1 diffie-hellman (dh) key exchange, 2 authentication – ZyXEL Communications 5 Series User Manual
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ZyWALL 5/35/70 Series User’s Guide
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Chapter 18 IPSec VPN
See the field descriptions for information about specific encryption algorithms, authentication
algorithms, and DH key groups. See
for more information about
DH key groups.
18.3.1.1 Diffie-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 174 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.
18.3.1.2 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 175 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.
Note: The ZyWALL and the remote IPSec router must use the same pre-shared key.