SSL certificates are a key technical component for enabling websites to upgrade from the HTTP protocol to the HTTPS protocol. They are digital certificates that establish an encrypted connection between browsers and servers, ensuring the security and confidentiality of data transmitted. In an era where network security is of increasing importance, SSL certificates have become an essential element in website development, especially for websites that involve user login or online transactions. They not only protect sensitive information but also enhance user trust and have a positive impact on search engine optimization (SEO). Therefore, understanding the working principle of SSL certificates, their types, and how to select and deploy them is crucial for website owners and developers.
The working principle of SSL certificates
The core of the SSL/TLS protocol is the use of a combination of asymmetric and symmetric encryption to establish a secure communication channel. This process is primarily accomplished through the “SSL handshake.”
The combination of asymmetric encryption and symmetric encryption
The SSL/TLS protocol cleverly combines the advantages of two different encryption methods. Asymmetric encryption, typically using RSA or ECC algorithms, is used at the beginning of the handshake process for authentication and the exchange of session keys. The key feature of asymmetric encryption is that different keys are used for encryption and decryption: a public key and a private key. The public key can be made available to the public and is used for encryption, while the private key is kept strictly confidential by the server and is used for decryption.
Recommended Reading The Ultimate SSL Certificate Guide: Types, How It Works, and Best Practices for Deployment。
Symmetric encryption, such as the AES algorithm, is used to encrypt the actual application layer data during transmission after the handshake process is completed. Its main characteristic is that the same key is used for both encryption and decryption, which results in fast computation speeds and makes it suitable for encrypting large amounts of data. One of the key purposes of the SSL handshake is to enable the client and server to securely negotiate a symmetric session key that is known only to them both.
The complete SSL/TLS handshake process
When a user enters an HTTPS URL in the browser’s address bar, the SSL/TLS handshake process begins. First, the client sends a “Client Hello” message to the server, which includes the SSL/TLS versions it supports, a list of available encryption protocols (cryptosuites), and a random number.
The server responds with a “Server Hello” message, selects a protocol version and encryption suite that are supported by both parties, and then sends its own random number as well as a digital certificate. This certificate contains the server’s public key, as well as a digital signature issued by a trusted certificate authority (CA).
After receiving the certificate, the client verifies its validity. It uses the local CA root certificate or the root certificate built into the operating system to verify the signature of the server certificate. Once the verification is successful, the client trusts the identity of the server.
Next, the client generates a “pre-master key” and encrypts it using the public key from the server’s certificate, then sends it to the server. The server decrypts the key with its own private key to obtain the pre-master key. At this point, both the client and the server have two random numbers (the client’s random number and the server’s random number) as well as the pre-master key. They each use the same algorithm to calculate the final symmetric session key.
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At the end of the handshake, both parties exchange a “Finished” message encrypted using the newly generated session key to verify the correctness of the key. Thereafter, all application-layer data will be encrypted and transmitted using this symmetric session key.
The main types of SSL certificates and how to choose them
Based on different verification levels and use cases, SSL certificates are mainly divided into three categories. Choosing the right type of certificate is an important step in ensuring a balance between security and cost.
The main differences between DV, OV, and EV certificates are as follows:
Domain Name Validation (DV) certificates are the most basic type of security certificate. The Certificate Authority (CA) only verifies the applicant’s control over the domain name, typically by checking the email address of the domain administrator or by setting specific DNS records. This type of certificate verification is fast and inexpensive, and is mainly used for personal websites, blogs, or test environments. It provides basic encryption capabilities. The browser’s address bar will display a lock icon and the “HTTPS” prefix.
Organizational Validation (OV) certificates require a more rigorous verification process. In addition to verifying domain name ownership, the Certificate Authority (CA) also examines the authenticity and legitimacy of the applying organization, for example by checking the company’s information in the business registration system. OV certificates include the name of the applying organization, which can be displayed to users when they click on the lock icon in their browsers. These certificates are suitable for corporate websites and general commercial websites, providing a higher level of credibility.
Extended Validation (EV) certificates undergo the most stringent verification processes. The Certificate Authority (CA) must conduct a comprehensive background check on the organization, following globally unified standards. Websites that have EV certificates will display a green status in the address bar of certain browsers, along with the company name. This provides the highest level of trust for websites that require high security, such as those in the financial and e-commerce sectors.
