Does IKE support Perfect Forward Secrecy?

IKE provides an optional property called Perfect Forward Secrecy (PFS) that allows each phase 2 key to be derived independently through a separate Diffie-Hellman exchange. With PFS, if a single key is compromised, the integrity of subsequently generated keys is not affected.

To implement PFS, configure these 2 parameters on the IpDynVpnAction statement that defines the control information for the phase 2 negotiation:

(1) InitiateWithPFS: Specifies whether perfect forward secrecy (PFS) is used as initiator of the security association, and if so, what Diffie-Hellman group is used. The default is None.

Result: For negotiations using IKE version 1, the InitiateWithPfs selection is used when sending the proposal. For negotiations using IKE version 2, all PFS selections specified on the AcceptablePfs list are included when sending the proposal, but the InitiateWithPfs selection is sent as the first choice.

Guideline: If you are using encryption or authentication algorithms with a 128-bit key, use Diffie-Hellman groups 5,14,19,20, or 24. If you are using encryption or authentication algorithms with a key length of 256 bits or greater, use Diffie-Hellman group 21.

(2) AcceptablePFS: Specifies acceptable Diffie-Hellman groups to use for perfect forward secrecy (PFS). The default is None.

Result: For negotiations using IKE version 1, the AcceptablePfs list is used when the z/OS IKE daemon is the responder for a security association. For negotiations using IKE version 2, the AcceptablePfs list is used in both initiator and responder modes.

Guideline: If you are using encryption or authentication algorithms with a 128-bit key, use Diffie-Hellman groups 5,14,19,20, or 24. If you are using encryption or authentication algorithms with a key length of 256 bits or greater, use Diffie-Hellman group 21. Rule: The InitiateWithPfs Diffie-Hellman group must be specified as one of the values in the AcceptablePfs parameter.

z/OSMF Configuration Assistant permits the user to select, when configuring Perfect Forward Security (PFS) for the IPSec phase 2 Security Association negotiation, either the default value of None (do not use Perfect Forward Security), or one or more PFS groups.