ZyXEL Communications P-661HW Series User Manual

P-661H/HW Series User’s Guide

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Chapter 13 VPN Screens

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.

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 ZyXEL Device. 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).

Click Advanced in the 

 screen to open this screen.