In today's internet environment, data security is a top concern for both users and website owners. When you see a lock icon in the browser address bar or when a website address starts with “https”, it means that the website is using an SSL certificate to protect your connection. This technology is the cornerstone of online security, ensuring the privacy and security of our activities such as online shopping, logging into accounts, or transmitting sensitive information.
The core definition and composition of an SSL certificate
An SSL certificate, whose full name is Secure Sockets Layer Certificate, has now evolved into its successor, the TLS certificate. However, the industry still commonly refers to it as an SSL certificate. It is a digital file whose primary function is to establish an encrypted and authenticated communication channel between the user's browser (client) and the website server.
A standard SSL certificate contains several key components of information. The first component is information about the certificate holder, which may include the domain name, company name, and location for different types of certificates. The second component is information about the certificate issuing authority, the trusted entity that issued the certificate. Most importantly, the certificate contains a pair of asymmetric encryption keys: a public key and a private key. The public key is included in the certificate file and can be distributed publicly; the private key, on the other hand, is kept secret by the website server and must not be disclosed. In addition, the certificate also includes its validity period, serial number, and a digital signature used to verify the integrity of the certificate.
Recommended Reading What is an SSL certificate? You’ll understand after reading this article.。
The working principle of the SSL/TLS handshake
The key process by which an SSL certificate comes into effect is known as the “SSL/TLS handshake.” This is a complex protocol interaction that occurs automatically in the background the moment a user visits a website, with the aim of establishing an encrypted connection securely.
The client initiates a “greeting” action.”
When you enter an HTTPS URL in a browser for the first time, the client (the browser) sends a “ClientHello” message to the server. This message includes the SSL/TLS protocol versions that the client supports, a list of available encryption algorithm suites, and a randomly generated string.
Server response and certificate presentation
After receiving the greeting, the server responds with a “ServerHello” message, which specifies the protocol version and encryption suite to be used by both parties. The server also sends a randomly generated string. Subsequently, the server transmits its SSL certificate to the client; this certificate contains the server’s public key.
Client-side verification of certificates
This is a crucial step in establishing trust. The client (either a browser or an operating system) comes with a pre-installed list of trusted certificate authorities (CAs). The client uses the public key of the CA’s root certificate to verify the digital signature on the server’s certificate. This verification process ensures that the certificate was issued by a trusted CA, that it is still valid, and that the domain name specified in the certificate matches the domain name of the website being visited. If the verification fails, the browser will display a clear warning to the user.
Key exchange and establishment of an encrypted communication channel
After the verification is successful, the client generates a random string called the “pre-master key” and encrypts it using the public key from the server’s certificate. The encrypted pre-master key is then sent to the server. Only the server, which possesses the corresponding private key, can decrypt it and obtain the original pre-master key. Subsequently, both the client and the server use the two previously exchanged random strings, along with the pre-master key, to independently generate identical “session keys.” All subsequent communications between them will be encrypted and decrypted using this symmetric session key, thereby establishing a fast and secure transmission channel.
Recommended Reading SSL Certificate: What is an SSL Certificate and a Detailed Explanation of How It Works。
The main types of SSL certificates
Based on the level of verification and the scope of application, SSL certificates are mainly divided into three categories to meet the security and trust requirements of different scenarios.
Domain Name Validation Certificate
DV (Domain Validation) certificates are the type with the lowest level of verification and the fastest issuance process. The Certificate Authority (CA) only verifies the applicant’s control over the domain name, typically by sending a verification email to the domain’s WHOIS email address or by requiring the setting of specific DNS records. DV certificates provide basic encryption capabilities, enabling websites to use HTTPS and displaying a lock icon in the address bar. They are very suitable for personal websites, blogs, or testing environments.
Organization validation certificate
OV certificates offer a higher level of credibility. In addition to verifying the ownership of the domain name, the CA (Certificate Authority) also conducts a manual check on the authenticity of the applying organization, for example, by verifying its registration information in government databases. Once the review is successful, the issued certificate will include the official name of the company. OV certificates are suitable for corporate websites and commercial platforms, as they reassure users that the website is associated with a verified and legitimate entity.
