HOWTO: Create Your Own Self-Signed Certificate with Subject Alternative Name Using OpenSSL in Ubuntu Bash for Window

My main development workstation is a Windows 10 machine, so we’ll approach this from that viewpoint.

Recently, Google Chrome started giving me a warning when I open a site that uses https and self-signed certificate on my local development machine due to some SSL certificate issues like the one below:

or one that is described in this forum post which I originally got.

I made my self-signed certificate using MAKECERT utility previously. Apparently, this tool does not support creating self-signed SSL certificate with Subject Alternative Name (SAN). If anyone knows different, please let me know.

So, after doing some searches, it seems that OpenSSL is the best solution for this.

If you are trying to use OpenSSL on Windows like me, you will probably be scratching your head on where to start. Build from the repository? Ouch. That’s what they called yak shaving. I just want to quickly create the self-signed certificate, not build a factory that can do it. Sure, there is binary installation available here, but after getting it installed and trying to figure out how to make it run nicely with PowerShell, I gave up.

Luckily, Windows 10 now has the ability to run Ubuntu Bash and after playing around with it, this seems to be the best way forward when using openssl.

Setup Ubuntu on Windows 10

To set it up, follow the instruction here.

Install OpenSSL

To install openssl run the following command from the bash shell:

sudo apt-get install openssl

Once installed, you are ready to create your own self-signed certificate.

Creating Self-Signed Certificate

I am using this OpenSSL Ubuntu article as the base, but there are some modifications along the way, so I’ll just explain the way I did it here. If you need further information, please visit that article.

The original article is using SHA1 but we really need to move to something else that is stronger like SHA256. If you are using SHA1 as suggested, you will be getting the “Your connection is not private” page in Chrome.

Creating Your Working Environment

We will use your Ubuntu user profile root directory (~/ which points to /home/jchandra in my case) to do this work. If you use anything else, you might need to customize the caconfig.cnf and localhost.cnf content below.

To create your environment, run the following in Bash:

cd ~/ && mkdir myCA && mkdir -p myCA/signedcerts && mkdir myCA/private && cd myCA

This will create the following directories under your user profile root folder:

  • ~/myCA contains CA certificate, certificates database, generated certificates, keys, and requests.
  • ~/myCA/signedcerts contains copies of each signed certificate.
  • ~/myCA/private contains the private key.

Create the Certificate Database

To create the database, enter the following in Bash:

echo '01' > serial && touch index.txt

Create Certificate Authority Configuration File

Create caconfig.cnf using vim or nano or whatever Linux text-editor of your choice.

To create it using vim, do the following:

vim caconfig.cnf

To create it using nano do the following:

nano caconfig.cnf

The content of caconfig.cnf should be like so:

# Default configuration to use when one is not provided on the command line.

[ ca ]
default_ca = local_ca
# Default location of directories and files needed 
# to generate certificates.

[ local_ca ]
# replace jchandra with your Ubuntu username
dir = /home/jchandra/myCA
certificate = $dir/cacert.pem
database = $dir/index.txt
new_certs_dir = $dir/signedcerts
private_key = $dir/private/cakey.pem
serial = $dir/serial
# Default expiration and encryption policies for certificates
default_crl_days = 365
default_days = 1825
# sha1 is no longer recommended, we will be using sha256
default_md = sha256
policy = local_ca_policy
x509_extensions = local_ca_extensions
# Copy extensions specified in the certificate request
copy_extensions = copy
# Default policy to use when generating server certificates.
# The following fields must be defined in the server certificate.
# It is the correct content.
[ local_ca_policy ]
commonName = supplied
stateOrProvinceName = supplied
countryName = supplied
emailAddress = supplied
organizationName = supplied
organizationalUnitName = supplied
# x509 extensions to use when generating server certificates
[ local_ca_extensions ]
basicConstraints = CA:false
# The default root certificate generation policy 
[ req ]
default_bits = 2048
default_keyfile = /home/jchandra/myCA/private/cakey.pem
# sha1 is no longer recommended, we will be using sha256
default_md = sha256
prompt = no
distinguished_name = root_ca_distinguished_name
x509_extensions = root_ca_extensions
# Root Certificate Authority distinguished name
[ root_ca_distinguished_name ]
commonName = InvoiceSmashDev Root Certificate Authority
stateOrProvinceName = NSW
countryName = AU
emailAddress =
organizationName = Coupa InvoiceSmash
organizationalUnitName = Development
[ root_ca_extensions ]
basicConstraints = CA:true

Caveats for caconfig.cnf:

  1. In [ local_ca ] section, make sure you replace jchandra with your Ubuntu username that you created when you setup Ubuntu on Windows 10. Mine is dir = /home/jchandra/myCA since that’s my Ubuntu username.
    NOTE: DO NOT USE ~/myCA. It does not work..
    Similarly, change the default_keyfile setting in [ req ] section to be the same.
  2. Leave the [ local_ca_policy ] section alone. commonName = supplied, etc. are correct and not to be overwritten.
  3. In [ root_ca_distinguished_name ] section, replace all values to your own settings.

Creating Your Test Certificate Authority

Certificate Authority (CA) Certificate is the certificate that you will use to sign other self-signed certificate. You need one before you can create other self-signed certificates.

