Configuring Advanced Ike Settings - ZyXEL Communications ZyWALL 10W User Manual

ZyWALL Series Internet Security Gateway
15.12.3 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.
15.12.4 Perfect Forward Secrecy (PFS)
Enabling PFS means that the key is transient. The key is thrown away and replaced by a brand new key using
a new Diffie-Hellman exchange for each new IPSec SA setup. With PFS enabled, if one key is compromised,
previous and subsequent keys are not compromised, because subsequent keys are not derived from previous
keys. The (time-consuming) Diffie-Hellman exchange is the trade-off for this extra security.
This may be unnecessary for data that does not require such security, so PFS is disabled (None) by default in
the ZyWALL. Disabling PFS means new authentication and encryption keys are derived from the same root
secret (which may have security implications in the long run) but allows faster SA setup (by bypassing the
Diffie-Hellman key exchange).

15.13 Configuring Advanced IKE Settings

Select Advanced at the bottom of the VPN IKE screen. This is the VPN IKE- Advanced screen as shown
next.
VPN Screens 15-21