IPSec - why does IKE need two phases

Within the framework of IPSec, IKE uses two phases to establish a secure tunnel.

Engineers could have designed the process to contain only one phase, however, there would be some disadvantages to doing so:

If everything were handled in a single phase, you would have the following problems:

• Lack of Security: Without Phase 1, sensitive negotiations in Phase 2 would be exposed to the network. Anyone could eavesdrop on key exchange, algorithm choices, and potentially attack the session. Phase 1 ensures that subsequent exchanges are secured.
• Separation of Responsibilities: By splitting the process into two phases, each phase handles specific tasks (authentication and secure channel creation in Phase 1, and data encryption in Phase 2). This modular design is more flexible and allows reusability. For example, once Phase 1 is complete, multiple Phase 2 tunnels can be established between the same peers without re-authenticating.
• Efficiency: The secure channel established in Phase 1 is reused for multiple sessions. Without this separation, the overhead of repeatedly negotiating and securing the connection would slow down the process, reducing efficiency in environments with frequent connections or multiple tunnels.

So, having two phases in IPsec ensures security, efficiency, and flexibility in managing tunnels and encrypted connections.