ZyXEL Communications USG FLEX H Series - Page 161

Chapter 11 IPSec VPN
USG FLEX H Series User’s Guide
161
• MD5 (Message Digest 5) produces a 128-bit digest to authenticate packet data.
• SHA1 (Secure Hash Algorithm) produces a 160-bit digest to authenticate packet data.
• SHA256 (Secure Hash Algorithm) produces a 256-bit digest to authenticate packet data.
• SHA512 (Secure Hash Algorithm) produces a 512-bit digest to authenticate packet data.
See Diffie-Hellman (DH) Key Exchange on page 161 for more information about DH key groups.
Diffie-Hellman (DH) Key Exchange
The Zyxel Device 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 114 IKE SA: Main Negotiation Mode, Steps 3 - 4: DH Key Exchange
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 Zyxel Device 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 Zyxel Device 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 Zyxel Device and remote IPSec router selected in previous steps.
Figure 115 IKE SA: Main Negotiation Mode, Steps 5 - 6: Authentication (continued)
You have to create (and distribute) a pre-shared key. The Zyxel Device and remote IPSec router use it in
the authentication process, though it is not actually transmitted or exchanged.
Note: The Zyxel Device and the remote IPSec router must use the same pre-shared key.
Diffie-Hellman key exchange
3
4
X
Y
Step 5:
pre-shared key
Zyxel Device identity, consisting of
- ID type
Step 6:
pre-shared key
Remote IPSec router identity, consisting of
- ID type