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Asymmetric Cryptography: RSA-OAEP, Elliptic Curves, ECDH Key Agreement and Hybrid Encryption

To encrypt data for a party you've never shared a key with, you use hybrid encryption: the recipient's public key carries only a symmetric key and AES-GCM encrypts the data. With real outputs we cover why OAEP is mandatory for RSA, the 190-byte limit, the small keys of elliptic curves, why an ECDH shared secret must go through HKDF, PEM and encrypted PKCS#8, and Bookstore's purchase order file sent to a supplier.