keyexchange

Key exchange is the process by which two or more parties establish a shared cryptographic key over an insecure channel. The goal is to enable confidential communication or secure authentication without requiring that the parties share a secret ahead of time. Key exchange is often differentiated from key_transport, where one party creates and sends a key to another.

The best-known key-exchange method is the Diffie-Hellman protocol, introduced in 1976, which allows two parties to agree on a common secret by exchanging values derived from a base and a public modulus. The security rests on the difficulty of the discrete logarithm problem. RSA can be used for key exchange by encrypting a randomly generated symmetric key with the recipient's public key, but this approach does not provide forward secrecy unless combined with other measures. Modern practice favors authenticated key exchanges, often using elliptic-curve variants such as ECDH or Diffie-Hellman with ephemeral parameters (DHE/ECDHE).

In practice, key exchange is embedded in protocols such as Transport Layer Security (TLS) and Secure Shell (SSH). The exchange negotiates a shared session key used to encrypt subsequent traffic. Authentication is typically provided by digital certificates or host keys, establishing trust and preventing impersonation. Ephemeral variants help achieve forward secrecy, ensuring past communications remain secure if a server's private key is compromised later.

Security considerations include protecting against man-in-the-middle attacks, ensuring authentication, and resisting replay and side-channel vulnerabilities. Proper implementation and up-to-date cryptographic parameters are essential, as is a robust public key infrastructure or trusted certificates to bind identities to public keys.

See also: key agreement, key agreement protocol, Diffie-Hellman, Elliptic Curve Diffie-Hellman, TLS, SSH, forward secrecy.