This tutorial will guide you in setting up authentication using TLS/SSL Client Certificates. It is a simple one, as it does not delve into details about integration with server-side apps. Instead, it simply gives you instructions on how to set up Client Certificates as a means to prevent unwanted parties from accessing your website.
For example, one scenario where it may come in useful is limiting access to sensitive things on the server, like phpMyAdmin. phpMyAdmin, for example, handles sensitive data (such as database authentication) and does so in plain HTTP, which may pose several security risks. Even if the data were encrypted, someone with access to the application might find vulnerabilities in it and exploit the relatively high privileges it has to compromise the server. The solution to this is to also limit who has access to the application at all. A possible solution, which I’ve used, is to limit access only to the local machine and use SSH or a VPN tunnel to access phpMyAdmin. A better solution would be to use TLS/SSL Client Certificates. They operate on the connection level and provide both encryption and authentication. They are easier to set up than VPN tunnels and easier to use.
Note that limiting access based on a TLS/SSL Client Certificate can only be done on the subdomain level, because it happens as part of the connection, before any specific HTTP request can be made.
Most of the tutorial is not HTTP server-specific; however, the server configuration part relates to Nginx. As other servers (such as Lighttpd) use a very similar configuration for Client Certificates, adapting the instructions should be straightforward.
Creating a CA
The two commands below will create a CA private key and a corresponding self-signed certificate for you to sign the TLS client certificates with.
openssl genpkey -algorithm RSA -pkeyopt rsa_keygen_bits:2048 -aes-128-cbc -out ca.key
openssl req -new -x509 -days 365 -sha256 -key ca.key -out ca.pem
The first command will ask you for a passphrase for the key. It is used to protect access to the private key. You can decide not to use one by dropping the -aes-128-cbc option from the command.
The second command will ask you to provide some details to be included in the certificate. Those details will be sent to the browser by the web server to let it know which client certificate to send back when authenticating.
Server Configuration
Upload the ca.pem that was just generated to your server. You should not upload the private key (ca.key).
The following instructions are for Nginx.
ssl_client_certificate /path/to/ca.pem;
ssl_verify_client on; # we require client certificates to access
Assuming you already enabled TLS/SSL for the specific subdomain, your configuration should look something like this:
server {
server_name subdomain.example.com;
# SSL configuration
#
listen 443 ssl;
listen [::]:443 ssl;
ssl_certificate /etc/nginx/example.pem;
ssl_certificate_key /etc/nginx/example.key;
ssl_client_certificate /etc/nginx/ca.pem;
ssl_verify_client on;
After reloading the server, check that everything is configured correctly by trying to access your site via HTTPS. It should report “400 Bad Request” and say that “No required SSL certificate was sent”.
Creating a Client Certificate
The following commands will create the private key used for the client certificate (client.key) and a corresponding Certificate Signing Request (client.csr), which the owner of the CA certificate can sign (which, in the case of this tutorial, will be you).
openssl genpkey -algorithm RSA -pkeyopt rsa_keygen_bits:2048 -out client.key
openssl req -new -key client.key -sha256 -out client.csr
You will be asked again to provide some details, this time about you. Those details will be available to the server once your browser sends it the client certificate. You can safely leave the “challenge password” empty1.
You can add the flag -aes-128-cbc to the first command if you want the private key for the client certificate to be encrypted. If you opt for it, you will be prompted for a passphrase just like before.
Signing a Client Certificate
The next step is to sign the certificate signing request from the last step. It is a good practice to review it and make sure all the details are as expected, so you do not sign anything you would not intend to.
openssl req -in client.csr -text -verify -noout | less
If everything looks just fine, you can sign it with the following command.
openssl x509 -req -days 365 -in client.csr -CA ca.pem -CAkey ca.key
-set_serial 0x`openssl rand 16 -hex` -sha256 -out client.pem
You will be prompted for the passphrase for ca.key if you chose one in the first step.
Installing Client Key
Now comes the final part, where we take the signed client certificate, client.pem, and combine it with the private key so it can be installed in our browser.
openssl pkcs12 -export -in client.pem -inkey client.key -name "Sub-domain certificate for some name" -out client.p12
Adjust the -name parameter to your liking. It will be used to identify the certificate in various places, such as the browser. If your private key was encrypted, you will be prompted to enter a passphrase for it. Encryption for certificates in p12 format is mandatory, so you will be prompted to enter a password for the generated file as well. It is OK to reuse the same password here, as those files are practically equivalent. Once imported to your browser, you will not need the password for normal usage, until you would like to import it to another browser.
GlobalSign provides instructions on how to actually install the p12 client certificate for browsers in Linux and Windows.
References
