IPsec with public key authentication on strongSwan >= 5.0.0

Setup

Generate an RSA keypair

root@debian:~# mkdir /etc/ipsec.d/public
root@debian:~# ipsec pki --gen --type rsa --outform pem --size 4096 > /etc/ipsec.d/private/mykey.pem
root@debian:~# ipsec pki --pub --in /etc/ipsec.d/private/mykey.pem --outform pem > /etc/ipsec.d/public/mykey.pub
root@debian:~# echo ": RSA mykey.pem" >> /etc/ipsec.secrets

Exchange public keys with your peer

Display the public key. Send the key data to your peer.

 root@debian:~# more /etc/ipsec.d/public/mykey.pub
 -----BEGIN PUBLIC KEY-----
 MIICIjANBgkqhkiG9w0BAQEFAAOCAg8AMIICCgKCAgEA2/FWIJuVUtfsLovavNp+
 nPSsT2mQoNK3ZUUwuEKfBjT7mhijdXRHh1SAtIaU2aen5+d5q6e27vMCCYOQLagn
 9CkKatBq54zGNvDSzQEpz0mIsaBx9xjvhsgqAmKCTpLtKuMz6cZbH8y8o9/ZZ8Kv
 +Jht67T8BDKXczgOg5IIaX84UpCrlSgmnSvKYKu3PXnt91bZ66HaDZJjPf9aiMNc
 fvuUqVfFWnsV2zI6HFvG/uwkqLalsnPaAwVeIWl2Ovy2Jzdj0GRLSYx87eneSBo+
 7tjlURQTudAj1+53SFOkBcCPSnzPYpIC3hBfZ8Zw8r/25moW3xf8TlLLJqgAh50Y
 tVyvyVSv1MKYBdjZcFsEXUceC5LI9JZryB/Serq0R+4//ZiR3LEtetVKNvco9bcI
 JHXr88HM2XeYRfRPAB6wembIEMKYdwIhwYAPPAtL+lDHtZBiBAIAp0y0FhaozSzl
 MSry8tbJR2fD/i8/yXr5isVfjJZdw8WK0LAd8a8zvmNIFKgiKWjoDgIycM5HrRD+
 rY0Br9xONkNdgB7Lz/wPEyUsiIiZpawM/S4taX7ExK4Wi3pdxkOHLn2ZyaWKsdhX
 PpCkdMfSOJ0SqCUcVze+xD8GlInUQsPgbDGvxT73jT6Ie+wSA94Cgs3mq7FS6cNo
 ZAASv7cT9DG+xfQmjrJC9SUCAwEAAQ==
 -----END PUBLIC KEY-----

Convert your peer's public key to the PEM format using the pubkey-converter script, if necessary.

Configuration

Configure the phase 1 IKE parameters

In this example, we'll use the following settings:

Key	Value
Encryption	AES-128
Hash	HMAC-SHA1
DH Group	5 (modp1536)
Lifetime	28800 seconds
Peer address	192.0.2.2
Local address	192.0.2.1

Add your peer's public key

 root@debian:~# cat << EOF > /etc/ipsec.d/public/peerkey.pub
 -----BEGIN PUBLIC KEY-----
 MIICIjANBgkqhkiG9w0BAQEFAAOCAg8AMIICCgKCAgEAuQ1hX3+AEiLis4p5jvmY
 IfEgaq9488GU2nkuR1gZK4/CphrccmztgADU/TkiE5IOOo7zKPparcl8dZwJfX+j
 9oIEfvIHqWbF8rIkmJDAn5ZQANDsVKSVvu8nqNZhAslBkFUw1Il6KJrdZEwV4ILL
 jZmPjN/TIpzPStufeKLfUr7OodFnJXdvWGW1OwRVR8YPZP+13S2nbaqohJZYulTz
 EcEaWFhXRKzm1dxXqt2H0S93mv5CT0JzVUAv35lB39NMqcRwrg5Fsw0z448nlenS
 pIDnP8AIPm5shNf9kZJCksOE9GfPMz5KZDOoR3rWPvvapY/Zs+SBz8drPwHrZahG
 KjxV2Txl7XYkFEWvre+rpiWSidIsnzizEBQ1ea6Inwd6Y29uzmQLhus1OPfhnCIk
 AVycK+stCZObPl6abomk/avaT+FwnjRgSZzkotlbA2IHgEjF26C4a3ZUGBwDZ+7r
 U2A7DKbHRN84f62BcMyiZpavXPvk4AUQD5+lRItbFDDeZYtoIdHb9gmMcNHWsGL8
 YVxF2A8IAvr7Q1TQfwm2WPAWvjai2rbl6w8ZYiQBPUwkFtUATXU7XyGcLmjCGJGg
 HZoAqPHddX1z7HosFId+SSc7ugCnUNtwSYHq+DczBVdTa65aPv758zxeKzUqJBKy
 mP4HkvHlEmXHP2oAQ4G6PTkCAwEAAQ==
 -----END PUBLIC KEY-----
 EOF

Configure a connection policy in ipsec.conf for your peer

 root@debian:~# cat << EOF >> /etc/ipsec.conf
 conn MYPEER
     # peer IPs
     left=192.0.2.1
     right=192.0.2.2
     # phase 1 parameters
     ike=aes128-sha1-modp1536!
     ikelifetime=28800s
     # authentication
     authby=pubkey
     leftrsasigkey=/etc/ipsec.d/public/mykey.pub
     rightrsasigkey=/etc/ipsec.d/public/peerkey.pub
 EOF

All done! Configure the phase 2 parameters as you otherwise would.

Full GRE/IPsec example

root@debian:~# ip addr show dev gre1
11: gre1@NONE: <POINTOPOINT,NOARP,UP,LOWER_UP> mtu 1400 qdisc noqueue state UNKNOWN
    link/gre 192.0.2.1 peer 192.0.2.2
    inet 10.1.2.0/31 scope global gre1
       valid_lft forever preferred_lft forever
    inet6 fe80::200:5efe:6825:1c22/64 scope link
       valid_lft forever preferred_lft forever
root@debian:~# more /etc/ipsec.conf
# ipsec.conf - strongSwan IPsec configuration file

config setup

conn %default
    keyexchange=ikev1
    dpdaction=restart

conn MYPEER
    # peer IPs
    left=192.0.2.1
    right=192.0.2.2
    # phase 1 parameters
    ike=aes128-sha1-modp1536!
    ikelifetime=28800s
    # authentication
    authby=pubkey
    leftrsasigkey=/etc/ipsec.d/public/mykey.pub
    rightrsasigkey=/etc/ipsec.d/public/peerkey.pub
    # phase 2 parameters
    esp=aes128-sha1-modp1536!
    lifetime=3600s
    type=transport
    leftprotoport=gre
    rightprotoport=gre
    # startup
    auto=route
    keyingtries=%forever

If your peer is using a Cisco router and is behind NAT, then you might need to add the following option:

    rightid=NATIP

strongSwan5Example