Figure 206 Ike Sa: Main Negotiation Mode, Steps 3 - 4: Dh Key Exchange - ZyXEL Communications ZYWALL 35 User Manual

Chapter 18 IPSec VPN
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.
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
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 206 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.
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Section 18.3.1.1 on page 356
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