Details
PFS generates each new encryption key independently from the previous key. Without PFS, compromise of one key will compromise all communications.
The Phase 2 (Quick Mode) Security Association (SA) is used to create an IPsec session key. Hence, its rekey or key regeneration procedure is very important. The Phase 2 rekey can be performed with or without Perfect Forward Secrecy (PFS). With PFS, every time a new IPsec Security Association is negotiated during the Quick Mode, a new Diffie-Hellman (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 Phase 1. Hence, if an older key is compromised at any time, it is possible that all new keys may be compromised.
The DH exchange is performed in the same manner as was done in Phase 1 (Main or Aggressive Mode). However, the Phase 2 exchange is protected by encrypting the Phase 2 packets with the key derived from the Phase 1 negotiation. Because DH negotiations during Phase 2 are encrypted, the new IPsec session key has an added element of secrecy.
Solution
Configure the ASA to specify PFS for the IPsec SA during IKE Phase 2 negotiation as shown in the example below.
ASA3(config)# crypto map IPSEC_CRYPTO_MAP 1 set pfs group5
Supportive Information
The following resource is also helpful.
This security hardening control applies to the following category of controls within NIST 800-53: System and Communications Protection.This control applies to the following type of system Cisco.
References
- 800-53|SC-8
- CAT|II
- CCI|CCI-002418
- Rule-ID|SV-239954r666268_rule
- STIG-ID|CASA-VN-000180
- Vuln-ID|V-239954