ZyXEL Communications 792H - Manual (Page 192)

Prestige 792H G.SHDSL Router
14-14 VPN Screens
Choose a Diffie-Hellman public-key cryptography key group (DH1 or DH2).
Set the IKE SA lifetime. This field allows you to determine how long an IKE SA should stay up
before it times out. An IKE SA times out when the IKE SA lifetime period expires. If an IKE
SA times out when an IPSec SA is already established, the IPSec SA stays connected.
In phase 2 you must:
Choose which protocol to use (ESP or AH) for the IKE key exchange.
Choose an encryption algorithm.
Choose an authentication algorithm
Choose whether to enable Perfect Forward Secrecy (PFS) using Diffie-Hellman public-key
cryptography – see section 14.10.3. Select None (the default) to disable PFS.
Choose Tunnel mode or Transport mode.
Set the IPSec SA lifetime. This field allows you to determine how long the IPSec SA should
stay up before it times out. The Prestige automatically renegotiates the IPSec SA if there is
traffic when the IPSec SA lifetime period expires. The Prestige also automatically renegotiates
the IPSec SA if both IPSec routers have keep alive enabled, even if there is no traffic. If an
IPSec SA times out, then the IPSec router must renegotiate the SA the next time someone
attempts to send traffic.
14.10.1 Negotiation Mode
The phase 1 Negotiation Mode you select determines how the Security Association (SA) will be established
for each connection through IKE negotiations.
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.
14.10.2 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.