When deploying an HTTPS certificate for your website, the first step is usually a file called a CSR. This file may look like a complex text file, but it's the crucial starting point for obtaining a trusted digital certificate. Understanding what a CSR is and how it works helps you complete the entire certificate configuration process more securely and smoothly.

A CSR is an abbreviation for Certificate Signing Request. Essentially, it's a digital file containing specific information. Its core purpose is to prove to a Certificate Authority (CA) that you have control over a domain name and request that the CA issue you an SSL/TLS certificate. You can think of a CSR as a formal, structured application form containing your identity information and a public "lock," while the CA's task is to verify the information and create an authoritative "instruction manual" for this lock.

Generating a CSR involves creating a pair of asymmetric keys. When you use a tool to generate a CSR, the system first creates a key pair locally: a private key and a public key. The private key must be kept strictly confidential and stored permanently on your server; the public key is then embedded in the CSR file and sent to the CA. Once the CA approves the request, the certificate they issue will include this public key. In this way, any visitor's browser can encrypt data using the public key in the certificate, while only your server, holding the corresponding private key, can decrypt it. The core role of the CSR in this process is to securely transmit your public key and identity information to the CA, while ensuring that the private key never leaves your machine.

A typical CSR file contains several key parts. The most important is the identification information you provide, known as the distinguishable name. This includes the country, province, city, organization name, department, and most importantly, the common name. For website certificates, the common name must be the fully qualified domain name you wish to protect. In addition, the CSR contains the aforementioned public key, as well as a digital signature of the entire content. This signature is crucial; it is calculated using the private key you just generated against the CSR content. When the CA receives the CSR, they can verify this signature using the public key within the CSR. Only if the signature verification is successful can the CA be certain that the person submitting the request truly possesses the private key matching the public key in the CSR, thus preventing unauthorized use of your domain to apply for certificates.

Generating a CSR is typically done using the OpenSSL command-line tool. Below is a basic generation command.

The command `openssl req -new -newkey rsa:2048 -nodes -keyout example.com.key -out example.com.csr`

After executing this command, the system will enter an interactive prompting phase, requiring you to enter information such as country, region, organization name, and common name. This process can also be automated by providing all parameters through a pre-configured text file, which is very useful for batch requests or automated deployments. Below is an example configuration file.

# Example.cnf Configuration File Contents

[ req ]

default_bits = 2048

distinguished_name = req_distinguished_name

req_extensions = req_ext

[ req_distinguished_name ]

countryName = CN

stateOrProvinceName = Beijing

localityName = Beijing

organizationName = My Company Ltd.

commonName = www.example.com

[ req_ext ]

subjectAltName = @alt_names

[ alt_names ]

DNS.1 = www.example.com

DNS.2 = example.com

The command to generate a CSR using the configuration file is as follows.

The command `openssl req -new -newkey rsa:2048 -nodes -keyout example.com.key -config example.cnf -out example.com.csr` generates a CSR file.

This file is Base64 encoded text, starting with `-----BEGIN CERTIFICATE REQUEST-----` and ending with `-----END CERTIFICATE REQUEST-----`. This is all you need to submit to the CA. Submission is usually done through a text box on the CA's website. You copy the entire CSR text block into it, and the CA's system decodes and verifies it.

Backstage, the CA receives the CSR and initiates the verification process. They first decode the CSR, extracting the domain name and organization information you provided. Then, they must verify that you do indeed have control of the domain. Common verification methods include sending a verification email to your domain administrator's email address or requiring you to place a specific verification file in the root directory of your website. For Enterprise Extended Validation Certificates (ECCs), the CA may also conduct manual verification by phone or check official business registration information. Only after ownership verification is successful will the CA use its own root certificate private key to sign the public key and identity information in your CSR, generating the final SSL certificate file.

After receiving the certificate file issued by the CA, the applicant needs to configure it on the web server along with the initially generated private key file. A typical Nginx configuration example is as follows:

nginx

server {

listen 443 ssl;

server_name www.example.com;

ssl_certificate /path/to/your_domain_certificate.crt;

ssl_certificate_key /path/to/your_private_key.key;

# Other SSL configuration...

}

A key security principle here is that the private key file must be properly stored along with the CSR file. While the CSR can be regenerated, if the private key is lost, the matching certificate becomes invalid. If the private key is leaked, the certificate must be revoked immediately, and the key pair and CSR must be regenerated.

Sometimes we need to examine the contents of a generated CSR to confirm the information is correct or to extract the public key. The following OpenSSL command can be used to decode and view detailed information about a CSR:

openssl req -in example.com.csr -noout -text

The output of this command will display all fields in the CSR, including the domain name you submitted, organization information, the algorithm and length of the public key, and any possible extended fields.

In the entire HTTPS deployment chain, the CSR plays a crucial role. It is the bridge between locally generated key pairs and publicly trusted certificates. Through a standardized, structured format, it protects the applicant's most sensitive private key from being transmitted while providing the certificate authority with all the necessary materials for verification and signing. Understanding the CSR allows you to better grasp the complete logic from key generation to certificate installation, making you more confident when dealing with certificate renewals, re-applications, or configuring certificates for multiple domains.