Which two statements correctly describe the differences between IPsec main mode and IPsec aggressive mode? (Choose two.)

The primary differences between IKEv1 Main Mode and Aggressive Mode relate to security, speed, and flexibility. Main Mode prioritizes security by using a six-packet exchange to establish a secure channel. This process protects the identities of the communicating peers, as their IDs are encrypted and exchanged only in the final two packets. Aggressive Mode prioritizes speed by using a condensed three-packet exchange. To achieve this, the initiator sends its peer ID in the first packet, unencrypted. This makes the exchange faster but exposes the initiator's identity. This behavior is essential for dialup VPNs using pre-shared keys, as the responder needs the ID to find the correct policy.

B. Main mode can be used for dialup VPNs, particularly when using digital certificates for authentication, as the certificate contains the necessary identity information. Aggressive mode is only mandatory for PSK-based dialup clients.
C. XAuth (Extended Authentication) is an IKEv1 extension for user authentication that can be used after the successful completion of either Main Mode or Aggressive Mode Phase 1 negotiations.

1. Fortinet FortiOS 7.0.0 Administration Guide:

IPsec VPN > IKE version 1 > Phase 1 negotiation: "Main mode uses six messages to establish the IKE SA... The identities of the peers are encrypted." This supports option D and the identity protection aspect of option A.

IPsec VPN > IKE version 1 > Phase 1 negotiation: "Aggressive mode uses three messages to establish the IKE SA... The peer ID of the initiator is sent in the first packet and is therefore in cleartext." This supports options D and A.

IPsec VPN > Dialup IPsec VPNs: "When configuring a dialup IPsec VPN

you must use aggressive mode if you are using a pre-shared key for authentication... Aggressive mode sends a peer ID in the first packet of the exchange

which allows the gateway to look up the pre-shared key." This confirms why option B is incorrect

as the limitation is specific to PSK.

2. NIST Special Publication 800-77

Guide to IPsec VPNs:

Section 4.3.1.1

IKE Main Mode: "Main Mode requires a total of six messages... Main Mode provides identity protection for the peers." This supports option D and the reasoning for A.

Section 4.3.1.2

IKE Aggressive Mode: "Aggressive Mode requires a total of three messages... Aggressive Mode does not provide identity protection for the peers." This supports options D and A. The document also clarifies that XAuth is a subsequent exchange

not tied to a specific Phase 1 mode

which invalidates option C.