1What a Secret and a Key Really AreModern security rests on one idea: publish the method, protect the key, and know which kind of key quantum computers threaten.Foundations 2What Encryption Really DoesEncryption is not one lock but two: a fast shared-key lockbox for data, and a public-key mailbox for trust.Foundations 3Classical vs Quantum ComputersA quantum computer is not a faster laptop; it is a fragile specialist whose rare strengths matter deeply for public-key security.Foundations 4Qubits, Superposition and EntanglementA plain guide to the three quantum ideas that make quantum computing powerful, strange, and hard to use.Foundations 5Shor, Grover and What Actually BreaksShor breaks public-key cryptography; Grover only reduces the margin for symmetric encryption and hashes.Foundations