Diffie-Hellman (DH) is a public-key cryptography protocol that allows two devices to establish a shared secret over an unsecure communications channel (ISAKMP Phase 1 / IPSec Phase 2)
Diffie-Hellman Group 1 — 768-bit DH Group.
Diffie-Hellman Group 2 — 1024-bit DH Group.
Diffie-Hellman Group 5 — 1536-bit DH Group.
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Perfect Forward Secrecy (PFS)
The PFS determines the length (complexity) of the IPSec "Session keys" (used in the encryption and decryption of data in the IPSec tunnel (IPSEC Security Association (SA) )), and is derived from the public and private keys.
PFS has four groups:
Group1: Specifies that IPSec should use the 768-bit Diffie-Hellman prime modulus group when performing the new Diffie-Hellman exchange.
Group2: Specifies that IPSec should use the 1024-bit Diffie-Hellman prime modulus group when performing the new Diffie-Hellman exchange.
Group5: Specifies that IPSec should use the 1536-bit Diffie-Hellman prime modulus group when performing the new Diffie-Hellman exchange.
Group7: Specifies that IPSec should use group7 (ECC) where the elliptical curve field size is 163-bits, for example, with the movianVPN client.
The crypto map set pfs command sets IPSec to ask for Perfect Forward Secrecy (PFS) when new security associations are requested for this crypto map entry. Alternatively, it asks that IPSec requires PFS when requests are received for new security associations.
To specify that IPSec not request PFS, issue the no crypto map set pfs command. This command is only available for ipsec-isakmp crypto map entries and dynamic crypto map entries.
Note: By default, PFS is not requested.
With PFS, every time a new security association is negotiated, a new Diffie-Hellman exchange occurs, which requires additional processing time.
PFS adds another level of security because if one key is ever cracked by an attacker, only the data sent with that key is compromised. During negotiation, the no crypto map set PFS command causes IPSec to request PFS when new security associations are requested for the crypto map entry.
The default (group1) is sent if the set pfs statement does not specify a group. If the peer initiates the negotiation and the local configuration specifies PFS, the peer must perform a PFS exchange or the negotiation fails.
If the local configuration does not specify a group, a default of group1 is assumed and an offer of either group1 or group2 is accepted. If the local configuration specifies group2, that group must be part of the peer offer or the negotiation fails.
