---
title: "Creating and Using a CA Certificate"
description: ""
date: "2023-01-27"
tags:
- SSL
draft: false
---

Even though there are plenty of free SSL certificate options, you may need to manage your own SSL certs for various reasons.

Below is how to create a CA certificate and issue certs signed with it, as well as some validation steps to make sure you did everything right.

All examples here use the [openssl](https://www.openssl.org/docs/man1.1.1/man1/) cli tool.


## Creating the Certificate Authority's Certificate and Keys

1.  Generate a private key for the CA:
```sh
$ openssl genrsa 4096 > ca-key.pem
```

2.  Generate the X509 certificate for the CA:
```sh
    $ openssl req -new -x509 -nodes -days 3650 -sha256 \
	   -subj "/C=US/ST=CO/O=My Org, Inc./OU=Engineering/CN=ca.example.internal" \
       -key ca-key.pem \
       -out ca-cert.pem
```

## Creating the Server's Certificate and Keys

1.  Generate the private key and certificate request:
```sh
$ openssl req -newkey rsa:4096 -nodes -days 3650 -sha256 \ # -newkey if you need a new private key
	-subj "/CN=mysite.example.internal" \ # exclude this and you will get interactive prompts
	-keyout server-key.pem \
	-out server-req.pem

# if you already have a private key
$ openssl req -new -nodes -days 3650 -sha256 \
	-subj "/CN=mysite.example.internal" \
	-key server-key.pem \
	-out server-req.pem

# if you want to add SANs
$ openssl req -new -nodes -days 3650 -sha256 \
	-subj "/CN=mydomain.com" \
	-reqexts SAN \
	-config <(cat /etc/ssl/openssl.cnf <(printf "\n[SAN]\nsubjectAltName=DNS:test.example.internal,DNS:www.example.internal")) \ # to add SANs
	-key server-key.pem \
	-out server-req.pem
```


2.  Generate the X509 certificate for the server:

> `-days` expiration can be no longer than 398 days (13 months) (thanks [Apple](https://support.apple.com/en-us/HT211025))
> https://www.globalsign.com/en/blog/maximum-ssltls-certificate-validity-now-one-year
> `-set_serial` is like an ID number for each cert. It must be a positive integer and unique to each cert signed by the CA.
> `date +%s` prints the current number of seconds since the Epoch (00:00:00 UTC, January 1, 1970). See [RFC 5280](https://datatracker.ietf.org/doc/html/rfc5280#section-4.1.2.2 "https://datatracker.ietf.org/doc/html/rfc5280#section-4.1.2.2") for details

```sh
$ openssl x509 -req -days 398 -set_serial $(date +%s) -sha256 \
   -in server-req.pem \
   -out server-cert.pem \
   -CA ca-cert.pem \
   -CAkey ca-key.pem

# if you want to add SANs
$ openssl x509 -req -days 398 -set_serial $(date +%s) -sha256 \
	-extfile <(printf "subjectAltName=DNS:my.domain.com") \
	-in server-req.pem \
	-out server-cert.pem \
	-CA ca-cert.pem \
	-CAkey ca-key.pem
```

## Verifying the Certificates

1.  Verify the server certificate:

```sh
    $ openssl verify -CAfile ca-cert.pem \
       ca-cert.pem \
       server-cert.pem
```

2.  Verify the client certificate:
```sh
    $ openssl verify -CAfile ca-cert.pem \
       ca-cert.pem \
       client-cert.pem
```

## Verify the certificate's content

```sh
openssl x509 -in mydomain.com.crt -text -noout
```