Single-domain, multi-domain, and wildcard certificates
In addition to the verification level, SSL certificates are also classified based on the number of domains they cover. A single-domain certificate only protects one fully qualified domain name (FQDN), for example: www.example.com Or example.com。
Recommended Reading What is an SSL certificate? A comprehensive guide from principles, types to the process of applying for and installing one.。
A multi-domain certificate, also known as a SAN (Subject Alternative Name) certificate, allows multiple different domain names to be added to the same certificate, whether they are the primary domain or subdomains. This facilitates the management of multiple related websites.
Wildcard certificates are particularly suitable for scenarios where there are a large number of subdomains. A single wildcard certificate can protect a domain name and all its subdomains at the same level. For example… *.example.com It can protect a.example.com、b.example.com、shop.example.com This is very valuable in large online platforms or SaaS services.
How to obtain and install an SSL certificate
The first step in enabling HTTPS for a website is to obtain and correctly install an SSL certificate. This process begins with generating a key pair and ends with deploying it to the server.
Generating a CSR (Certificate Signing Request) and Verifying It
The certificate application process is usually initiated on the server. First, a private key file and a Certificate Signing Request (CSR) file need to be generated on the server. The private key must be kept with utmost security; it represents the server’s identity. Once the private key is compromised, the security of the entire certificate is compromised as well.
The CSR (Certificate Signing Request) file contains the server’s public key as well as the applicant’s information, such as the domain name, organization name, and location. The applicant submits the CSR file to the selected certificate authority (CA). The CA will verify the submitted information based on the type of certificate being requested (DV, OV, or EV). For example, for a DV certificate, the CA will send a verification email to the domain name’s registered email address or require the setting of a specific DNS TXT record. Once the verification is successful, the CA will issue the signed SSL certificate file.
Server-side deployment and configuration
After receiving the certificate file issued by the CA, you need to deploy it together with the previously generated private key file to the web server software, such as Apache, Nginx, or IIS. Once the deployment is complete, make sure to check whether the HTTPS service is running properly. Using online SSL testing tools can help verify whether the certificate has been installed correctly, whether a secure encryption suite is being used, and whether there are any known vulnerabilities.
Deployment not only involves installing certificates but also crucial server configurations. For example, all HTTP requests should be redirected to HTTPS using a 301 redirect to enforce the use of secure connections; the HSTS (HTTP Strict Transport Security) header should be enabled to instruct browsers to access the website only via HTTPS for a specified period of time; and outdated, insecure SSL protocols such as SSLv2 and SSLv3 should be disabled, forcing the use of TLS 1.2 or higher versions.
The maintenance and management of SSL certificates
Deploying an SSL certificate is not a one-time solution. Effective lifecycle management is a crucial aspect of ensuring ongoing security, which includes updating the certificate when it expires, handling revocation processes, and considering performance considerations.
Certificate Lifecycle and Renewal
SSL certificates are not permanently valid. The current industry standard specifies that the maximum validity period of a certificate is 398 days (approximately 13 months). This duration is set by the CA/B forums to promote regular certificate rotation and security audits, thereby reducing the long-term risks associated with private key leaks. Once a certificate expires, it becomes invalid, causing security warnings on the website, which can significantly impact the user experience and the availability of the website.
因此，监控证书的到期日期至关重要。建议设置日历提醒，并在到期前30天开始准备续订。续订流程与初次申请类似，都需要生成新的CSR并提交给CA进行验证。理想的做法是，建立自动化的证书续订机制，尤其是在使用Let's Encrypt等提供的免费、自动化的证书服务时，可以有效避免因证书过期而导致的服务中断。
Certificate Revocation and Private Key Security
During the validity period of a certificate, if the server’s private key is accidentally lost or leaked, or if the information of the organization that owns the certificate changes (for example, the domain name is no longer in use), it is necessary to immediately apply to the CA (Certificate Authority) for the certificate to be revoked. Once revoked, the certificate will be added to the Certificate Revocation List (CRL) or marked as invalid through the Online Certificate Status Protocol (OCSP).
Maintaining the security of private keys is the cornerstone of the entire SSL/TLS security framework. Private keys should be stored on servers in an encrypted format, and access to them should be strictly controlled, allowing only the necessary service processes to read them. Under no circumstances should private keys be transmitted via insecure channels, or included in code repositories or backup files without being encrypted.
