Chapter 14 IPSec VPN

"Both routers must use the same encryption algorithm, authentication algorithm, and DH key group.

See the field descriptions for information about specific encryption algorithms, authentication algorithms, and DH key groups. See Section 14.3.1.1 on page 306 for more information about DH key groups.

14.3.1.1Diffie-Hellman (DH) Key Exchange

The ZyWALL and the remote IPSec router use a DH key exchange to establish a shared secret, which is used to generate encryption keys for IKE SA and IPSec SA. In main mode, the DH key exchange is done in steps 3 and 4, as illustrated below.

Figure 195 IKE SA: Main Negotiation Mode, Steps 3 - 4: DH Key Exchange

The DH key exchange is based on DH key groups. Each key group is a fixed number of bits long. The longer the key, the more secure the encryption keys, but also the longer it takes to encrypt and decrypt information. For example, DH2 keys (1024 bits) are more secure than DH1 keys (768 bits), but DH2 encryption keys take longer to encrypt and decrypt.

14.3.1.2 Authentication

Before the ZyWALL and remote IPSec router establish an IKE SA, they have to verify each other’s identity. This process is based on pre-shared keys and router identities.

In main mode, the ZyWALL and remote IPSec router authenticate each other in steps 5 and 6, as illustrated below. Their identities are encrypted using the encryption algorithm and encryption key the ZyWALL and remote IPSec router selected in previous steps.

Figure 196 IKE SA: Main Negotiation Mode, Steps 5 - 6: Authentication

The ZyWALL and remote IPSec router use a pre-shared key in the authentication process, though it is not actually transmitted or exchanged.

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ZyWALL 2WG User’s Guide