About Diffie-Hellman Groups
Diffie-Hellman (DH) groups determine the strength of the key used in the key exchange process. Higher group numbers are more secure, but require additional time to compute the key.
- DH Group 1: 768-bit group
- DH Group 2: 1024-bit group
- DH Group 5: 1536-bit group
- DH Group 14: 2048-bit group
- DH Group 15: 3072-bit group
- DH Group 19: 256-bit elliptic curve group
- DH Group 20: 384-bit elliptic curve group
Both peers in a VPN exchange must use the same DH group, which is negotiated during Phase 1 of the IPSec negotiation process. When you define a manual BOVPN tunnel, you specify the Diffie-Hellman group as part of Phase creation of an IPSec connection. This is where the two peers make a secure, authenticated channel they can use to communicate.
DH groups and Perfect Forward Secrecy (PFS)
In addition to Phase 1, you can also specify the Diffie-Hellman group to use in Phase 2 of an IPSec connection. Phase 2 configuration includes settings for a security association (SA), or how data packets are secured when they are passed between two endpoints. You specify the Diffie-Hellman group in Phase 2 only when you select Perfect Forward Secrecy (PFS).
PFS makes keys more secure because new keys are not made from previous keys. If a key is compromised, new session keys are still secure. When you specify PFS during Phase 2, a Diffie-Hellman exchange occurs each time a new SA is negotiated.
The DH group you choose for Phase 2 does not need to match the group you choose for Phase 1.
How to Choose a Diffie-Hellman Group
For branch office VPN tunnels and BOVPN virtual interfaces, the default DH group for both Phase 1 and Phase 2 is Diffie-Hellman Group 14.
If the speed for tunnel initialization and rekey is not a concern, you can use a higher DH group. Actual initialization and rekey speed depends on a number of factors. You might want to try one of the higher DH groups and decide whether the slower performance time is a problem for your network. If the performance is unacceptable, change to a lower DH group.
In Fireware v11.12.4 or lower, the default DH group is Diffie-Hellman Group 2.