Considerations for Optimizing HTTPS Performance
Enabling HTTPS does indeed increase the computational overhead on the server due to the additional encryption and decryption processes. However, with the advancement of modern hardware, this performance impact has become minimal. By enabling TLS session reuse and optimizing the selection of encryption suites (for example, preferring suites that support AES-GCM or ChaCha20-Poly1305, as well as ECDHE key exchange), the handshake latency and server load can be significantly reduced.
In addition, using the TLS 1.3 protocol can further reduce the handshake time, as it simplifies the handshake process. Using performance as an excuse to refuse to enable HTTPS is no longer valid in 2026.
summarize
In summary, SSL certificates are the cornerstone of building a secure and trustworthy internet. They establish a strong barrier between user browsers and servers through sophisticated encryption mechanisms, ensuring the confidentiality and integrity of data transmission. With a range of products available – from basic DV certificates to highly verified EV certificates, from single-domain certificates that protect a single domain to wildcard certificates that cover multiple domains – the market meets the security and business needs of various scenarios. Obtaining, deploying, and managing SSL certificates involves a comprehensive process that includes generating private keys, submitting for verification, configuring servers, and renewing certificates when they expire. In today’s internet environment, deploying effective SSL certificates on websites and configuring secure HTTPS connections has evolved from being an optional feature to a necessary measure for gaining user trust, enhancing a professional image, and even improving search rankings.
FAQ Frequently Asked Questions
Does an SSL certificate cost anything?
SSL证书有免费和付费两种类型。像Let's Encrypt提供的免费DV证书，完全能够满足个人网站或博客的基本加密需求。付费证书则提供了更高级别的组织验证（OV）、扩展验证（EV），以及更长的有效期、更完善的赔偿保障和技术支持，适合商业网站和企业级应用。
Is a website absolutely secure once an SSL certificate is installed?
That’s not the case. SSL/TLS primarily addresses the issue of data encryption during transmission, preventing data from being eavesdropped on or tampered with while being sent. It does not protect against other security vulnerabilities of a website, such as server breaches, SQL injection or cross-site scripting (XSS) vulnerabilities in the website’s code, or users setting overly simple passwords. An SSL certificate is a crucial component of a website’s overall security strategy, but it is not the entire solution.
Does my website content not contain any sensitive information? Do I still need an SSL certificate?
Yes, it is necessary. Firstly, modern browsers such as Chrome and Firefox mark websites that do not use HTTPS as “insecure,” which directly affects users’ first impressions and trust in those websites. Secondly, search engines like Google have made HTTPS a positive factor in their search ranking algorithms. Moreover, even for non-sensitive information, an encrypted connection prevents traffic from being intercepted, advertisements from being inserted, or malicious code from being installed, thus protecting the integrity of the website and the user experience.
How to view the SSL certificate information of a website?
When accessing an HTTPS website, most browsers display a lock icon on the left side of the address bar. Clicking on this lock icon usually reveals a message indicating that the connection is secure. By clicking on options such as “Certificate is valid” or similar phrases, you can open a certificate details window, where you can view information such as to whom the certificate was issued (the domain name), the issuer (the CA), the validity period of the certificate, and the encryption algorithm being used.
Can an SSL certificate be used on multiple servers?
Technically, it is possible. As long as the private keys are the same, the same certificate can be deployed on multiple servers that provide the same service, such as a cluster of web servers behind a load balancer. However, extra care is required in terms of management and security: the transmission and storage of private keys across multiple servers must be absolutely secure. If the private key of any one of the servers is compromised, all services that use that certificate will be affected, and the certificate must be immediately revoked and reissued.
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.
- CDN Technology Analysis: A Guide to Website Acceleration and Security Protection, from Beginner to Expert
- In-Depth Explanation of CDN Technology: From Principles to Practice – The Ultimate Guide to Improving Website Performance and Security
- Comprehensive Analysis of SSL Certificates: An Integrated Guide from Application to Deployment and Renewal
- SSL Certificate Comprehensive Analysis: From How It Works to Best Practices for Deployment
- In today's internet environment, data security is a critical issue that concerns both users and website owners.
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