Diffie-Hellman (Dh) Key Groups; Perfect Forward Secrecy (Pfs); Advanced Ike Settings - ZyXEL Communications P-202H V2 User Manual

• 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.

11.10.2 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.

11.10.3 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 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).

11.11 Advanced IKE Settings

Select Advanced at the bottom of the VPN-IKE screen. The following screen displays.
Chapter 11 VPN Screens
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