Public and Private Key Cryptography

Also called asymmetric cryptography: each user has a key pair.

• Public key: shared openly
• Private key: kept secret by the owner

Data encrypted with one key can only be decrypted with the other key of the pair, and the private key cannot practically be derived from the public key.

Working (confidentiality)

1. The sender encrypts the message with the receiver's public key.
2. Only the receiver's private key can decrypt it.

Digital signature (authentication and integrity)

The sender signs a hash of the message with the sender's private key; anyone can verify it with the sender's public key.

Features

• No need to share a secret key beforehand, which solves key distribution
• Supports confidentiality, authentication, integrity and non-repudiation
• Algorithms: RSA, ElGamal, ECC, Diffie-Hellman (key exchange)

Symmetric vs asymmetric

	Symmetric	Asymmetric
Keys	One shared secret key	Public and private key pair
Speed	Fast	Slower
Key distribution	Difficult	Easy
Examples	DES, AES	RSA, ECC

In practice, asymmetric cryptography exchanges a session key, and symmetric encryption protects the bulk data (as in TLS).