CA Setup

Create a Certificate Authority for issuing strongSwan VPN certificates.

A Certificate Authority (CA) is the trust anchor for your VPN. Every gateway and client certificate you issue will be signed by it, and every peer will verify its certificate against the CA certificate. This page walks through creating a minimal, self-contained root CA using pki.

1. Generate the CA private key

   Create an Ed25519 private key (recommended):

   pki --gen --type ed25519 --outform pem > strongswanKey.pem
   

   Alternatively, generate a 3072-bit RSA key:

   pki --gen --type rsa --size 3072 --outform pem > strongswanKey.pem
   

   Ed25519 produces compact 256-bit keys with 128-bit security and is supported by all modern IKEv2 implementations. RSA is more widely compatible with legacy systems.

   Key size guidance for RSA:

   Size	Security level	Recommended use
   2048 bits	~112-bit	Legacy / interop only
   3072 bits	~128-bit	Current standard
   4096 bits	~140-bit	High-security environments

2. Create the self-signed CA certificate

   Use pki --self to wrap the public key in a self-signed X.509 CA certificate with a 10-year lifetime (3652 days):

   pki --self --ca --lifetime 3652 \
       --in strongswanKey.pem \
       --dn "C=CH, O=strongSwan, CN=strongSwan Root CA" \
       --outform pem > strongswanCert.pem
   

   Key options:

   Option	Description
   --ca	Set the CA basic constraint and keyCertSign / CRLSign key usages
   --lifetime <days>	Validity period in days (default: 1095)
   --dn <distinguished name>	Subject DN in RFC 4514 format
   --outform pem	Output as Base64 PEM instead of binary DER

3. Inspect the CA certificate

   Verify the generated certificate:

   pki --print --in strongswanCert.pem
   

   Expected output:

   subject:  "C=CH, O=strongSwan, CN=strongSwan Root CA"
   issuer:   "C=CH, O=strongSwan, CN=strongSwan Root CA"
   validity:  not before May 18 08:32:06 2017, ok
              not after  May 18 08:32:06 2027, ok (expires in 3651 days)
   serial:    57:e0:6b:3a:9a:eb:c6:e0
   flags:     CA CRLSign self-signed
   subjkeyId: 2b:95:14:5b:c3:22:87:de:d1:42:91:88:63:b3:d5:c1:92:7a:0f:5d
   pubkey:    ED25519 256 bits
   keyid:     a7:e1:6a:3f:e7:6f:08:9d:89:ec:23:92:a9:a1:14:3c:78:a8:7a:f7
   subjkey:   2b:95:14:5b:c3:22:87:de:d1:42:91:88:63:b3:d5:c1:92:7a:0f:5d
   

   Confirm that flags includes CA and CRLSign, and that issuer equals subject (self-signed).

4. Store and distribute the CA files

   File	Where it goes	Who needs it
   strongswanKey.pem	Secure offline storage (not on any VPN gateway)	CA operator only
   strongswanCert.pem	/etc/swanctl/x509ca/ on every VPN endpoint	All gateways and clients

   Copy the CA certificate to each host:

   install -m 644 strongswanCert.pem /etc/swanctl/x509ca/
   

   Then reload credentials on each host:

   swanctl --load-creds
   

Intermediate CAs

For larger deployments, issue an intermediate CA certificate instead of signing end-entity certificates directly from the root. This limits exposure of the root key.

First issue an intermediate CA cert from your root CA:

# Generate the intermediate key
pki --gen --type ed25519 --outform pem > interKey.pem

# Issue the intermediate CA certificate
pki --issue --cacert strongswanCert.pem --cakey strongswanKey.pem \
    --ca --lifetime 1826 \
    --in interKey.pem \
    --dn "C=CH, O=strongSwan, CN=strongSwan Intermediate CA" \
    --outform pem > interCert.pem

The --ca flag on pki --issue sets the CA:TRUE basic constraint, making the issued certificate capable of signing further certificates.

Place both the intermediate certificate and the root CA certificate in /etc/swanctl/x509ca/. strongSwan will automatically build the chain.

Protect the CA private key rigorously. Anyone with access to strongswanKey.pem can issue arbitrary certificates that your VPN peers will trust. Store it on encrypted offline media and keep it air-gapped from production systems.