ZyXEL Communications Prestige 661H Series, User Manual

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Prestige 661H/HW Series User’s Guide
189 Chapter 15 VPN Screens
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 15.11.3 on page 190 . 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.
15.11.1 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.
15.11.2 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.