You only need to create this once and reuse it when you need to sign a new self-signed certificate. So don’t go creating this every single time you need a new self-signed certificate. The only time you need to recreate this is to renew it since the one we’re going to create here will expire in 1825 days from the creation date.

Run the following command so openssl will pick the settings automatically:

export OPENSSL_CONF=~/myCA/caconfig.cnf

Generate the Certificate Authority (CA) certificate:

openssl req -x509 -newkey rsa:2048 -out cacert.pem -outform PEM -days 1825

Enter and retype the password you wish to use to import/export the certificate.

NOTE: Remember this password, you will need it throughout this walk-through.

Once you are done, you should have the following files:

  • ~/myCA/cacert.pem : CA public certificate
  • ~/myCA/private/cakey.pem : CA private key

In Windows, we will be using .crt file instead, so create one using the following command:

openssl x509 -in cacert.pem -out cacert.crt

Creating Your Self-Signed Certificate with Subject Alternative Name (SAN)

Now that you have your CA, you can create the actual self-signed SSL certificate.

But first, we need to create the configuration file for it. So again, use vim or nano, etc. to create the file. In this example, I will call mine localhost.cnf since that's the server that I am going to be using to test my development code. You can call it whatever you want. Just make sure you use the right filename in the export command later on.

Below is the content of ~/myCA/localhost.cnf:

# localhost.cnf

[ req ]
prompt = no
distinguished_name = server_distinguished_name
req_extensions = v3_req

[ server_distinguished_name ]
commonName = localhost
stateOrProvinceName = NSW
countryName = AU
emailAddress =
organizationName = Coupa InvoiceSmash
organizationalUnitName = Development

[ v3_req ]
basicConstraints = CA:FALSE
keyUsage = nonRepudiation, digitalSignature, keyEncipherment
subjectAltName = @alt_names

[ alt_names ]
DNS.0 = localhost
DNS.1 = invoicesmash.local

Caveats for localhost.conf

  1. Change the values in [ server_distinguished_name ] section to match your own settings.
  2. In [ alt_names ] section, change the value for DNS.0 and DNS.1 to whatever you need. In my case, I test my web application using https://localhost:44300, therefore the correct value for me is DNS.0 = localhost. I am not sure what to do with DNS.1 so, I just changed it to DNS.1 = invoicesmash.local. If so happen that I have a host entry in my hosts file that matches this (mapped to IP Address, it should still work.

Once you created the configuration file, you need to export using the following command:

export OPENSSL_CONF=~/myCA/localhost.cnf

Now generate the certificate and key:

openssl req -newkey rsa:2048 -keyout tempkey.pem -keyform PEM -out tempreq.pem -outform PEM

Again, provide the password that you previously entered and wait for the command to complete.

Next, run the following to create the unencrypted key file:

openssl rsa < tempkey.pem > server_key.pem

You’ll be prompted for the password again. Enter it and wait for command to complete.

Now switch back the export to caconfig.cnf so we can signed the new certificate request with the CA:

export OPENSSL_CONF=~/myCA/caconfig.cnf

And sign it:

openssl ca -in tempreq.pem -out server_crt.pem

Password time again. Enter it and wait for command to complete. Just type in Y whenever it asks you for [y/n].

Now you should have your self-signed certificate and key:

  • ~/myCA/server_crt.pem : self signed SSL certificate
  • ~/myCA/server_key.pem : self signed SSL certificate private key

Converting the .pem files to .pfx for Windows Usage

In Windows, we mostly use .pfx and .crt files. Therefore, we need to convert the .pem file to .pfx. We'll use cat to combine server_key.pem and server_crt.pem into a file called hold.pem. Then we will do the conversion using openssl pkcs12 command as shown below. You can use whatever text you want to describe your new .pfx file in the -name parameter.

cat server_key.pem server_crt.pem > hold.pem 
openssl pkcs12 -export -out localhost.pfx -in hold.pem -name "InvoiceSmash Dev Self-Signed SSL Certificate"

Again, provide the password that you previously entered and wait for the command to complete.

Now you should have the following files that we will use in the next section.

  • ~/myCA/localhost.pfx : self signed SSL certificate in PKCS#12 format
  • ~/myCA/cacert.crt : CA certificate used to signed the self-signed certificate

Copy the PFX and CA Certificates from Ubuntu Bash to Windows Side

It seems it is forbidden to touch the Linux Subsystem from Windows side, but you can touch Windows side from inside Ubuntu Bash, so that’s what we are going to do.

To copy the files from Ubuntu bash, you need to know where you want to copy the files to on Windows side. For example, if I want to copy the files to C:\certificates folder, I'd do something like this:

cp {localhost.pfx,cacert.crt} /mnt/c/certificates

Install the New CA and Self-Signed Certificates in Windows

To install the CA and self signed certificates, all you need to do is double-click the file from where you copied them into from Windows File Explorer.

Once clicked, just follow the Install Certificate steps and you should be good.

For the CA Certificate (cacert.crt), make sure you install it to Local Machine, Trusted Root Certification Authorities.

For the self-signed certificate (localhost.pfx), install it to Local Machine, enter the password when prompted, and store it in Personal.

Now you can configure your application to use the new certificate. In my situation, I just need to configure the Azure Cloud Service project to use that certificate as pointed by this document.

I do not know your workflow, so it might be different.

That’s it.


Originally published at

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