Chapter 21 IPSec VPN
USG20(W)-VPN Series User’s Guide
360
Some USGs also offer stronger forms of AES that apply 192-bit or 256-bit keys to 128-bit blocks of
data.
In most USGs, you can select one of the following authentication algorithms for each proposal. The
algorithms are listed in order from weakest to strongest.
• 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 360
for more information about DH key groups.
Diffie-Hellman (DH) Key Exchange
The USG 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 234
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 USG 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 USG 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 USG and remote IPSec router selected in previous steps.
Diffie-Hellman key exchange
3
4
X
Y
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