Site-to-Site VPNs with Diffie-Hellman Groups 19 & 20 (Elliptic Curve)

Similar to my test with Diffie-Hellman group 14 shown here I tested a VPN connection with the elliptic curve Diffie-Hellman groups 19 and 20. The considerations why to use these DH groups are listed in the just mentioned post – mainly because of the higher security level they offer. I tested the site-to-site IPsec connections with a Juniper ScreenOS firewall and a Fortinet FortiGate firewall. (Currently, neither the Palo Alto nor the Cisco ASA support these groups.)

[This is one of many VPN tutorials on my blog. Please look here find the appropriate one.]

The numbers for the groups are specified in RFC 5114: Additional Diffie-Hellman Groups for Use with IETF Standards. And according to this document on p. 30 (from the “European Network of Excellence in Cryptology”), the bits of security for the elliptic curve groups are the following:

  • Group 19 = 256-bit EC = 128 bits of security
  • Group 20 = 384-bit EC = 192 bits of security

That is, both groups offer a higher security level than the Diffie-Hellman groups 14 (103 bits) or 5 (89 bits). When using group 20 in IPsec phase 2 (PFS) with AES-256, the security level of the whole VPN connection is really 192 bit!

Test Group 19

The config changes for my test VPN between the SSG and the FortiGate were trivial. These are the proposals I am using now:

The site-to-site IPsec tunnel came up without any problems. The SSG reveals the correct crypto algorithms used for this VPN with the following CLI commands:


However, I have not found a command on the FortiGate to display the Diffie-Hellman group that is used for a certain VPN. Does anyone know?!?


Test Group 20

And, of course, very similar for group 20 (while still only the SSG shows the active DH groups in its command line output):

Featured image “A Voice At The End Of The Line” by Jeremy Brooks is licensed under CC BY-NC 2.0.

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