4.4.3.4. How the PAD Is Used
Initial IKE Exchange
During the initial IKE exchange, the initiator and responder each assert their identity via the IKE ID payload and send an AUTH payload to verify the asserted identity. One or more CERT payloads may be transmitted to facilitate the verification of each asserted identity.
When an IKE entity receives an IKE ID payload, it uses the asserted ID to locate an entry in the PAD, using the matching rules described above. The PAD entry specifies the authentication method to be employed for the identified peer. This ensures that the right method is used for each peer and that different methods can be used for different peers. The entry also specifies the authentication data that will be used to verify the asserted identity. This data is employed in conjunction with the specified method to authenticate the peer, before any CHILD SAs are created.
Child SA Creation
Child SAs are created based on the exchange of traffic selector payloads, either at the end of the initial IKE exchange or in subsequent CREATE_CHILD_SA exchanges. The PAD entry for the (now authenticated) IKE peer is used to constrain creation of child SAs; specifically, the PAD entry specifies how the SPD is searched using a traffic selector proposal from a peer.
There are two choices:
- Either the IKE ID asserted by the peer is used to find an SPD entry via its symbolic name, OR
- Peer IP addresses asserted in traffic selector payloads are used for SPD lookups based on the remote IP address field portion of an SPD entry
It is necessary to impose these constraints on creation of child SAs to prevent an authenticated peer from spoofing IDs associated with other, legitimate peers.
Protection Against Spoofing
Note that because the PAD is checked before searching for an SPD entry, this safeguard protects an initiator against spoofing attacks.
Example: Assume that IKE A receives an outbound packet destined for IP address X, a host served by a security gateway. RFC 2401 [RFC2401] and this document do not specify how A determines the address of the IKE peer serving X. However, any peer contacted by A as the presumed representative for X must be registered in the PAD in order to allow the IKE exchange to be authenticated. Moreover, when the authenticated peer asserts that it represents X in its traffic selector exchange, the PAD will be consulted to determine if the peer in question is authorized to represent X.
Thus, the PAD provides a binding of address ranges (or name sub-spaces) to peers, to counter such attacks.