Watch a conference talk from TQC 2023 exploring the development of resource theory for quantum secret key, where Karol Horodecki presents groundbreaking research on defining and analyzing key costs in quantum states, channels, and devices. Learn about privacy dilution protocols that convert ideal privacy states into diluted ones, and discover how the regularized key formation equals quantum state key cost. Delve into mixed-state analogues of pure quantum states in privacy domains, examining entanglement measures and their relationships. Explore single-shot regime privacy costs, yield-cost relations, and fundamental findings for quantum channels and devices. The presentation, delivered at the 18th Conference on Theory of Quantum Computation, Communication and Cryptography at the University of Aveiro, covers essential concepts including separable states, private states, irreducible private states, and generalized private states through detailed visualization and performance analysis.