ZyXEL 2WG User Guide

Chapter 15 IPSec VPN Screens

ZyWALL 2WG User’s Guide

Figure 213   IKE SA: Main Negotiation Mode, Steps 1 - 2: IKE SA Proposal

The ZyWALL sends one or more proposals to the remote IPSec router. (In some devices, you 
can set up only one proposal.) Each proposal consists of an encryption algorithm, 
authentication algorithm, and DH key group that the ZyWALL wants to use in the IKE SA. 
The remote IPSec router selects an acceptable proposal and sends the accepted proposal back 
to the ZyWALL. If the remote IPSec router rejects all of the proposals (for example, if the 
VPN tunnel is not configured correctly), the ZyWALL and remote IPSec router cannot 
establish an IKE SA.

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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.

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 214   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.

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.