D-Link NetDefend DFL-210 User Manual page 244

9.3.2. Internet Key Exchange (IKE)
IKE Authentication
IKE DH (Diffie-Hellman) Group
IKE Lifetime
PFS
• Cast128
• 3DES
• DES
DES is only included to be interoperable with other older
VPN implementations. Use of DES should be avoided
whenever possible, since it is an old algorithm that is no
longer considered secure.
This specifies the authentication algorithms used in the IKE
negotiation phase.
The algorithms supported by NetDefendOS IPsec are:
• SHA1
• MD5
This specifies the Diffie-Hellman group to use when doing
key exchanges in IKE.
The Diffie-Hellman groups supported by NetDefendOS are:
• DH group 1 (768-bit)
• DH group 2 (1024-bit)
• DH group 5 (1536-bit)
Security of the key exchanges increases as the DH group bit
become larger, as does the time taken for the exchanges.
This is the lifetime of the IKE connection.
It is specified in time (seconds) as well as data amount
(kilobytes). Whenever one of these expires, a new phase-1
exchange will be performed. If no data was transmitted in the
last "incarnation" of the IKE connection, no new connection
will be made until someone wants to use the VPN connection
again. This value must be set greater than the IPsec SA
lifetime.
With PFS disabled, initial keying material is "created" during
the key exchange in phase-1 of the IKE negotiation. In
phase-2 of the IKE negotiation, encryption and authentication
session keys will be extracted from this initial keying
material. By using PFS, Perfect Forwarding Secrecy,
completely new keying material will always be created upon
re-key. Should one key be compromised, no other key can be
derived using that information.
PFS can be used in two modes, the first is PFS on keys, where
a new key exchange will be performed in every phase-2
negotiation. The other type is PFS on identities, where the
identities are also protected, by deleting the phase-1 SA every
time a phase-2 negotiation has been finished, making sure no
more than one phase-2 negotiation is encrypted using the
same key.
PFS is generally not needed, since it is very unlikely that any
encryption or authentication keys will be compromised.
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Chapter 9. VPN