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The answer arrived in the 1970s with , also known as Public-Key Cryptography. It is the invisible bedrock of every "https" website, encrypted chat, and digital signature we use today. How It Works: The Padlock and the Key
Primarily used for "key exchange," allowing two parties to create a shared secret over an insecure channel. The Quantum Threat
One of the oldest and most widely used, based on the difficulty of factoring giant prime numbers.