The secrets of IKE (Internet Key Exchange) 05 - Digital Signature Aggressive Mode

This is the 5th post in this IKE series. You can find the other posts of this series from the following:

01 - Introduction & IKE structure
02 - PSK Main Mode
03 - PSK Aggressive Mode
04 - Digital Signature Main Mode
05 - Digital Signature Aggressive Mode
06 - IKE Phase 2 Quick Mode

In this post, let's go through the Digital Signature Aggressive Mode. Many items have already been described in the PKI Aggressive Mode and Digital-Sign Main Mode posts. So I won't go through the details of them. Here I will just go through the special characters in Digital-Sign Aggressive Mode.

Same as in PSK Aggressive Mode, in Digital-Sign Aggressive Mode, three packets will be exchanged between the Initiator and Responder.


Packet 1 (Initiator -> Responder):

The Packet 1 contents are very similar to the one in PSK Aggressive Mode. Just the authentication method in SA Proposal here is "RSA signatures".

Packet 2 (Initiator <- Responder):

Before sending the 2nd Packet, the Responder will need to figure out which cert will be used for generating the SIG_R and sent to the Initiator for identity validation. 

In Main Mode, Initiator sends its cert first. So Responder can decide which cert to be used for this IKE session based on either the IKE ID of the Initiator or the Initiator's cert elements (such as Issuer's CN or Subject OU).

But in Aggressive Mode, Responder has to send its cert first. And because it has not receive the Initiator's cert at this point, it can only make the cert decision based on the Initiator's IDii. 

Packet 3 (Initiator -> Responder):

Once the Initiator receives the 2nd packet, it will validate Responder's Identity by checking the SIG_R. If everything is ok, it will send out the Packet 3 including its own cert and SIG_I.

After exchanging these three packets, the Digital-Sign Aggress Mode Phase 1 is completed.

Doesn't like in PSK, in Digital Signature authentication, IDii is no longer limited to IP address. The Main Mode which is consider to be more secure are highly recommended. And the Digital-Sign Aggressive Mode is less preferred. 

The pacp file for a example of Digital-Sign Aggressive Mode can be download from the following link:

Dig-Sign_AM.pcap

In the next post, we will discuss about the Quick Mode which is the Phase 2 in IKE.

Reference:

• RFC2409
• IPSec Tunnel Creation
• Internet Key Exchange (IKE)