Chapter 23 IPSec VPN

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The ZyWALL sends one or more proposals to the remote IPSec router. (In some devices, you can only set up one proposal.) Each proposal consists of an encryption algorithm, authentication algorithm, and DH key group that the ZyWALL wants to use in the IKE SA. The remote IPSec router selects an acceptable proposal and sends the accepted proposal back to the ZyWALL. If the remote IPSec router rejects all of the proposals, the ZyWALL and remote IPSec router cannot establish an IKE SA.

Note: Both routers must use the same encryption algorithm, authentication algorithm, and DH key group.

In most ZyWALLs, you can select one of the following encryption algorithms for each proposal. The algorithms are listed in order from weakest to strongest.

•Data Encryption Standard (DES) is a widely used method of data encryption. It applies a 56-bit key to each 64-bit block of data.

•Triple DES (3DES) is a variant of DES. It iterates three times with three separate keys, effectively tripling the strength of DES.

•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 400 for more information about 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

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ZyWALL USG 50 User’s Guide