What is an SSL certificate?

What is an SSL certificate?

An SSL certificate, whose full name is Secure Sockets Layer Certificate, has evolved into the more comprehensive TLS (Transport Layer Security) certificate. It is a type of digital certificate that serves a crucial purpose: to establish an encrypted and secure connection between the user's browser (or client) and the website server. You can think of it as the website server's “digital passport” or “identity card,” issued by a trusted third-party organization known as a certificate authority.

Once a website has been deployed with a valid SSL certificate, its URL changes from “http://” to “https://”, and a lock icon is displayed in the browser’s address bar. This is more than just a visual indicator; it also signifies that the data being transmitted is encrypted with high security, effectively preventing eavesdropping, tampering, or forgery. Therefore, SSL certificates are an essential foundation for establishing online trust, protecting data privacy, and verifying the authenticity of a website.

The core working principle of an SSL certificate

The SSL/TLS protocol establishes a secure connection through a process known as the “handshake.” Although this process is complex, its core objectives can be summarized as follows: authentication and key exchange.

Recommended Reading SSL Certificate Guide: From Beginner to Expert – Ensuring the Security of Your Website’s Data Transmission。

The combination of asymmetric encryption and symmetric encryption

The SSL handshake cleverly combines two encryption methods. First, the server sends its SSL certificate (which contains the public key) to the client. The client uses a built-in, trusted CA (Certificate Authority) root certificate to verify the authenticity of the server’s certificate. Once the verification is successful, the client generates a random “session key.”

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Detailed explanation of the handshake process

The client uses the server’s public key to encrypt the session key and then sends it back to the server. Since only the server, which possesses the corresponding private key, can decrypt this information, the security of the session key transmission is ensured. Once the server decrypts the session key, both parties use this shared session key for symmetric encryption to encrypt all subsequent communication content. Symmetric encryption is faster and more suitable for the encrypted transmission of large amounts of data, while the initial asymmetric encryption process securely transfers the symmetric encryption key.

The main types of SSL certificates are:

Based on the level of validation and the features they provide, SSL certificates are mainly divided into the following categories to meet the needs of different scenarios.

Domain Validation Certificate

DV (Domain Validation) certificates are the type of certificate with the lowest level of verification and the fastest issuance process. Certification authorities (CAs) only verify the applicant's ownership of the domain name, for example, by checking domain name resolution records or the specified email address. These certificates provide basic encryption for websites and display a lock icon in the address bar, but they do not display the company name. They are commonly used for personal websites, blogs, or testing environments.

Organizational validation type certificate

OV (Organic Trust) certificates provide a higher level of verification. In addition to verifying the ownership of the domain name, the CA (Certificate Authority) also confirms the actual existence of the applying company by checking its registration information with the relevant authorities (such as the business registration department). The certificate details will include the verified name of the company. This helps to demonstrate to visitors that there is a legitimate entity behind the website, thereby enhancing trust. OV certificates are suitable for corporate websites and commercial websites.

Recommended Reading A Comprehensive Analysis of SSL Certificates: The Cornerstone of Your Website’s Security and Reputation。

Extended Validation Certificate

EV certificates are the most rigorously verified and have the highest level of trust. Certification Authorities (CAs) follow strict review processes, which include in-depth checks of a company’s legal, physical, and operational existence. Websites that successfully deploy EV certificates will display a lock icon in the address bar, as well as the company’s name in green, in most browsers. This provides the highest level of visual trust for websites with high security requirements, such as those in the financial and e-commerce sectors.

Multiple domain and wildcard certificates

In addition to different verification levels, there are also certificates categorized by their functionality. Multi-domain certificates allow you to protect multiple completely different domain names with a single certificate. Wildcard certificates, on the other hand, enable you to protect a main domain name and all its subdomains at the same level; for example, `*.example.com` can protect `blog.example.com`, `shop.example.com`, and so on, making management much more flexible and efficient.

Recommended Reading SSL Certificate Tutorial: From Beginner to Expert – Ensuring the Security of Website Data Transmission。

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Why must websites deploy SSL certificates?

The deployment of SSL certificates has shifted from being a “best practice” to a “mandatory requirement,” and its importance is evident on multiple levels.

First and foremost, data encryption and privacy protection are of utmost importance. They ensure that sensitive information such as passwords, credit card numbers, personal details, and chat records is transmitted in encrypted form, making it impossible to decipher even if it is intercepted. This fundamentally safeguards the privacy of users.

Secondly, it provides identity authentication and protection against phishing attacks. Certificates of the OV (Organizational Validation) and EV (Extended Validation) types verify the true identity of website operators, helping users distinguish between legitimate websites and maliciously forged phishing sites, thus preventing them from being deceived.

Once again, it is of utmost importance for search engine optimization (SEO). Major search engines such as Google and Baidu have long recognized HTTPS as a positive indicator in their ranking algorithms. Websites without an SSL certificate will be at a disadvantage in search results.

