ZyXEL NBG-460N User Guide - Page 208

Chapter 15 IPSec VPN 15.6.3 Diffie-Hellman (DH) Key Exchange The NBG-460N 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 127 IKE SA: Main Negotiation Mode, Steps 3 - 4: DH Key Exchange 3 4 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. 15.6.4 Authentication Before the NBG-460N 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 NBG-460N 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 NBG-460N and remote IPSec router selected in previous steps. Figure 128 IKE SA: Main Negotiation Mode, Steps 5 - 6: Authentication 5 208 6 The NBG-460N and remote IPSec router use a pre-shared key in the authentication process, though it is not actually transmitted or exchanged. Note: The NBG-460N and the remote IPSec router must use the same pre-shared key. Router identity consists of ID type and ID content. The ID type can be IP address, domain name, or e-mail address, and the ID content is a specific IP address, NBG-460N User's Guide