IPSec VPN Concepts
IPSec
IKE Phase 2
Under the protection of ISAKMP Security Association, IKE Phase 2 performs parameters negotiations to establish the
IPsec Security Association which protects the subsequent IPSec VPN communications. IKE Phase 2 is processed in
one mode called Quick Mode (New Group Mode is not supported by FortiWAN). Similar to Phase 1, in IKE Phase 2,
another proposal of encryption and authentication algorithms is negotiated, shared secret keys are derived, and the
negotiation sessions are authenticated. The negotiated encryption and authentication algorithms, derived secret keys
and other necessary parameters, which are the successful outcome of IKE Phase 2, constitute the IPSec Security
Association. So that the security association between two IPSec VPN gateways is established, and the VPN
communications are so that protected.
Perfect Forward Secrecy, PFS
Perfect Forward Secrecy is a property of communication security that past session keys can not be compromised by
the compromise of long-term keys if a session key is associated to the long-term key in some way. Actually, the shared
secret key we introduced in IKE Phase 2 is derived by calculation with the secret key derived in IKE Phase 1 and some
insecure (is public to any third-party) parameters (a Diffie-Hellman exchange is not involved in the calculation), if PFS
is not enabled for IKE Phase 2. Once the secret key of IKE Phase 1 is compromised to an attacker, all the secret
session keys derived in IKE Phase 2 might become compromised. With enabling PFS, the calculation of secret keys
involves a new Diffie-Hellman exchange. The private key material of Diffie-Hellman exchange protects the session
secret keys of IKE Phase 2 from the compromise of IKE Phase 1's keys. However, system performance might be
concerned if Diffie-Hellman exchange is performed twice (Phase 1 and Phase 2 individually) for a establishment of
IPsec Security Association.
How IPSec VPN Works
So far we have a overview of IPSec concept and how the Security Associations are established. Before a further
discussion, here is the IPSec VPN's operation broken down into five main steps:
1. The initial packet matching correspondent IPSec VPN policies and attempting to pass through the IPSec VPN
gateway triggers the IKE processes to establish Security Associations.
2. During IKE Phase 1, IKE proposals are negotiated, secret keys are shared and the two IPSec endpoints are
authenticated. The ISAKMP SA is established for IKE Phase 2.
3. IKE Phase 2 negotiates new parameters and calculates new secret keys. The IPSec SA is established for VPN
communications.
4. Communications over the two IPSec VPN gateways are protected according on the security parameters and keys
stored in Security Association database. Data packets are encapsulated with ESP header and new IP header,and
transferred over the IPSec VPN tunnel.
5. IPSec SAs terminate by timing out.
Modes of IPSec VPN data transmission
IPSec transfers the encrypted or authenticated IP packets (ESP or AH encapsulated packets) in a host-to-host
transport mode, as well as in a tunneling mode. Packet exchanges during IKE Phase 1 and Phase 2 are nothing about
the two modes.
FortiWAN Handbook
Fortinet Technologies Inc.
177
