ZyXEL Communications ZyWall USG 2000 User Manual page 360

Chapter 21 IPSec VPN
• Advanced Encryption Standard (AES) is a newer method of data encryption that
also uses a secret key. AES applies a 128-bit key to 128-bit blocks of data. It is
faster than 3DES.
Some ZyWALLs also offer stronger forms of AES that apply 192-bit or 256-bit keys
to 128-bit blocks of data.
In most ZyWALLs, 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.
See Diffie-Hellman (DH) Key Exchange on page 360
DH key groups.
Diffie-Hellman (DH) Key Exchange
The ZyWALL 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 222 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 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.
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Diffie-Hellman key exchange
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for more information about
3
4
ZyWALL USG 2000 User's Guide
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