What Is an SSL Certificate? How Does SSL Work?

Posted on May 14, 2024

SSL/TLS certificates are important for protecting online communication between websites and users. These digital certificates work as identity cards, checking the authenticity of a website and creating an encrypted connection to safeguard sensitive data. In this article, we will explain what SSL/TLS certificates are, how they function, and their role in maintaining online security and privacy.

Key Takeaways

  • SSL/TLS certificates serve as digital identity cards, verifying the authenticity of a website and establishing an encrypted connection to protect sensitive data.
  • SSL/TLS certificates play a crucial role in online security by protecting private data, strengthening customer confidence, supporting regulatory compliance, and improving SEO.
  • SSL/TLS certificates use encryption, authentication, and digital signatures to secure communication between a web browser and a server through the SSL/TLS handshake process.
  • SSL/TLS certificates come in different validation levels (Extended Validation, Organization Validation, and Domain Validation) and domain types (Single Domain, Wildcard, and Multi-Domain) to cater to various security needs and website configurations.
  • Certificate Authorities issue SSL/TLS certificates after verifying the domain and owner details, and these certificates have limited validity periods to reduce security risks, making timely renewal essential for maintaining uninterrupted website security.

What is an SSL Certificate?

SSL/TLS certificates are digital files that enable secure communication between a website and a user's web browser. They serve as digital identity cards, verifying the authenticity of a website and establishing an encrypted connection to protect sensitive data.

Digital Identity Cards

SSL/TLS certificates work like digital identity cards for websites. When you visit a website secured with an SSL/TLS certificate, your browser checks the certificate to verify the site's identity. This process makes sure that the website is real and not an imposter trying to steal your information.

For example, when you visit a banking website like https://www.examplebank.com, your browser will check the SSL/TLS certificate to confirm that the website is owned and operated by Example Bank. This helps prevent phishing attacks where bad actors create fake websites to trick you into revealing your login credentials or other sensitive information.

SSL/TLS certificates are part of a larger system called the public key infrastructure (PKI). PKI provides a framework for secure communication by using digital certificates and public-key encryption. In this system, a trusted third party called a certificate authority (CA) issues SSL/TLS certificates to websites after verifying their identity and ownership.

Importance of SSL Certificates

SSL/TLS certificates play an important role in online security by:

  1. Protecting private data: SSL/TLS certificates enable encryption of all communication between your browser and the website. This means that sensitive information like passwords, credit card numbers, and personal details are protected from interception by hackers or other bad actors.

    For instance, when you enter your credit card details on an e-commerce website with an SSL/TLS certificate, the information is encrypted before being sent over the internet. Even if a hacker manages to intercept the data, they would only see gibberish and would not be able to decipher the actual credit card number.

  2. Strengthening customer confidence: Websites with SSL/TLS certificates display a padlock icon in the browser's address bar, indicating a secure connection. This visual cue reassures you that your data is safe, increasing your trust in the website and your willingness to share information or make purchases.

  3. Supporting regulatory compliance: Many industries, such as e-commerce and healthcare, have regulations that require websites to secure user data. For example, the Payment Card Industry Data Security Standard (PCI DSS) mandates the use of SSL/TLS certificates for websites that process credit card transactions.

    Other regulations that may require SSL/TLS certificates include:

    • Health Insurance Portability and Accountability Act (HIPAA) for healthcare websites
    • General Data Protection Regulation (GDPR) for websites serving European Union citizens
    • Federal Information Security Management Act (FISMA) for U.S. government websites
  4. Improving SEO: Search engines like Google prioritize websites that use SSL/TLS certificates, as they consider them more secure and trustworthy. Websites with SSL/TLS certificates often rank higher in search results, potentially increasing their visibility and traffic.

How SSL Certificates Work

SSL/TLS certificates use key principles to secure communication between a web browser and a server. These principles include encryption, authentication, and digital signatures.


Encryption is the process of scrambling data so that only the recipient can decrypt it. SSL/TLS certificates use public key cryptography, which involves a public key and a private key. The public key is shared, while the private key is kept secret by the web server. When a browser wants to send information to the server, it encrypts the data using the server's public key. Only the server can decrypt the data using its private key, ensuring that the information remains secure even if intercepted.

Here's an example of how encryption works:

  1. Alice wants to send a message to Bob.
  2. Bob shares his public key with Alice.
  3. Alice encrypts her message using Bob's public key and sends it to him.
  4. Bob uses his private key to decrypt the message.

Even if someone intercepts the encrypted message, they won't be able to read it without Bob's private key.


Authentication is the process of verifying the identity of the web server. SSL/TLS certificates contain information about the website owner, such as their name and address. This information is verified by a trusted third party called a certificate authority (CA) before the certificate is issued. When a browser connects to a website with an SSL/TLS certificate, it checks the certificate's information to ensure that the website is who it claims to be.

