ZyXEL Communications Internet Security Gateway ZyWALL 2 Series - Manual (Page 229)

ZyWALL 2 Series User’s Guide
VPN Screens 14-23
IPSec SA lifetime period expires. The ZyWALL also automatically renegotiates the IPSec SA if both
IPSec routers have keep alive enabled, even if there is no traffic. If an IPSec SA times out, then the IPSec
router must renegotiate the SA the next time someone attempts to send traffic.
14.12.1 X-Auth and IKE
X-Auth (Extended Authentication) inserts a new exchange between IKE phases 1 and 2 for client
authentication.
14.12.2 Negotiation Mode
The phase 1 Negotiation Mode you select determines how the Security Association (SA) will be established
for each connection through IKE negotiations.
Main Mode ensures the highest level of security when the communicating parties are negotiating
authentication (phase 1). It uses 6 messages in three round trips: SA negotiation, Diffie-Hellman
exchange and an exchange of nonces (a nonce is a random number). This mode features identity
protection (your identity is not revealed in the negotiation).
Aggressive Mode is quicker than Main Mode because it eliminates several steps when the
communicating parties are negotiating authentication (phase 1). However the trade-off is that faster speed
limits its negotiating power and it also does not provide identity protection. It is useful in remote access
situations where the address of the initiator is not know by the responder and both parties want to use pre-
shared key authentication.
14.12.3 Pre-Shared Key
A pre-shared key identifies a communicating party during a phase 1 IKE negotiation. It is called “pre-shared”
because you have to share it with another party before you can communicate with them over a secure
connection.
14.12.4 Diffie-Hellman (DH) Key Groups
Diffie-Hellman (DH) is a public-key cryptography protocol that allows two parties to establish a shared
secret over an unsecured communications channel. Diffie-Hellman is used within IKE SA setup to establish
session keys. 768-bit (Group 1 - DH1) and 1024-bit (Group 2 – DH2) Diffie-Hellman groups are supported.
Upon completion of the Diffie-Hellman exchange, the two peers have a shared secret, but the IKE SA is not
authenticated. For authentication, use pre-shared keys.