Hybrid Cryptosystems
In practice, public-key crypto used to secure and distribute session keys, which are then used with private-key crypto to secure message traffic.
Bob sends Alice his public key.
Alice generates random session key K, encrypts it using Bob’s public key, and sends it to Bob.
Bob decrypts Alice’s message using his private key to recover session key.
Both encrypt their communications using same session key.
Public-key crypto solves important key-management problem.