Create your own HTTPS certificate for your domains on Localhost

This focuses on generating the certificates for loading local virtual hosts hosted on your computer, for development only.

Do not use self-signed certificates in production !
For online certificates, use Let's Encrypt instead (tutorial).

Certificate authority (CA)

Generate RootCA.pem, RootCA.key & RootCA.crt:

openssl req -x509 -nodes -new -sha256 -days 1024 -newkey rsa:2048 -keyout RootCA.key -out RootCA.pem -subj "/C=US/CN=Example-Root-CA"
openssl x509 -outform pem -in RootCA.pem -out RootCA.crt

Note that Example-Root-CA is an example, you can customize the name.

Domain name certificate

Let's say you have two domains fake1.local and fake2.local that are hosted on your local machine
for development (using the hosts file to point them to 127.0.0.1).

First, create a file domains.ext that lists all your local domains:

authorityKeyIdentifier=keyid,issuer
basicConstraints=CA:FALSE
keyUsage = digitalSignature, nonRepudiation, keyEncipherment, dataEncipherment
subjectAltName = @alt_names
[alt_names]
DNS.1 = localhost
DNS.2 = fake1.local
DNS.3 = fake2.local

Generate localhost.key, localhost.csr, and localhost.crt:

openssl req -new -nodes -newkey rsa:2048 -keyout localhost.key -out localhost.csr -subj "/C=US/ST=YourState/L=YourCity/O=Example-Certificates/CN=localhost.local"
openssl x509 -req -sha256 -days 1024 -in localhost.csr -CA RootCA.pem -CAkey RootCA.key -CAcreateserial -extfile domains.ext -out localhost.crt

Note that the country / state / city / name in the first command can be customized.

You can now configure your webserver, for example with Apache:

```SSLEngine on
SSLCertificateFile "C:/example/localhost.crt"
SSLCertificateKeyFile "C:/example/localhost.key"```

Trust the local CA

At this point, the site would load with a warning about self-signed certificates.
In order to get a green lock, your new local CA has to be added to the trusted Root Certificate Authorities.

Windows 10: Chrome, IE11 & Edge

Windows 10 recognizes .crt files, so you can right-click on RootCA.crt > Install to open the import dialog.

Make sure to select "Trusted Root Certification Authorities" and confirm.

You should now get a green lock in Chrome, IE11 and Edge.

Windows 10: Firefox

There are two ways to get the CA trusted in Firefox.

The simplest is to make Firefox use the Windows trusted Root CAs by going to about:config,
and setting security.enterprise_roots.enabled to true.

The other way is to import the certificate by going
to about:preferences#privacy > Certificats > Import > RootCA.pem > Confirm for websites.