How to generate TLS certificates (Windows)?

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Introduction[edit | edit source]

Transport Layer Security (TLS) is a cryptographic protocol that provides communications security over a computer network and is also the successor to SSL.

The TLS protocol aims primarily to provide privacy and data integrity between two communicating computer applications. When secured by TLS, connections between a client and a server have one or more of the following properties:

  • The connection is private (or secure) because symmetric cryptography is used to encrypt the data transmitted. The keys for this symmetric encryption are generated uniquely for each connection and are based on a shared secret negotiated at the start of the session. The server and client negotiate the details of which encryption algorithm and cryptographic keys to use before the first byte of data is transmitted. The negotiation of a shared secret is both secure (the negotiated secret is unavailable to eavesdroppers and cannot be obtained, even by an attacker who places themselves in the middle of the connection) and reliable (no attacker can modify the communications during the negotiation without being detected).
  • The identity of the communicating parties can be authenticated using public-key cryptography. This authentication can be made optional, but is generally required for at least one of the parties (typically the server).
  • The connection ensures integrity because each message transmitted includes a message integrity check using a message authentication code to prevent undetected loss or alteration of the data during transmission.

This article provides a guide on how to generate your own TLS certificates and keys for OpenVPN connection that uses TLS authentication. This guide is aimed at Windows users.

Step 1: installing OpenVPN software[edit | edit source]

  • The first thing that we'll need to do is install the necessary OpenVPN software. You can download the OpenVPN installer file from here.
    Simply run the downloaded file and follow the instructions of the installation guide.
  • Important note: at one point before the installation begins, you will be prompted to select which components should be included in the installation. Make sure to select EasyRSA as it will be needed later on to generate keys and certificates. You can leave the rest as defaults:
      Openvpn windows server setup step 1 openvpn installation part 1 v2.png

Step 2: preparing EasyRSA[edit | edit source]

  • Now we can start preparing to generate certificates and keys. For this we'll be using the EasyRSA application that was installed along with OpenVPN.
    EasyRSA commands have to be executed via the Windows Command Prompt. It can be opened by typing cmd in the Windows search bar (Windows button + S). When you launch it, make sure you run it as administrator:
      How to generate tls certificates windows part 1 v3.png
  • Change the current directory to the EasyRSA folder. To do so, execute this command:
      cd "C:\Program Files\OpenVPN\easy-rsa"
  • Initialize the OpenVPN configuration with the following command:
  • Open the vars.bat file with the Notepad text editor:
      notepad vars.bat
  • This is the template file for generating certificates, i.e., the information stored here will be offered as default values during certificate generation. Locate and edit the following lines in accordance with your needs:
      set KEY_COUNTRY=US
      set KEY_CITY=SanFrancisco
      set KEY_ORG=OpenVPN
      set KEY_EMAIL=mail@host.domain
  • You can also set the key size for the Diffie Hellman parameters:
      set DH_KEY_SIZE=2048
  • Once you're done, save the file and close the editor
  • Run the following commands:

Step 3: generating certificates and keys[edit | edit source]

  • Now we can start generating the certificates and keys. Begin with the certificate authority (CA) - the root certificate file that will be used to sign other certificates and keys:

    NOTE: you can press the "Enter" key when prompted to enter the values set in the vars.bat file earlier. Doing this will set the values to the default specified in vars.bat. However, you should type in a meaningful Common Name.

  • Next, build the server certificate and key:
      build-key-server server

    NOTE: once again, don't forget to specify a different Common Name (use the name "server" for easier management purposes). When prompted the sign and commit the certificate, type y and press "Enter".

  • Next, build certificates and keys for the clients:
      build-key Client1

    TIP: use the same Common Name as the certificate name (Client1 in this example). This will help you differentiate between clients easier. Pick meaningful names like "toms_PC", "company_maintenance", etc. Repeat this step as many times as you need, depending on the client quantity.

  • Lastly, generate Diffie Hellman parameters: