ZyXEL Communications 792H - Manual (Page 197)

Prestige 792H G.SHDSL Router
VPN Screens 14-19
Table 14-8 VPN IKE: Advanced
LABEL DESCRIPTION
Authentication
Algorithm
Select SHA1 or MD5 from the drop-down list box. MD5 (Message Digest 5) and
SHA1 (Secure Hash Algorithm) are hash algorithms used to authenticate packet
data. The SHA1 algorithm is generally considered stronger than MD5, but is slower.
Select MD5 for minimal security and SHA-1 for maximum security.
SA Life Time
(Seconds)
Define the length of time before an IKE SA automatically renegotiates in this field. It
may range from 60 to 3,000,000 seconds (almost 35 days).
A short SA Life Time increases security by forcing the two VPN gateways to update
the encryption and authentication keys. However, every time the VPN tunnel
renegotiates, all users accessing remote resources are temporarily disconnected.
Encapsulation Select Tunnel mode or Transport mode from the drop-down list box.
Perfect Forward
Secrecy (PFS)
Perfect Forward Secrecy (PFS) is disabled (None) by default in phase 2 IPSec SA
setup. This allows faster IPSec setup, but is not so secure. Choose DH1 or DH2 from
the drop-down list box to enable PFS. DH1 refers to Diffie-Hellman Group 1 a 768 bit
random number. DH2 refers to Diffie-Hellman Group 2 a 1024 bit (1Kb) random
number (more secure, yet slower).
Apply Click Apply to save your changes back to the Prestige and return to the VPN IKE
screen.
Cancel Click Cancel to return to the VPN IKE screen without saving your changes.
14.12 Manual Key Setup
Manual key management is useful if you have problems with IKE key management.
14.12.1 Security Parameter Index (SPI)
An SPI is used to distinguish different SAs terminating at the same destination and using the same IPSec
protocol. This data allows for the multiplexing of SAs to a single gateway. The SPI (Security Parameter
Index) along with a destination IP address uniquely identify a particular Security Association (SA). The SPI
is transmitted from the remote VPN gateway to the local VPN gateway. The local VPN gateway then uses
the network, encryption and key values that the administrator associated with the SPI to establish the tunnel.