ZyXEL Communications ISG50-ISDN User Manual page 388

Chapter 24 IPSec VPN
Diffie-Hellman (DH) Key Exchange
The ISG50 and the remote IPSec router use DH public-key cryptography to establish a shared
secret. The shared secret is then used to generate encryption keys for the IKE SA and IPSec SA. In
main mode, this is done in steps 3 and 4, as illustrated next.
Figure 254 IKE SA: Main Negotiation Mode, Steps 3 - 4: DH Key Exchange
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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 ISG50 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 ISG50 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 ISG50 and remote IPSec router selected in previous steps.
Figure 255 IKE SA: Main Negotiation Mode, Steps 5 - 6: Authentication (continued)
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Diffie-Hellman key exchange
3
4
Step 5:
pre-shared key
ISG50 identity, consisting of
- ID type
- content
Step 6:
pre-shared key
Remote IPSec router identity, consisting of
- ID type
- content
5
6
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ISG50 User's Guide