ipsec - IKE v1

IKEv1 phase 1:

IKEv1 main mode uses 6 messages. 

message 1:

The initiator (peer that wants to build the tunnel) will send the first message. This is a proposal for the security association. Above you can see that the initiator uses IP address 192.168.12.1 and is sending a proposal to responder (peer we want to connect to) 192.168.12.2. IKE uses UDP port 500 for this. In the output above you can see an initiator SPI (Security Parameter Index), this is a unique value that identifies this security association.

We can see the IKE version (1.0) and that we are using main mode. The domain of interpretation is IPsec and this is the first proposal. In the transform payload you can find the attributes that we want to use for this security association.

message 2:

When the responder receives the first message from the initiator, it will reply. This message is used to inform the initiator that we agree upon the attributes in the transform payload. You can also see that the responder has set its own SPI value.

message 3:

 Since our peers agree on the security association to use, the initiator will start the Diffie Hellman key exchange. In the output above you can see the payload for the key exchange and the nonce.

message4:

 The responder will also send his/her Diffie Hellman nonces to the initiator, our two peers can now calculate the Diffie Hellman shared key.

message 5:

The last two messages are encrypted so we can’t see its contents anymore. These two are used for identification and authentication of each peer. The initiator starts.

message 6:

And above we have the 6th message from the responder with its identification and authentication information. IKEv1 main mode has now completed and we can continue with IKE phase 2.

IKEv1 aggressive mode

IKEv1 aggressive mode only requires three messages to establish the security association. It’s quicker than main mode since it adds all the information required for the DH exchange in the first two messages. Main mode is considered more secure since identification is encrypted, aggressive mode does this in clear-text.

message 1:

 The first message is from the initiator (192.168.12.1) to the responder (192.168.12.2).  You can see the transform payload with the security association attributes , DH nonces and the identification (in clear text) in this single message.

message 2:

 The responder now has everything in needs to generate the DH shared key and sends some nonces to the initiator so that it can also calculate the DH shared key. It also calculates a hash that is used for authentication.

message 3:

Both peers have everything they need, the last message from the initiator is a hash that is used for authentication.

Our IKE phase 1 tunnel is now up and running and we are ready to continue with IKE phase 2.

IKEv1 phase 2:

The IKE phase 2 tunnel (IPsec tunnel) will be actually used to protect user data. There is only one mode to build the IKE phase 2 tunnel which is called quick mode.

message 1:

This negotiation happens within the protection of our IKE phase 1 tunnel so we can’t see anything. Just for the sake of completeness, here’s what it looks like in wireshark:

message 2:

message 3:

 Once IKE phase 2 has completed, we are finally ready to protect some user data.