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OpenSSL Creating a Certificate Authority (CA)

In this post we’ll look at how to create our own Certificate Authority (CA) using OpenSSL.

  1. Generating a Private Key
  2. Generating a Certificate
  3. Using the CA

Generating a Private Key

The first step is to create a Private key for our certificate. We can choose either an RSA key or an Elliptic Curve key.

List OpenSSL Supported Elliptic Curves

The following command will display a list of supported OpenSSL elliptic curves. We’ll need to choose one of these curves when we are generating our Private key.

openssl ecparam -list_curves

Elliptic Curve Private Key

openssl ecparam -genkey -name secp384r1 -out ca.key
  • secp384r1 is the name of the curve we are using.
  • ca.key is the name of the output file where we want to store the Private key.

RSA Private Key

The following command will generate a 2048-bit RSA key:

openssl genrsa -out ca.key

2048-bits is the standard size. A custom key size can be specified by giving an extra argument. The following command will generate a 4096-bit RSA key:

openssl genrsa -out ca.key 4096

Generating a Certificate

CA Configuration File

We need to specify some parameters for our CA before we can create it.

Create a file called ca.conf and add the following:

basicConstraints = CA:TRUE
keyUsage = cRLSign, keyCertSign
distinguished_name = req_distinguished_name
prompt = no
C   = AU
ST  = Victoria
L   = Melbourne
CN  = My Personal Root CA
  • Customize the C, ST, L and CN to your desired values.

The CN (Common Name) field will be displayed when your computer shows the certificate path:

Creating the Certificate

We are now ready to create the certificate using the private key and config:

openssl req -x509 -new -sha512 -nodes -key ca.key -days 7307 -out ca.crt -config ca.conf
  • -x509 output a Certificate instead of a Certificate Signing Request (CSR).
  • -sha512 specifies the hash function that will be used to sign the certificate. Other values may be used here such as sha256 and sha384.
  • -nodes means the certificate should be unencrypted.
  • -key ca.key specifies the Private Key we are signing this with.
  • -days 7307 specifies the number of days the certificate is valid for. 7307 days ≈ 20 years.
  • -out ca.crt specifies the output file name.
  • -config ca.conf specifies the configuration file we wish to use.

Using the CA

Now that we have our own Certificate Authority, we can use it sign certificates for web-servers so they can use TLS. I have a guide to do this here.

If we want to use the CA and not have certificate warnings, then it will have to be installed as a Trusted Root CA on your computer/browser.

This post is licensed under CC BY 4.0 by the author.