ZyXEL Communications ZyWALL USG 50 User Manual page 407

keys for the IKE SA and IPSec SA. In main mode, this is done in steps 3 and 4, as
illustrated next.
Figure 238 IKE SA: Main Negotiation Mode, Steps 3 - 4: DH Key Exchange
X
DH public-key cryptography 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, 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 keys take longer
to encrypt and decrypt.
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. The identities are also encrypted using the
encryption algorithm and encryption key the ZyWALL and remote IPSec router
selected in previous steps.
Figure 239 IKE SA: Main Negotiation Mode, Steps 5 - 6: Authentication (continued)
You have to create (and distribute) a pre-shared key. The ZyWALL and remote
IPSec router use it 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.
ZyWALL USG 50 User's Guide
Diffie-Hellman key exchange
3
4
Step 5:
pre-shared key
ZyWALL identity, consisting of
- ID type
- content
Step 6:
pre-shared key
Remote IPSec router identity, consisting of
- ID type
- content
Chapter 23 IPSec VPN
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