ZyXEL Communications ZyXEL ZyWALL 2WE - Manual (Page 308)

ZyWALL 2 and ZyWALL 2WE
27-14 VPN/IPSec Setup
27.10.3 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.
27.10.4 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).
27.11 Configuring Advanced IKE Setup
Select one of the VPN rules in the VPN Summary screen and click Edit to configure the rule’s settings. The
basic IKE rule setup screen opens
Set the Key Management field to IKE and click the Advanced button to display the advanced IKE VPN
rule setup screen.