SSL certificate deployment is a fundamental part of enabling HTTPS encrypted connections. This process involves applying for, verifying, installing, and maintaining an SSL certificate. Understanding key considerations can help avoid common mistakes and ensure successful security configuration on the first try.

Before starting the application, clear planning is crucial. First, you need to determine the domain name you will use for the certificate application. Is it a single domain (e.g., `www.example.com`), a wildcard domain (`*.example.com`), or a multi-domain certificate containing multiple domains? Wildcard certificates protect all sibling subdomains under the main domain, making management easier, but they are usually more expensive. Multi-domain certificates are suitable for scenarios with multiple unrelated domains. Most commercial certificate authorities also offer free single-domain DV certificates, suitable for personal websites or testing environments.

Second, choose a reliable certificate authority. Well-known CAs have their root certificates widely pre-installed on major devices and browsers worldwide, ensuring compatibility. The application process begins with generating a private key and a certificate signing request on your server. The private key is fundamental to the security system and must be generated entirely on your own server and kept strictly confidential; it must never be sent to the CA or any third party. Losing or disclosing your private key renders your certificate completely insecure.

The command to generate a CSR is typically as follows (using OpenSSL as an example):

openssl req -new -newkey rsa:2048 -nodes -keyout your_domain.key -out your_domain.csr

When executing this command, you will be prompted to enter relevant information, including your country, province/city, organization name, and most importantly, your common name. For single-domain certificates, the "common name" must be the complete domain name you want to protect (e.g., `www.example.com` or `example.com`). Please ensure that this information is accurate; even a single incorrect character will invalidate the certificate.

The generated CSR file is a text file that begins with "—–BEGIN CERTIFICATE REQUEST—–". You will need to submit the contents of this CSR file when purchasing or applying for a certificate from a CA. Also, please back up and save the generated private key file (`.key`).

After submitting the CSR, the CA must verify your ownership or control of the domain name. This is a crucial security step to prevent others from obtaining malicious certificates for your domain. There are three main methods for DV certificate verification:

1. DNS Resolution Verification: The CA provides a specific TXT record value, which you need to add to your domain's DNS resolution settings. This is the most common and usually the most convenient method.

2. HTTP File Verification: The CA provides a file with a specific name and content, which you need to place in a specified path in your website's root directory (e.g., `/.well-known/pki-validation/`) so that the CA can access it via HTTP.

3. Email Verification: A verification email is sent to the domain's specific administrative email address (e.g., `admin@example.com`).

Key Considerations: DNS verification records may take several minutes to several hours to become effective; HTTP file verification requires that your port 80 or 443 be publicly accessible to the CA server, and that temporary directories are not blocked by redirects or access control rules. Ensure that your server is configured to allow this access during verification.

After successful verification, the CA will issue a certificate. You will typically receive a compressed package containing multiple files, the most important of which include:

Domain certificate file (e.g., `your_domain.crt`): This is your primary certificate.

Intermediate certificate chain file (e.g., `ca_bundle.crt`): This is the intermediate certificate linking your certificate to the CA root certificate. During installation, the primary certificate and intermediate certificate chain must be correctly combined; otherwise, some browsers will report an "incomplete certificate chain" error.

On an Nginx server, during installation, you need to specify the paths to the certificate and private key in the `server` block of the configuration file:

ssl_certificate /path/to/your_domain_chain.crt;` # Usually the certificate file with the intermediate chain combined
ssl_certificate_key /path/to/your_domain.key;

For Apache servers, the configuration uses the `SSLCertificateFile` and `SSLCertificateKeyFile` directives.

A crucial detail is that in Nginx, the file pointed to by the `ssl_certificate` directive should be the file containing your primary certificate and the combined intermediate certificate chain. You can use a text editor to append the contents of the intermediate certificate chain provided by the CA (usually `ca_bundle.crt`) to the end of your primary certificate file (`your_domain.crt`) to create a new file.

After installation and configuration, use

nginx -t

or

apachectl configtest

to test the configuration file syntax, and then reload the service:

systemctl reload nginx

or

systemctl reload apache2

After reloading the service, verification is necessary. Access your `https://` domain in a browser and check for a padlock icon in the address bar. It is highly recommended to use a professional online SSL checking tool (such as SSL Labs' SSL Server Test), which provides a detailed report, including certificate validity, protocol configuration, cipher suite strength, and whether there are any issues with incomplete certificate chains.

Long-term maintenance hinges on setting up expiration reminders and renewing promptly. Certificates have a fixed expiration date. It is essential to renew and install the new certificate before it expires; otherwise, the website will be inaccessible due to the expired certificate. It is recommended to automate the renewal process (e.g., using the Certbot tool) and set up multiple reminders before expiration.