How to create your own root CA for Solid

This text explains how to create and use a certificate authority (CA) with self-signed root certificate for use on a local Solid server with domain names that end in solid.localhost. Certificates are created with OpenSSL, which is free software. The following is known to work under GNU/Linux and with Docker.

Note! The following installs a new root certificate, which should only be done on test systems. With such settings, one can create valid certificates for any domain.

As explained in Solid’s readme a multi-user installation requires the use of wildcard certificates (as a new hostname is created for each user). A wildcard certificate is a certificate for a domain name such as * and is acceptable for all servers under the domain (e.g.,,

Initially, I tried to use domain names such as alice.localhost and bob.localhost with a wildcard certificate for *.localhost. This did not work as localhost is a top-level domain, for which wildcard certificates are not accepted by browsers (without clear error message). Thus, I decided to go for solid.localhost as domain name for use with Solid on my machine.

1 Create CA with OpenSSL

The openssl program can use default values from a configuration file to reduce typing efforts. Mine are available in this directory, which also contains the files (keys, certificates, …) created during the following steps. Copy the files openssl.cnf and openssl-wildcard.cnf to some directory in which you want to create your CA and its certificates.

  1. Files for the CA will be located in the new sub-directory CA.

    mkdir CA
  2. Create certificate authority.
    • Create directory structure and necessary files ("unique_subject = no" in index.attr allows to create multiple certificates per name; the initial serial number is arbitrary).

      cd CA; mkdir certs crl newcerts private; touch index.txt; echo "unique_subject = no" > index.attr; echo 314159 > serial
    • Invoke openssl to create self-signed root certificate. (When asked, use and write down a short pass phrase; remember that all this is only for testing. Hit return to accept default values from configuration file. Note the output options passed to openssl.)

      openssl req -config ../openssl.cnf -new -x509 -days 3650 -newkey rsa:4096 -sha256 -extensions v3_ca -out certs/my_cacert.crt -keyout private/my_key.crt
    • A real CA would publish its self-signed certificate and have it embedded in operating systems and browsers by default. Do this manually:
      • Make new root certificate known to operating system (afterwards, you can find the new certificate under /etc/ssl/certs/ and in /etc/ssl/certs/ca-certificates.crt)

        sudo cp certs/my_cacert.crt /usr/local/share/ca-certificates/
        sudo update-ca-certificates
        • Note that some software uses that certificate store, while other does not. Browsers usually come with their own certificate store (see next step). For node.js you may need to point to the certificate in an environment variable, as is done in for a Solid server with Docker.
      • Import new root certificate into browser (e.g., with Firefox: Preferences → Privacy & Security → Certificates → View Certificates → Authorities → Import). You may want to use a separate browser profile for such experiments.
  3. Create server key pair and certificate signing request (CSR). Such commands would really be executed by the organization owning the server, not by the CA; the resulting request (containing the public key) is then turned into a signed certificate by the CA in the next step; the CA must never learn the private key.

    Here, the config file openssl-wildcard.cnf is used, which contains a section subjectAltName = @alt_names with hard-coded names to generate a wildcard certificate for *.solid.localhost. When asked, the challenge password can be empty.

    cd ..
    openssl genrsa -out solid_key.crt 4096
    openssl req -config openssl-wildcard.cnf -sha256 -new -key solid_key.crt -out solid_csr.crt
  4. Sign CSR, again with wildcard information. Our new CA does this. Use the pass phrase written down in step 2.

    openssl ca -config openssl-wildcard.cnf -extensions v3_req -notext -md sha256 -in solid_csr.crt -out solid.crt
  5. Create certificate chain. The organization owning the server would do this. The server needs to be told where to find it (and the corresponding private key).

    cat solid.crt CA/certs/my_cacert.crt > solid.chain.crt

    For Solid with Docker below, copy the CA certificate to the current directory:

    cp CA/certs/my_cacert.crt .

2 Use CA with Solid (manual approach)

Use key and certificate when initializing your Solid server.

Make sure that host names under solid.localhost are resolved to your local machine. The IPv4 address of localhost is Add additional lines like this to /etc/hosts: *.localhost *.solid.localhost

Some operating systems seem to ignore such wildcard entries in the hosts file, others accept them. Try ping solid.localhost with the above settings. If that works, everything is fine. If not, either add entries with full names such as the following ones, or install dnsmasq (as in the Docker image mentioned in the next section). solid.localhost alice.solid.localhost bob.solid.localhost <more names as necessary>

3 Use CA with Solid in Docker

The above setup is bundled in this Docker image for the node Solid server.

Run as follows (maybe replace $PWD with a directory of your choice, where you want to collect Solid data):

docker run -it --cap-add=NET_ADMIN --dns= -p 8443:8443 --name my-solid -v $PWD:/opt/solid

Finally, explore your own POD: https://solid.localhost:8443

4 Clean up

If you imported the above CA certificate into your browser, make sure to delete it at the end of your experiments. View certificates (as for import above), scroll down to “University of Muenster”, delete “Solid operator”.

License Information

This document is part of a larger course. Source code and source files are available on GitLab under free licenses.

Except where otherwise noted, the work “How to create your own root CA for Solid”, © 2019, 2021 Jens Lechtenbörger, is published under the Creative Commons license CC BY-SA 4.0.

Created: 2021-12-03 Fri 12:11