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-----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-----END PUBLIC KEY-----

Convert your peer's public key to the Base64 RFC 3110 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

Note: strongSwan < 5.0.0 will read PEM-formatted private keys, but requires public keys to be added to the configuration in Base64 RFC 3110 format. Convert your host's public key from PEM to Base64 RFC 3110 for inclusion in ipsec.conf as described below.

Configure a connection policy in ipsec.conf for your peer. The leftrsasigkey attribute is your host's public key in Base64 RFC 3110 format enclosed in double quotes, and rightrsasigkey is your peer's key.

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="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"
    rightrsasigkey="0sAwEAAbkNYV9/gBIi4rOKeY75mCHxIGqvePPBlNp5LkdYGSuPwqYa3HJs7YAA1P05IhOSDjqO8yj6Wq3JfHWcCX1/o/aCBH7yB6lmxfKyJJiQwJ+WUADQ7FSklb7vJ6jWYQLJQZBVMNSJeiia3WRMFeCCy42Zj4zf0yKcz0rbn3ii31K+zqHRZyV3b1hltTsEVUfGD2T/td0tp22qqISWWLpU8xHBGlhYV0Ss5tXcV6rdh9Evd5r+Qk9Cc1VAL9+ZQd/TTKnEcK4ORbMNM+OPJ5Xp0qSA5z/ACD5ubITX/ZGSQpLDhPRnzzM+SmQzqEd61j772qWP2bPkgc/Haz8B62WoRio8Vdk8Ze12JBRFr63vq6YlkonSLJ84sxAUNXmuiJ8HemNvbs5kC4brNTj34ZwiJAFcnCvrLQmTmz5emm6JpP2r2k/hcJ40YEmc5KLZWwNiB4BIxduguGt2VBgcA2fu61NgOwymx0TfOH+tgXDMomaWr1z75OAFEA+fpUSLWxQw3mWLaCHR2/YJjHDR1rBi/GFcRdgPCAL6+0NU0H8JtljwFr42otq25esPGWIkAT1MJBbVAE11O18hnC5owhiRoB2aAKjx3XV9c+x6LBSHfkknO7oAp1DbcEmB6vg3MwVXU2uuWj7++fM8Xis1KiQSspj+B5Lx5RJlxz9qAEOBuj05"
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="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"
    rightrsasigkey="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"
    # phase 2 parameters
    esp=aes128-sha1!
    pfs=yes
    pfsgroup=modp1536
    lifetime=3600s
    type=transport
    leftprotoport=gre
    rightprotoport=gre
    # startup
    auto=route
    keyingtries=%forever