In addition, increasing user trust and conversion rates directly leads to commercial benefits. Browsers clearly mark websites that are not HTTPS as “insecure,” which can result in a loss of users. However, a prominent lock icon and “secure” notifications can significantly boost user confidence, thereby increasing conversion rates for actions such as registration and placing orders.

Finally, this is the foundation for meeting compliance requirements and the demands of modern technologies. Many industry regulations (such as PCI DSS, the data security standard for the payment card industry) and new web technologies (such as HTTP/2 and Service Workers) either mandate the use of HTTPS or require it to function at their best performance levels.

How to choose and install an SSL certificate?

Choosing the right certificate and installing it correctly are crucial steps in ensuring that security measures are effectively implemented.

Select the certificate type based on your requirements.

个人博客或展示类网站可以选择免费的DV证书（如Let‘s Encrypt）或低成本的DV证书。企业官网、内部管理系统建议使用OV证书，以展示企业身份。电商、金融、在线支付等对信任要求极高的平台，应考虑EV证书。如果拥有多个域名或需要频繁添加子域名，则应选择多域名证书或通配符证书以简化管理。

Process for Obtaining and Installing Certificates

Certificates are typically obtained through cloud service providers, hosting providers, or specialized certificate resellers. The installation process generally consists of three steps: First, generate a CSR (Certificate Signing Request) file on the server, which contains your public key and organizational information; next, submit the CSR to a Certificate Authority (CA) for review and issuance; finally, you will receive the certificate file issued by the CA. You then need to deploy this certificate, along with your private key, on your web server software (such as Nginx, Apache, or IIS), and configure the system to enforce HTTPS connections.

The keys to certificate management: renewal and monitoring

SSL certificates have a clear expiration date, usually one year. Once a certificate expires, the website becomes inaccessible, and security warnings are displayed, which can severely damage a website’s reputation. It is essential to set up reminders or use automatic renewal tools to ensure that certificates are renewed on time. Additionally, it is important to monitor the status of revoked certificates to prevent risks such as the theft of private keys.

summarize

SSL certificates are the cornerstone of internet security today. They protect the privacy and integrity of every data exchange by using encrypted connections and authentication mechanisms. From improving search engine rankings to gaining user trust, from meeting compliance requirements to enabling modern web technologies, deploying HTTPS has become a standard practice for website operations. Understanding the different types of SSL certificates, their working principles, and deployment methods, selecting the right certificate based on one’s business needs, and maintaining it properly are essential skills that every website owner should possess. In an era where network security is of increasing importance, a valid SSL certificate serves as a website’s first line of defense and a symbol of trust in the digital world.

FAQ Frequently Asked Questions

What is the difference between free SSL certificates and paid ones?

免费证书（如Let‘s Encrypt）通常是DV类型，提供与付费DV证书相同强度的加密功能，适合个人或小型项目。主要区别在于：免费证书有效期较短（通常90天），需频繁续期；一般不含技术支持或赔付保障；而付费证书提供更长的有效期、技术支持、更高的赔付金额（如因证书问题导致损失），以及OV/EV等需要人工审核的更高级别验证，能展示企业身份。

After the SSL certificate was installed, why does some content on the website still appear as insecure?

This situation is known as a “mixed content” issue. Although the main page is loaded via HTTPS, some of the resources referenced on the page (such as images, JavaScript scripts, and CSS style sheets) are still loaded using insecure HTTP links. As a result, the browser determines that the page is not completely secure and displays a warning. The solution is to find the relevant code and change all the references to these resources to use the HTTPS protocol, or to use a relative protocol (for example, `//example.com/image.jpg`).

Can an SSL certificate be used on multiple servers?

Sure, but you need to pay attention to the security of the certificate’s private key. The same certificate can be deployed on multiple servers (for example, on different nodes within a load balancing cluster). The important thing is that these servers must provide services using the same domain name. A more secure approach is to generate a separate CSR (Certificate Signing Request) and key pair for each server, or to apply for a separate certificate for each server’s domain name. This will prevent the leakage of the private key from putting all servers at risk.

What are the consequences if an SSL certificate expires?

After the certificate expires, when users visit your website, the browser will disconnect and display a clear security warning message (such as “Your connection is not private”), preventing users from continuing to access the site. This can lead to interruptions in website services, a very poor user experience, and a significant damage to the website’s credibility and brand image. Search engines may also lower the website’s ranking as a result. Therefore, it is essential to set up automatic renewal or regular reminders.

Can wildcard certificates protect all subdomains?

Wildcard certificates can protect all subdomains at a specified level. For example, a wildcard certificate issued for `*.example.com` can protect `blog.example.com` and `shop.example.com`, but not `dev.blog.example.com` (which is a second-level subdomain). To protect second-level subdomains, you need to apply for a multi-level wildcard certificate such as `*.*.example.com` (which is less common and more expensive), or you can apply for separate certificates for each second-level subdomain.