Strong Swan works using the PKCS#11 driver. Basically follow these steps:

  1. Generate a key on Nitrokey via pkcs11-tool. In this example it’s a 4096 bit RSA key.

$ pkcs11-tool --module /usr/lib/x86_64-linux-gnu/pkcs11/opensc-pkcs11.so -l -k --key-type rsa:4096 --id 10 --label 'Staging Access'
  1. Generate a certificate signing request via openssl + pkcs11 module

$ openssl
OpenSSL> engine dynamic -pre SO_PATH:/usr/lib/x86_64-linux-gnu/engines-1.1/pkcs11.so -pre ID:pkcs11 -pre LIST_ADD:1 -pre LOAD -pre MODULE_PATH:/usr/lib/x86_64-linux-gnu/pkcs11/opensc-pkcs11.so
OpenSSL> req -engine pkcs11 -sha256 -new -key id_10 -keyform engine -out user@email.com-staging-cert.csr -subj '/C=GB/L=Cambridge/O=Organization/OU=Staging Access/CN=user@email.com/emailAddress=user@email.com'
  1. Sign the certificate with your certificate authority

  2. Convert the certificate to DER

$ openssl x509 -in user@email.com-staging-cert.csr -out user@email.com-staging-cert.der -outform DER
  1. Import the certificate into the Nitrokey via pkcs11-tool

$ pkcs11-tool --module /usr/lib/x86_64-linux-gnu/pkcs11/opensc-pkcs11.so -l -y cert -w user@email.com-staging-cert.der --id 10 --label 'Staging Access'
  1. Configure Strongswan to load opensc-pkcs11 module then to load the certificate on Nitrokey. Edit /etc/strongswan.d/charon/pkcs11.conf and add the following module:

modules {
   Nitrokey {
       path = /usr/lib/x86_64-linux-gnu/pkcs11/opensc-pkcs11.so
  1. Initiate the VPN connection via IPSec/Strongswan, then prompt for Nitrokey PIN

  2. VPN is now connected