Certificates Quickstart

This section is not a full-blown tutorial on how to use the strongSwan pki tool. It just lists a few points that are relevant if you want to generate your own certificates and certificate revocation lists (CRLs) for use with strongSwan.

Generating a CA Certificate

The pki --gen command

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

generates an elliptic Edwards-Curve key with a cryptographic strength of 128 bits. Alternative key types are ecdsa, classic rsa and ed448.

Never store the private key of the Certification Authority (CA) on a host with constant direct access to the Internet (e.g. a VPN gateway) since a theft of this master signing key will completely compromise your PKI.

The corresponding public key is packed into a self-signed CA certificate with a lifetime of 10 years (3652 days) using the pki --self command

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

which can be listed with the pki --print command

pki --print --in strongswanCert.pem

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

If you prefer the CA private key and X.509 certificate to be in binary DER format then just omit the --outform pem option.

The /etc/swanctl/x509ca directory stores all required CA certificates either in binary DER or in Base64 PEM format. Irrespective of the file suffix the correct format will be determined by strongSwan automagically.

Generating an End Entity Certificate

Again we are using the command

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

to generate an Ed25519 private key for the host moon. Alternatively you could type

pki --gen --type rsa --size 3072 > moonKey.der

to generate a traditional 3072 bit RSA key and store it in binary DER format. Depending on the key type, the private host or user key is stored in the appropriate subdirectory (ecdsa, rsa, pkcs8 or private) of the /etc/swanctl directory.

As an alternative, a TPM 2.0 Trusted Platform Module available on every recent Intel platform could be used as a virtual smartcard to securely store an RSA or ECDSA private key. For details, refer to the TPM 2.0 tutorial.

In a next step the pki --req command

pki --req --type priv --in moonKey.pem \
          --dn "C=CH, O=strongswan, CN=moon.strongswan.org" \
          --san moon.strongswan.org --outform pem > moonReq.pem

creates a PKCS#10 certificate request that has to be signed by the CA. Through the [multiple] use of the --san parameter any number of desired subjectAlternativeNames can be added to the request. These can be of the form

--san sun.strongswan.org     # fully qualified host name
--san carol@strongswan.org   # RFC822 user email address
--san            # IPv4 address
--san fec0::1                # IPv6 address

It is strongly recommended to include the hostname of a VPN gateway as a subjectAltName in its certificate. Depending on your clients there may be additional requirements imposed on gateway certificates, for instance, the Windows certificate requirements or those for iOS and macOS clients.

Based on the certificate request the CA issues a signed end entity certificate with the pki --issue command

pki --issue --cacert strongswanCert.pem --cakey strongswanKey.pem \
            --type pkcs10 --in moonReq.pem --serial 01 --lifetime 1826 \
            --outform pem > moonCert.pem

If the --serial parameter with a hexadecimal argument is omitted then a random serial number is generated. Some third party VPN clients require that a VPN gateway certificate contains the TLS Server Authentication Extended Key Usage (EKU) flag which can be included with the following option

--flag serverAuth

If you want to use the dynamic CRL fetching feature described in one of the following sections then you may include one or several crlDistributionPoints in your end entity certificates using the --crl parameter

--crl  http://crl.strongswan.org/strongswan.crl
--crl "ldap://ldap.strongswan.org/cn=strongSwan Root CA, o=strongSwan,c=CH?certificateRevocationList"

The issued host certificate can be listed with

pki --print --in moonCert.pem

subject:  "C=CH, O=strongSwan, CN=moon.strongswan.org"
issuer:   "C=CH, O=strongSwan, CN=strongSwan Root CA"
validity:  not before May 19 10:28:19 2017, ok
           not after  May 19 10:28:19 2022, ok (expires in 1825 days)
serial:    01
altNames:  moon.strongswan.org
flags:     serverAuth
CRL URIs:  http://crl.strongswan.org/strongswan.crl
authkeyId: 2b:95:14:5b:c3:22:87:de:d1:42:91:88:63:b3:d5:c1:92:7a:0f:5d
subjkeyId: 60:9d:de:30:a6:ca:b9:8e:87:bb:33:23:61:19:18:b8:c4:7e:23:8f
pubkey:    ED25519 256 bits
keyid:     39:1b:b3:c2:34:72:1a:01:08:40:ce:97:75:b8:be:ce:24:30:26:29
subjkey:   60:9d:de:30:a6:ca:b9:8e:87:bb:33:23:61:19:18:b8:c4:7e:23:8f

End entity certificates are usually stored in the /etc/swanctl/x509 directory.

