Technical Tip: Understanding Diffie-Hellman groups and IKEv1 vs IKEv2 in FortiGate VPNs

Description

This article describes the role of Diffie-Hellman groups in IPsec VPN key negotiation and highlights the differences between IKEv1 and IKEv2. It includes best practices for selecting key exchange parameters on FortiGate devices.

Scope

FortiGate devices configured with IPsec VPNs using IKEv1 or IKEv2 protocols for site-to-site or remote access.

Solution

Diffie-Hellman groups.

Diffie-Hellman (DH) groups are the key-exchange strength setting used in IKE (Phase 1) and optionally for PFS in ESP (Phase 2). Both peers must have at least one matching group.

Diffie-Hellman groups define the cryptographic strength used during key exchange. Larger group numbers offer higher security but may increase CPU load.

IKEv1 vs IKEv2 comparison:

Note: FortiGate is not compatible with MOBIKE devices for IKEv2.

CLI example: IKEv2 with DH group 20:

config vpn ipsec phase1-interface
    edit "vpn-to-remote"
        set interface "wan1"
        set ike-version 2
        set dhgrp 20
        set proposal aes256-sha256
        set psksecret ENC xxxxxxxx
        set remote-gw 198.51.100.1
    next
end

CLI example: IKEv1 with DH group 5:

config vpn ipsec phase1-interface
    edit "vpn-legacy"
        set interface "wan1"
        set ike-version 1
        set dhgrp 5
        set proposal 3des-sha1
        set psksecret ENC xxxxxxxx
        set remote-gw 192.0.2.10
    next
end

Key notes:

• Groups 1, 2, 5: Legacy, avoid unless absolutely required for backward compatibility.

• Group 14 (MODP 2048): Baseline safe if elliptic curve groups are not available.

• Group 15 (MODP 3072): Stronger than 14 but heavier; still considered acceptable if ECC is not usable.

• Groups 19 & 20 (ECP): Elliptic curve groups, efficient and recommended.

• Group 31 (Curve25519): Modern and highly recommended for both security and performance.

On FortiGate, when configuring IPsec Phase 1 or 2, it is best practice to select Groups 14, 19, 20, or 31, depending on the peer's compatibility.

Best practices:

• Use IKEv2 for all new deployments due to protocol efficiency, crash recovery, and better EAP and mobility support.

• Group 14 is the minimum recommended DH group. Group 19 or 31 is preferred for stronger security and performance.

• Verify compatibility with third-party vendors when changing DH groups.

GUI configuration path:

• Select 'VPN' -> 'IPsec Tunnels'.

• Select 'Create New' or edit an existing tunnel.

• Under Phase 1 settings, select the appropriate IKE version and DH group under 'Advanced'.

Related article:
Technical Tip: How to check if Diffie-Hellman(DH) group is the same on both peer units