ZyXEL Communications 792H - Manual (Page 193)

Prestige 792H G.SHDSL Router
VPN Screens 14-15
14.10.3 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 Prestige. 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).
14.11 Configuring Advanced IKE Settings
Click Advanced in the VPN IKE screen. This is the VPN IKE- Advanced screen as shown next.