we show that the key cost is bounded from below by the regularized relative entropy of entanglement, which implies the irreversibility of the privacy creation-distillation process for a specific class of states. We further focus on mixed-state analogues of pure quantum states in the domain of privacy, and we prove that a number of entanglement measures are equal to each other for these states, similar to the case of pure entangled states. The privacy cost and distillable key in the single-shot regime exhibit a yield-cost relation, and basic consequences for quantum devices are also provided. Importantly, our results presented here will remain valid even if entangled states with zero distillable key were shown to exist.<\/jats:p>","DOI":"10.22331\/q-2026-05-06-2098","type":"journal-article","created":{"date-parts":[[2026,5,6]],"date-time":"2026-05-06T09:25:34Z","timestamp":1778059534000},"page":"2098","update-policy":"https:\/\/doi.org\/10.22331\/q-crossmark-policy-page","source":"Crossref","is-referenced-by-count":0,"title":["Cost of quantum secret key"],"prefix":"10.22331","volume":"10","author":[{"given":"Karol","family":"Horodecki","sequence":"first","affiliation":[{"name":"Institute of Informatics, National Quantum Information Centre, Faculty of Mathematics, Physics and Informatics, University of Gda\u0144sk, Wita Stwosza 57, 80-308 Gda\u0144sk, Poland"},{"name":"International Centre for Theory of Quantum Technologies, University of Gda\u0144sk, Wita Stwosza 63, 80-308 Gda\u0144sk, Poland"},{"name":"School of Electrical and Computer Engineering, Cornell University, Ithaca, New York 14850, USA"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Leonard","family":"Sikorski","sequence":"additional","affiliation":[{"name":"Institute of Informatics, National Quantum Information Centre, Faculty of Mathematics, Physics and Informatics, University of Gda\u0144sk, Wita Stwosza 57, 80-308 Gda\u0144sk, Poland"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Siddhartha","family":"Das","sequence":"additional","affiliation":[{"name":"q4i, Centre for Quantum Science and Technology (CQST), Center for Security, Theory and Algorithmic Research (CSTAR), International Institute of Information Technology Hyderabad, Gachibowli 500032, Hyderabad, Telangana, India"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Mark M.","family":"Wilde","sequence":"additional","affiliation":[{"name":"School of Electrical and Computer Engineering, Cornell University, Ithaca, New York 14850, USA"}],"role":[{"role":"author","vocabulary":"crossref"}]}],"member":"9598","published-online":{"date-parts":[[2026,5,6]]},"reference":[{"key":"0","doi-asserted-by":"publisher","unstructured":"Eric Chitambar and Gilad Gour. ``Quantum resource theories''. 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