| Diffie-Hellman (DH) is a public -key cryptography scheme allowing two parties to establish a shared secret over an insecure communications channel. IKE uses Diffie-Hellman to create keys used to encrypt both the Internet Key Exchange (IKE) and IPSec communication channels. The process works by two peers both generating a private and a public key and then exchanging their public keys with each other. The peers produce the same shared secret by using each other’s public key and their own private key using the DH algorithm.
With Perfect Forward Secrecy (PFS), every time a new IPSec SA is negotiated during the Quick Mode, a new DH exchange occurs. The new DH shared secret will be included with original keying material (SYKEID_d, initiator nonce, and responder nonce from Phase 1) for generating a new IPSec session key. If PFS is not used, the IPSec session key will always be completely dependent on the original keying material from the Phase-1. Hence, if an older key is compromised at any time, it is possible that all new keys may be compromised.
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