Инструкция для ZYXEL P-334 EE

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Prestige 334 User’s Guide

Chapter 15 VPN Screens

168

• Choose an encryption algorithm.
• Choose an authentication algorithm
• Choose whether to enable Perfect Forward Secrecy (PFS) using Diffie-Hellman public-

key cryptography – see

Section Perfect Forward Secrecy (PFS)

. 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.

15.11.3 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.