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The IPSec tunnel creation process involves 2 steps:

The ISAKMP Phase

The IPSec Phase

What is the reasoning behind having the two phases? From what i see, the first phase is already encrypted using pre-negotiated keys and the tunnel established is used to negotiate the parameters for the second tunnel.

If the first phase is considered secure, why the need for the second phase. And if the first phase isn't secure, can't an attacker theoretically compromise it and gain the information about the second phase thereby compromising it as well?

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up vote 4 down vote accepted

Different parties are being authenticated during each of the phases of negotiation. During the first phase, the parties being authenticated are the ISAKMP servers/hosts, while during the second phase, users or application level programs are being authenticated.

If you look at the RFC of ISAKMP, in the section 2.3 (Negotiation phase) it talks about advantages of such implementation:

While the two-phased approach has a higher start-up cost for most simple scenarios, there are several reasons that it is beneficial for most cases.

First, entities (e.g. ISAKMP servers) can amortize the cost of the first phase across several second phase negotiations. This allows multiple SAs to be established between peers over time without having to start over for each communication.

Second, security services negotiated during the first phase provide security properties for the second phase. For example, after the first phase of negotiation, the encryption provided by the ISAKMP SA can provide identity protection, potentially allowing the use of simpler second-phase exchanges. On the other hand, if the channel established during the first phase is not adequate to protect identities, then the second phase must negotiate adequate security mechanisms.

Third, having an ISAKMP SA in place considerably reduces the cost of ISAKMP management activity - without the "trusted path" that an ISAKMP SA gives you, the entities (e.g. ISAKMP servers) would have to go through a complete re-authentication for each error notification or deletion of an SA.

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