In today's internet world, website security has become a fundamental cornerstone. Whether it's conducting online transactions, logging into accounts, or simply browsing information, users expect their data to be protected. One of the core technologies for achieving this security goal is the SSL/TLS protocol, and its physical manifestation is the SSL certificate.
An SSL certificate is a digital file that serves as a website’s “digital identity card” and “encrypted envelope.” It is issued by a trusted certificate authority and performs two main functions: verifying the identity of the website owner and establishing a highly secure encrypted communication channel between the user’s browser and the website server. When you visit a website that uses a valid SSL certificate, a lock icon will appear in the browser’s address bar, and the URL will start with “https://,” where the “s” stands for “secure.”
Recommended Reading The Ultimate Guide to SSL Certificates: Types, Options, Installation and Deployment。
The working principle of SSL certificates
The SSL/TLS protocol establishes a secure connection through a process known as the “handshake.” Although this process is complex, its main goal is to securely exchange information between the client (browser) and the server, and to negotiate a session key that is only known to both parties. This session key is then used to encrypt subsequent communications.
The combination of asymmetric encryption and symmetric encryption
The SSL handshake cleverly combines two encryption techniques. In the initial phase of the handshake, asymmetric encryption (such as RSA or ECC) is used. The server sends its SSL certificate (which contains its public key) to the browser. The browser then uses the public key from the certificate to encrypt a randomly generated “pre-master key” and sends it back to the server, which decrypts it using its own private key. Since the private key is always kept secret by the server, even if a third party intercepts the encrypted pre-master key, they will not be able to decrypt it.
Both parties then use this pre-master key to independently generate the same “session key.” All subsequent communication during the session will be conducted using a faster symmetric encryption algorithm (such as AES), with the session key serving as the key for encryption and decryption. This approach not only ensures the security of the key exchange but also guarantees the efficiency of data transmission.
Recommended Reading SSL certificates in detail: from the type of choice to the Nginx server installation and configuration of the whole guide。
Certificate Validation and Chain of Trust
After receiving the server’s certificate, the browser does not trust it immediately. Instead, it performs a series of verifications: it checks whether the certificate has expired, whether it has been revoked, and whether the domain name listed in the certificate matches the website being visited. The most critical step is to verify whether the issuer of the certificate (the Certificate Authority, or CA) is trusted.
Browsers and operating systems come with a pre-installed list of trusted root certificate authorities (CAs). Server certificates are usually not issued directly by the root CA but by intermediate CAs. Browsers verify the signatures along the trust chain in the order of “server certificate -> intermediate CA certificate -> root CA certificate”. As long as the chain ultimately leads to a pre-installed, trusted root certificate, the browser establishes trust, and the lock icon is displayed.
The main types of SSL certificates
Based on different verification levels and features, SSL certificates are mainly classified into the following categories to meet the security and trust requirements of various scenarios.
Recommended Reading SSL Certificate Explained: From principle to deployment, comprehensive protection of website security。
Domain Validation Certificate
DV (Domain Validation) certificates are the fastest-to-issue and lowest-cost type of certificate. The Certificate Authority (CA) only verifies the applicant's ownership of the domain name (usually through email or DNS records). They provide basic encryption capabilities but do not verify the actual identity of the company or organization. DV certificates are suitable for personal websites, blogs, or testing environments.
Organizational validation type certificate
OV certificates build upon the DV (Domain Validation) process by conducting additional rigorous checks on the authenticity and legitimacy of the applying organization (such as a company or government agency). The Certificate Authority (CA) verifies official documents, including business registration information. The certificate details include the verified name of the organization, which helps to demonstrate the entity behind the website and enhances its credibility. These certificates are suitable for corporate websites and commercial platforms.
Extended Validation Certificate
EV certificates are the most rigorously verified and highest-security level of certificates. In addition to completing the organization verification required for OV-level certificates, the CA (Certificate Authority) conducts additional in-depth audits to ensure that the organization is a legally existing entity. The most distinctive feature of EV certificates is that, in browsers that support them, the website address bar will directly display the company’s name in green, which serves as the highest level of trust indicator. These certificates are commonly used by financial institutions and large e-commerce platforms.
Classification according to coverage
In addition to the verification level, certificates can also be classified according to the number of domains covered: single-domain certificates (protecting a specific domain), multi-domain certificates (one certificate protecting multiple different domains), and wildcard certificates (protecting a main domain and all its subdomains at the same level, such as *.example.com).
Recommended Reading What is an SSL certificate? A comprehensive guide from principles to selection and installation。
How to obtain and install an SSL certificate
Deploying an SSL certificate is a systematic process that involves generating a key pair and ultimately configuring it on the server.
The process of applying for and issuing certificates
First, you need to generate a private key and a Certificate Signing Request (CSR) on your web server. The CSR contains your public key, organizational information, and other details; it serves as the “application form” you use to request a certificate from a Certificate Authority (CA).
Then, submit the CSR (Certificate Signing Request) to the selected certificate authority, and complete the corresponding verification process based on the type of certificate you have chosen.
After the verification is successful, the CA will issue the certificate file (usually in . crt or . pem format) and provide it to you, along with any intermediate certificates that may be required.
Installing and configuring on common servers
The installation process varies depending on the server software, but the core steps are always the same: uploading the private key, server certificate, and intermediate certificate files to the server, as well as modifying the configuration files.
