IPsec, a Tutorial--Part X

IKE Phase II Negotiation
The goal of IKE Phase II negotiation is establishment of IPsec SAs between two endpoints. IKE uses Diffie-Hellman key exchange to negotiate the shared secret key to be used in the encryption cipher specified in the IPsec transform set. The IPsec SA can include the shared Diffie-Hellman key used to encrypt the ISAKMP SA, or it can be renegotiated over the ISAKMP SA during Phase II negotiation.

Quick Mode
IKE Phase II negotiation is done in only one mode, quick mode. Due to the fact that Phase II negotiation's goal is establishment of an IPsec SA, quick mode exchange must inform both crypto endpoints of the IPsec mode to use ESP and AH and all other relevant variables needed to populate the IPsec SA. To do this, quick mode uses a two-step exchange composed of four messages sent between initiator (James) and responder (Charlie), as illustrated in Figure 26.

After IKE Phase II negotiation has successfully completed quick mode exchange, both crypto endpoints should have three established security associations in their SADB:

• ISAKMP SA--This is a bidirectional SA that is used to dynamically establish a secure channel for the negotiation of IPsec SAs.
• Outbound IPsec SA--This unidirectional SA is used to provide protection services offered by IPsec to traffic that is to be transmitted to the remote tunnel endpoint, identified by the SPI within the IPsec header.
• Inbound IPsec SA--This unidirectional SA is used to process inbound IPsec traffic from a remote crypto endpoint. Again, this traffic is identified by the SPI that was inserted into the IPsec header by the transmitting IPsec endpoint.

PFS
PFS guarantees that session keys are generated independently from previous session keys. With PFS enabled, would-be attackers are unable to use old session keys that have been compromised to compromise the integrity and confidentiality of current and future session keys. PFS does this by forcing renegotiation of shared Diffie-Hellman keys whenever a new negotiation of ISAKMP and IPsec SAs As PFS is a feature based on Diffie-Hellman, the strength of PFS relies on the prime modulus size used to derive the shared secret keys. There are there prime modulus sizes offering increasing level of security (groups 1, 2, and 5). PFS is enabled when configuring the IPsec crypto map, as illustrated in Example 15.

Page 2: next page
Page 1 | 2