Real-world cryptographic implementations: Signal's Double Ratchet, PGP, WireGuard, and Zero-Knowledge Proofs.

PGP encrypts emails and files using a hybrid model:
1. Plaintext is encrypted with a fast, random symmetric key (e.g. AES).
2. The symmetric key is encrypted using the recipient's public key (RSA) and appended to the message.
3. Recipient uses their private key to decrypt the symmetric key, then decrypts the message payload.

A modern VPN protocol that uses Cryptographic Key Routing. It binds tunnel IP addresses to peer public keys, using Curve25519 (ECDH), ChaCha20-Poly1305 (encryption), and BLAKE2s (hashing) to provide faster, simpler connections than IPsec or OpenVPN.

ZKP is a cryptographic method that allows one party (the prover) to prove to another party (the verifier) that they know a secret value without revealing any information about the secret itself (e.g., proving identity without revealing the password).