How can I install a free SSL certificate using let’s encrypt?

You can get an SSL certificate for free via Let’s Encrypt. Here is how you do it using certbot:

curl https://dl.eff.org/certbot-auto > /usr/local/bin/certbot-auto
chmod a+x /usr/local/bin/certbot-auto

certbot-auto certonly --webroot --webroot-path /opt/openproject/public -d openprojecct.mydomain.com

This requires your OpenProject server to be available from the Internet on port 443 or 80. If this works the certificate (cert.pem) and private key (privkey.pem) will be created under /etc/letsencrypt/live/openproject.mydomain.com/. Configure these for OpenProject to use by running openproject reconfigure and choosing yes when the wizard asks for SSL.

Now this Let’s Encryt certificate is only valid for 90 days. To renew it automatically all you have to do is to add the following entry to your crontab (run crontab -e):

0 1 * * * certbot-auto renew --quiet --post-hook "service apache2 restart"

This will execute certbot renew every day at 1am. The command checks if the certificate is expired and renews it if that is the case. The web server is restarted in a post hook in order for it to pick up the new certificate.