ZyXEL Communications Prestige 792H, User Manual

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Prestige 792H User’s Guide
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.
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,