HP-UX IPSec Version A.03.00 Administrator's Guide

The traffic selectors specify the endpoints for the IPsec SA by IP address, protocol, and port
number.
• Message 2: Responder sends accepted IPsec SA proposal, SPI, and traffic IDs
In message 2, the responder sends back the accepted IPsec SA proposal and the SPI for the
responder's inbound IPsec SA (for packets to the responder from the initiator).
• Message 3: Initiator sends hash message to prove liveness
In Message 3, the initiator contains a hash of data sent by the responder in message 2. This
proves that the first message is “live” and not a previous message captured by an attacker
and replayed.
Some implementations send an optional fourth message from the responder (a NOTIFY message);
HP-UX IPsec accepts this message but does not send it unless requested by the initiator.
IKEv2 IKE and IPsec SA Message Flow
IKEv2 uses four messages to establish an IKE SA and an initial IPsec SA pair (data used to
negotiate the initial IPsec SA pair is included with the messages used to complete the IKE SA
negotiation):
Figure A-4 IKEv2 SA Negotiations
• Message 1: Initiator sends IKE SA proposals and Diffie-Hellman public value
In message 1, the node initiating the IKE exchange (the initiator) sends IKE SA proposals,
which contain IKE SA parameters, and its Diffie-Hellman public value.
• Message 2: Responder sends accepted IKE SA proposal and Diffie-Hellman public value
In message 2, the peer node (the responder) sends back its accepted IKE SA proposal and
its Diffie-Hellman public value.
The Diffie-Hellman exchange enables the IKE entities to each generate the same, shared
secret value that they use to generate encryption and authentication keys. The keys are used
for the IKE SA and the IPsec SAs. After the shared secret is generated, subsequent IKE
messages in the exchange are secure (encrypted). However, the IKE IDs are not authenticated
until Messages 3 and 4 in the exchange.
• Message 3: Initiator sends IKE ID, authentication data, IPsec SA proposals, and IPsec
traffic IDs
In message 3, the initiator sends information to authenticate the IKE SA and to establish the
initial IPsec SA pair.
The initiator sends its IKE ID type and value (HP-UX IPSec uses IP addresses as ID values
by default).
If the IKE authentication method is RSA signatures, the initiator sends the certificate for the
local system, a digital signature calculated using its private key, and a request for the peer's
certificate. If the IKE authentication method is preshared keys, the initiator sends a hash
value calculated using the preshared key.
The initiator also includes information for establishing the initial IPsec SA pair: IPsec SA
proposals, and IPsec traffic selectors. The traffic selectors specify the endpoints for the IPsec
SA by IP address, protocol, and port number.
172 Product Specifications