Another kind of authentication (pre-shared key) works with Main Mode. The key can only be identified by the IP address of the peers since the hash is computed before a relevant address field.

Aggressive Mode will allow a wider range of identifiers of the pre-shared secret. In addition, Aggressive Mode allows two parties to maintain multiple, different pre-shared keys and identify the correct one for a particular exchange.

Quick Mode is essentially an SA negotiation and an exchange of nonces that provides replay protection. The nonces are used to generate fresh key material and prevent replay attacks. An optional Key Exchange payload can be exchanged to allow for an additional Diffie-Hellman exchange and exponentiation per Quick Mode. Using the key exchange payload with Quick Mode is optional, but must be supported.