For Apache servers, you need to specify the paths for `SSLCertificateFile`, `SSLCertificateKeyFile`, and `SSLCertificateChainFile` in the virtual host configuration.
For Nginx servers, the `ssl_certificate` and `ssl_certificate_key` directives are configured within the `server` block.
After the configuration is complete, restart the server to apply the changes. Then, visit `https://your-domain-name` to test whether everything has been set up successfully.
Necessary tasks after installation
The installation of a certificate is not a one-time solution that solves all problems permanently. First of all, it is necessary to set up a 301 redirect from HTTP to HTTPS to ensure that all traffic is directed through a secure connection. Secondly, SSL certificates have an expiration date (usually one year), and they must be renewed before they expire; otherwise, the website will display security warnings. It is crucial to monitor the certificate’s expiration date and set up reminders for renewal. Additionally, implementing security headers such as HTTP Strict Transport Security (HSTS) can further enhance the security of the website.
The importance of SSL certificates for websites
The benefits of deploying SSL certificates far exceed those of simple encryption; they have become an essential part of modern websites.
The most direct benefit of this technology is the encryption of data during transmission, which prevents sensitive information from being eavesdropped on or tampered with. This ensures the security of users’ passwords, credit card numbers, and personal information.
It proves the legitimate identity of the website to users, especially through OV (Organizational Validation) and EV (Extended Validation) certificates, which effectively prevent phishing attacks and build user trust. This is crucial for e-commerce and online services.
Major search engines such as Google have clearly identified HTTPS as a positive indicator for search rankings. Using SSL certificates helps to increase the visibility of a website in search results.
Modern browsers have increasingly strict security policies. For websites that do not use HTTPS, a “not secure” warning may be displayed, and certain features may even be restricted. The performance advantages of the HTTP/2 protocol also generally require the use of HTTPS.
Many regulations, such as the Payment Card Industry Data Security Standard and the European Union's General Data Protection Regulation, require the encryption of data in transit. SSL certificates are a fundamental component for meeting these compliance requirements.
summarize
SSL certificates have evolved from an optional, advanced feature to a essential component for ensuring the security of online communications, building user trust, and meeting business requirements. Understanding their principles helps us appreciate the value of encryption and authentication. Distinguishing between different types of SSL certificates allows us to select the right product for various scenarios. Mastering the process of installing and configuring them is a crucial step in putting security theories into practice. In an era that places increasing emphasis on privacy and security, enabling HTTPS for your website and deploying the appropriate SSL certificate is no longer a matter of choice, but a fundamental responsibility and commitment to all your visitors.
FAQ Frequently Asked Questions
Are SSL certificates and TLS certificates the same thing?
Yes, what we commonly refer to as an “SSL certificate” today actually refers to a certificate based on the TLS protocol. Due to the long history of the SSL protocol, the name “SSL” is still widely used; however, the technical basis behind it is the more secure and modern TLS protocol. The certificate itself is protocol-independent and can be used for both SSL and TLS connections.
What is the difference between a free SSL certificate and a paid one?
免费证书（如Let‘s Encrypt颁发的）通常是DV证书，提供了与付费DV证书相同强度的加密功能，非常适合个人网站或博客。主要区别在于，免费证书有效期较短，需要频繁续期；一般没有商业保障；并且通常只提供基础的技术支持。付费证书则提供OV、EV等更高级别的验证，包含身份保险，提供专业的技术支持和更长的可选有效期。
Can an SSL certificate be used for multiple domain names?
Sure, but it depends on the type of certificate. A single-domain certificate can only protect one specific domain name. A multi-domain certificate allows you to include multiple different domain names in the same certificate. A wildcard certificate, on the other hand, can protect a main domain name and all its subdomains at the same level. You need to choose the appropriate type based on your actual needs.
The website has an SSL certificate installed, so why does the browser still indicate that it is not secure?
There could be several reasons for this situation. The most common one is that the webpage contains mixed content that uses the HTTP protocol for loading insecure resources, such as images, scripts, or style sheets. As a result, the browser considers the entire page to be insecure. Other possible causes include an expired certificate, a mismatch between the certificate and the domain name being visited, or the absence of a valid intermediate certificate chain. It is necessary to investigate the issue based on the specific error messages provided by the browser.
How long is the validity period of an SSL certificate, and what should I do if it expires?
According to industry regulations, the maximum validity period of SSL certificates has been reduced to one year. Once a certificate expires, website visitors will receive a severe “unsecure” warning, and the connection will be blocked by their browsers.
You must renew the certificate before it expires. The process is similar to applying for a new certificate: generate a new CSR (Certificate Signing Request), submit a renewal request to the CA (Certificate Authority), obtain the new certificate file after verification, replace the old certificate file on the server, and restart the web service. It is recommended to set up a calendar reminder or use a certificate monitoring tool to automate this process.
What's next, what's next?
Extended reading and practical knowledge
The following are related to the topic of this article and are suitable for further in-depth reading. Prioritize starting with the article that is closest to your current problem, and gradually expanding to surrounding topics usually works better.
- What is an SSL certificate? A comprehensive explanation from its principles to the process of applying for and using it.
- What is an SSL certificate? A comprehensive guide to understanding the principles, types, and installation procedures of digital certificates.
- In-depth Analysis of SSL Certificates: From Beginner to Expert – Comprehensive Protection for Website Security
- What is an SSL certificate and how does it work
- Comprehensive Guide to SSL Certificates: From Principles and Types to Practical Details on Deployment and Management
English