Create a distinct private key and a matching X.509 end entity certificate signed by your CA for each peer, i.e. for all VPN clients and VPN gateways in your network and store the peer’s private key and X.509 certificate in the /etc/swanctl directory of the corresponding host.

PKCS#12 Container

Usually a Windows, Android, macOS or iOS based VPN client needs its private key, its host or user certificate and the CA certificate. The most convenient way to load this information is to put everything into a PKCS#12 container:

openssl pkcs12 -export -inkey carolKey.pem \
               -in carolCert.pem -name "carol" \
               -certfile strongswanCert.pem -caname "strongSwan Root CA" \
               -out carolCert.p12

The strongSwan pki tool currently is not able to create PKCS#12 containers so that openssl must be used.

Apple and Android clients can only process PKCS#12 containers that are protected with 3DES and the legacy KDF. So when using openssl pkcs12 with OpenSSL 3.0 and newer, the -legacy option is required.

Omitting the -certfile argument from the command above is indicated in the following situation:

  • On Android 4.4 and later, you may get a warning ("Network may be monitored by an unknown third party") if the PKCS#12 file contains a CA certificate. To avoid that, create the PKCS#12 container without a CA certificate. Then import the PKCS#12 file into the Android trust store as usual and the CA certificate file directly into the strongSwan app. Or install both files via the configuration profile.

It’s also possible to add additional CA certificates with multiple -certfile arguments (e.g. for multi-level X.509 trust chains).

The openssl pkcs12 command requires the certificates and the private key to be in PEM format. DER format is not accepted. Either use --outform pem with the pki commands above to generate all files in PEM format or convert them later on with the command

openssl x509 -inform der -outform pem -in cert.der -out cert.pem

for certificates and

openssl ec -inform der -outform pem -in key.der -out key.pem

for private ECDSA keys or

openssl rsa -inform der -outform pem -in key.der -out key.pem

for private RSA keys.

Generating a CRL

An empty CRL that is signed by the CA can be generated with the pki --signcrl command

pki --signcrl --cacert strongswanCert.pem --cakey strongswanKey.pem \
              --lifetime 30 > strongswan.crl

If you omit the --lifetime option then the default value of 15 days is used. CRLs can either be uploaded to a HTTP or LDAP server or put in binary DER or Base64 PEM format into the /etc/swanctl/x509crl directory from where they are loaded into the charon daemon with the swanctl --load-creds command.

Revoking a Certificate

A specific end entity certificate is revoked with the command

pki --signcrl --cacert strongswanCert.pem --cakey strongswanKey.pem \
              --lifetime 30 --lastcrl strongswan.crl \
              --reason key-compromise --cert moonCert.pem > new.crl

Instead of the certificate file (in our example moonCert.pem), the serial number of the certificate to be revoked can be indicated using the --serial parameter. The pki --signcrl --help command documents all possible revocation reasons but the --reason parameter can also be omitted. The content of the new CRL file can be listed with the command

pki --print --type crl --in new.crl

issuer:   "C=CH, O=strongSwan, CN=strongSwan Root CA"
update:    this on May 19 11:13:01 2017, ok
           next on Jun 18 11:13:01 2017, ok (expires in 29 days)
serial:    02
authKeyId: 2b:95:14:5b:c3:22:87:de:d1:42:91:88:63:b3:d5:c1:92:7a:0f:5d
1 revoked certificate:
  01: May 19 11:13:01 2017, key compromise

Local Caching of CRLs

The strongswan.conf option

charon {
  cache_crls = yes

activates the local caching of CRLs that were dynamically fetched from an HTTP or LDAP server. Cached copies are stored in /etc/swanctl/x509crl using a unique filename formed from the issuer’s subjectKeyIdentifier and the .crl suffix.

With the cached copy the CRL is immediately available after startup. When the local copy has become stale, an updated CRL is automatically fetched from one of the defined CRL distribution points during the next IKEv2 authentication.