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IPSec VPN Concepts IPSec
Security Association
To support secure communications (data encryption and authentication) between two VPN gateways, the common
security attributes must be shared in advance, which are the cryptographic and authentication algorithms, encryption
secret key and other necessary parameters. A common set of the security attributes maintained by two IPSec VPN
gateways for an IPSec VPN tunnel is what called Security Association (SA), which is used to provide a secure channel
and protect the communications between the two site networks. Each of the two IPSec VPN gateways
encrypts/decrypts data according to the established Security Association. The process to establish a Security
Association involves sharing and negotiation of the security attributes.
IKE key exchange
Internet Key Exchange (IKE) is the protocol used to establish a Security Association (SA), which is included in the
IPSec protocol suite. The purposes of IKE are to
l
Negotiate an encrypt algorithm and an authentication algorithm
l
Generate a shared secret key to encrypt/decrypt IPSec VPN communications (data transmission).
Both are used by IPSec VPN to provide secure communications between two endpoints.
IKE consists of two phases, Phase 1 and Phase 2. The purpose of IKE Phase 1 is to establish a secure and
authenticated channel, which is actually a Security Association (called ISAKMP SA as well), between two entities for
further IKE Phase 2 negotiations. With the protection of ISAKMP SA, Phase 2 will then be performed to establish the
final Security Association (called IPSec SA as well) used to protect the VPN communications (data transmission)
between two sites. In other words, before users' VPN communication starts (data packet being transferred to each
other), the correspondent IKE Phase 1 and Phase 2 must be done to establish the SAs between the two VPN
gateways. With the established SA between two VPN gateways, privacy and authenticity are so that guaranteed to the
VPN communications (by encryption and authentication). Basically, IKE Phase 1 authenticates a remote peer and sets
up a secure channel for going forward Phase 2 negotiations to establish the IPSec SA.
IKE Phase 1
Before we talk about the details of IKE Phase 1, let us have an overview on Phase 1's Identity Verification
(Authentication). The endpoint who begins the IKE Phase1 negotiation makes a declaration of who it is to the opposite
endpoint, and the opposite endpoint verifies the identity. FortiWAN's IPSec employs a
pre-shared key to achieve the
identity verification. The pre-shared key is a common key (similar to a password) pre-shared between the two entities
who join in the Phase 1 negotiations. This pre-shared key is used for verification of the declared identity in a
cryptographic system (MAC calculation of the identity). This mechanism is on the premise that the pre-shared key is
never compromised to the third-party. Although it looks like a password, the pre-shared key, also known as a shared
secret, is never sent by either endpoint during the processes of authentication. Actually, the pre-shared key is involved
in the calculations of encryption keys, which is actually used for the authentication, at each endpoint.Unmatched pre-
shared keys result in unmatched encryption keys, and indirectly cause the authentication in IKE Phase 1 failed.
Now back to the IKE Phase 1. Phase 1 achieves the following objectives to establish ISAKMP Security Association:
IKE Proposals negotiation
An IKE proposal is a set of necessary parameters for negotiations to establish a Security Association. The negotiation
initiator offers opposite endpoint the proposals of the suggested encryption and authentication algorithms, the time-
period that keys should remain active, and the strength of the keys used in Diffie-Hellman key exchange process. The
opposite endpoint chooses an appropriate proposal and responds it to the initiator, so that the algorithms and other
parameters used to protect data transmission between two endpoints are determined.
FortiWAN Handbook
Fortinet Technologies Inc. 175