How to generate a Certificate Request (CSR)

This tutorial describes how to generate your own certificate request for use in an online SSL certificate request.

Mbed TLS includes the core and applications for generating keys and certificate requests without relying on other libraries and applications, offering users a command-line alternative to OpenSSL for generating their keys and certificate requests.

This article assumes you have compiled and installed the Mbed TLS library and example programs on your system.

Certificate Request for use with SSL vendors

Whenever you request a certificate from one of the SSL vendors, you are asked to enter a CSR. CSR is short for Certificate Signing Request and is often in the PEM format.

To generate a certificate request, you need a private-public key pair. The public key is put in the certificate request in addition to some identifying information (such as website domain, address, and country). By submitting your request, you ask the SSL vendor to sign that request with their CA key and generate a full certificate from it.

The CA will determine the validity of the certificate they generate based on how much you paid them.

Generating a RSA key file

The first step for generating a certificate request, is to generate a private-public key pair for the certificate.

For generating key files, Mbed TLS includes the gen_key application in programs/pkey.

This key generation application accepts the following arguments:

 usage: gen_key param=<>...

 acceptable parameters:
    type=rsa              default: rsa
    rsa_keysize=%d        default: 4096
    filename=%s           default: keyfile.key
    format=pem|der        default: pem

The following command generates a 2048 bit RSA key file, as explained here:

programs/pkey/gen_key type=rsa rsa_keysize=2048 filename=example.com.key

Generating certificate request

For generating and writing certificate files, Mbed TLS includes the cert_req application in programs/x509.

Before generating the certificate request you need to determine the different values that need to go in it.

Key to use in the certificate

First and foremost a certificate binds a public-private key pair to an identity. To indicate which key to use in the certificate request you use the filename argument, like so filename=example.com.key.

Subject name

Each certificate request needs a subject name (the identity that is being signed). Each CA vendor has different requirements for which items are required in a certificate request.

If you want to request a certificate for the example.com domain name from the organization Example Ltd in the country UK, you should use subject_name=CN=example.com,O=Example\ Ltd,C=UK on the command-line.

Note: If you want to use a space in one of the names you have to either nullify it (issuer_name=CN=my\ server) or contain the entire argument in quotation marks ("issuer_name=CN=my server").

Note: Commas inside names need to be nullified with a backslash as well. You need to protect the backslash from your shell, for example, issuer_name=CN=my\\\,server or issuer_name='CN=my\,server'.

The available items you can put in a subject_name (that you support) are:

C = Country
CN = Common Name
L = Locality
O = Organization
OU = Organizational Unit
R = e-mail address
ST = State
serialNumber
postalAddress
postalCode

Command to generate a certificate request

So the full command for generating a certificate request for example.com.key with the name CN=example.com,O=Example Ltd,C=UK would be:

programs/x509/cert_req filename=example.com.key                          \
                       subject_name=CN=example.com,O=Example\ Ltd,C=UK \
                       output_file=example.com.req

The file example.com.req now contains your PEM-encoded certificate request. It will look something like:

You can copy and paste the PEM content into the website for the certificate vendor.