Extended Validation Certificates
EV certificates provide the highest level of verification and the most recognizable symbol of trust. Certification Authorities (CAs) conduct thorough offline reviews of the organizations applying for these certificates, including verifying their legal, physical, and operational existence. Websites that use EV certificates will display a lock icon in modern browsers, as well as the company’s name in green directly in the address bar. Financial institutions and large e-commerce platforms often use EV certificates to maximize user confidence.
In addition, based on the number of domains they cover, there are single-domain certificates, multi-domain certificates, and wildcard certificates. Wildcard certificates are particularly convenient, as a single certificate can protect a main domain and all its subdomains at the same level.
Why must websites deploy SSL certificates?
The deployment of SSL certificates has evolved from a best practice to a mandatory requirement for website operations, with its necessity primarily stemming from security, trust, and business considerations.
Recommended Reading Comprehensive Guide to SSL Certificates: From Principles and Types to Practical Details on Deployment and Management。
From a security perspective, SSL certificates prevent data from being eavesdropped on or stolen during transmission through encryption. Without HTTPS, the passwords, credit card numbers, and chat messages you enter are transmitted over the network in plain text, making them highly susceptible to interception by attackers. SSL also ensures that data is not altered during transmission, guaranteeing that the information received by the user is the exact same as the original content sent by the server.
In terms of building user trust, modern browsers mark all websites that do not use HTTPS as “insecure.” This prominent warning significantly increases users’ concerns and the rate of page abandonment, which can be fatal for e-commerce or service-oriented websites. Conversely, a website that displays a lock icon or a green address bar immediately conveys a sense of security and professionalism, thereby increasing users’ willingness to stay on the page and complete transactions.
For search engine optimization (SEO), mainstream search engines such as Google and Baidu have long considered HTTPS to be a positive factor in their ranking algorithms. Websites that use SSL certificates may achieve higher rankings in search results, thereby attracting more organic traffic. Additionally, many modern web technologies, such as certain performance features of HTTP/2 and geolocation APIs, require websites to operate in an HTTPS environment in order to function properly.
summarize
An SSL certificate is by no means a simple technical component; it is a core infrastructure for building a trustworthy internet ecosystem. Its functionality is based on sophisticated asymmetric encryption and rigorous trust chain verification, creating an invisible bridge of security between users and servers. ranging from DV certificates, which provide basic encryption, to EV certificates that demonstrate the highest level of identity credibility, different types of SSL certificates meet a variety of security requirements. In today’s digital environment, deploying effective SSL certificates has become a prerequisite for protecting user data, establishing brand trust, improving search rankings, and making full use of modern web technologies. It is a fundamental responsibility that every responsible website operator must fulfill.
FAQ Frequently Asked Questions
What will happen if the SSL certificate expires?
SSL certificates have a clear expiration date, usually one year or less. Once they expire, browsers will fail to establish a secure connection, displaying a serious error warning to the user, indicating that the connection is not secure and preventing access to the website. This can result in the website being inaccessible, significantly impacting the user experience and business operations. Therefore, it is essential to regularly monitor SSL certificates and renew them in a timely manner.
I already have an SSL certificate, so why does the browser still indicate that the connection is not secure?
This situation is commonly referred to as a “mixed content” issue. Although the web page itself is loaded via HTTPS, some of the resources referenced within the page—such as images, style sheets, and JavaScript scripts—are still linked using the insecure HTTP protocol. As a result, the browser considers the entire page to be insecure and displays a warning. The solution is to change all the links to these resources to use the HTTPS protocol.
Are free SSL certificates (such as Let's Encrypt) reliable?
从加密强度上来说，免费的DV证书与付费的DV证书是同等可靠的，它们都提供相同的加密级别。Let’s Encrypt等免费CA的推出极大地推动了HTTPS的普及。主要区别在于，免费证书通常有效期很短，需要频繁自动续签；且一般不含技术支持或价值担保。对于需要组织验证或扩展验证的正式商业网站，仍需选择付费的OV/EV证书。
Can one SSL certificate protect multiple subdomains?
Sure, but you need to choose the correct type of certificate. A regular single-domain certificate can only protect one complete domain name. If you need to protect a main domain name and all its subdomains at the same level, you should choose a wildcard certificate. If you need to protect multiple completely different domain names, you will need to choose a multi-domain certificate.
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
- Comprehensive Guide to SSL Certificates: From Principles and Types to Practical Details on Deployment and Management
- Comprehensive Guide to SSL Certificates: Types, Prices, and Answers to Common Deployment Questions
English