Here's an example of how authentication works:

Step Description
1 Bob requests an SSL/TLS certificate from a CA.
2 The CA verifies Bob's identity and issues a certificate.
3 When Alice connects to Bob's website, her browser receives the SSL/TLS certificate.
4 Alice's browser checks the certificate's information to verify the website belongs to Bob.

If the certificate's information doesn't match the website, the browser will warn Alice that the site may not be trustworthy.

Digital Signatures

Digital signatures ensure the certificate hasn't been altered. When a CA issues an SSL/TLS certificate, it signs the certificate with its own private key, creating a unique digital signature. When a browser receives the certificate, it verifies the digital signature using the CA's public key. If the signature doesn't match, the browser knows the certificate has been altered and warns the user.

Here's an example of how digital signatures work:

  1. The CA creates a hash of the certificate's contents.
  2. The CA encrypts the hash using its private key, creating a digital signature.
  3. The digital signature is attached to the certificate.
  4. When a browser receives the certificate, it creates its own hash of the contents.
  5. The browser decrypts the digital signature using the CA's public key and compares it to the hash it created.
  6. If the hashes match, the browser knows the certificate is valid.

SSL Handshake Process

The SSL/TLS handshake process establishes a secure connection between a web browser and a server. The process begins when the browser connects to the web server and requests information. The server responds by sending its SSL/TLS certificate, which contains the server's public key.

The browser verifies the certificate by checking its information and digital signature. If the certificate is valid, the browser generates a random session key and encrypts it using the server's public key. The browser sends the encrypted session key to the server.

The server decrypts the session key using its private key and sends an acknowledgment to the browser. Both the browser and the server now have the same session key, which they use to encrypt and decrypt all communication. This ensures that any data exchanged between the browser and the server remains secure.

Here's a summary of the SSL/TLS handshake process:

Step Client (Browser) Server
1 Sends a request to connect securely -
2 - Sends its SSL/TLS certificate
3 Verifies the certificate -
4 Generates a session key, encrypts it with the server's public key, and sends it -
5 - Decrypts the session key with its private key
6 - Sends an acknowledgement to the client
7 Encrypts communication with the session key Encrypts communication with the session key

SSL/TLS certificates help websites protect sensitive information and ensure the privacy of their users' data. SSL/TLS is used by many websites, including online stores, banks, and social media platforms, to secure user information such as passwords, credit card numbers, and personal details. By using an SSL/TLS certificate, websites can establish a secure connection and encrypt data transmitted between the web browser and the server, making online transactions safer.

Types of SSL Certificates

SSL/TLS certificates come in different types, each with varying levels of validation and domain coverage. Understanding these types can help you choose the right certificate for your website based on your security needs and budget.

Validation Levels

SSL/TLS certificates are classified into three validation levels:

  1. Extended Validation (EV): EV SSL/TLS certificates provide the highest level of encryption and validation. They are often used by websites that handle sensitive data, such as financial institutions and e-commerce platforms. To obtain an EV certificate, the applicant must go through a rigorous verification process, which includes proving their legal identity and business registration. EV certificates display the company name in the browser's address bar, providing visual assurance to users.

    For example, when you visit a banking website like Bank of America (https://www.bankofamerica.com/), you will see the bank's name displayed in the address bar, indicating that the website uses an EV SSL/TLS certificate.

  2. Organization Validation (OV): OV SSL/TLS certificates offer a moderate level of validation. They are suitable for businesses and organizations that want to prove their identity and domain ownership. To obtain an OV certificate, the applicant must provide proof of their organization's identity and demonstrate control over the domain. OV certificates display the company name in the certificate details, which can be viewed by users.

    An example of a website using an OV SSL/TLS certificate is the University of California, Berkeley (https://www.berkeley.edu/). When you view the certificate details, you will see the university's name listed as the organization.

  3. Domain Validation (DV): DV SSL/TLS certificates have the lowest level of validation. They only verify that the applicant controls the domain name, without any additional checks on their identity or business. DV certificates are often used for blogs, personal websites, or non-commercial projects. They are the easiest and quickest to obtain, but they do not display any company information in the certificate details.

    A personal blog or a small business website, such as a local coffee shop, might use a DV SSL/TLS certificate to secure their website without the need for extensive validation.

Here's a comparison table of the three validation levels:

Validation Level Encryption Identity Verification Issuance Time Display in Browser Ideal Use Cases
Extended Validation (EV) High Extensive 1-5 days Company name E-commerce, financial services
Organization Validation (OV) High Moderate 1-3 days Company name in certificate details Business websites, company intranets
Domain Validation (DV) High Low Minutes to hours No company information Blogs, personal websites

Domain Types

SSL/TLS certificates also differ based on the number and type of domains they protect:

  1. Single Domain: A single domain SSL/TLS certificate secures one specific domain or subdomain. For example, a single domain certificate can protect www.example.com or blog.example.com, but not both at the same time. This type of certificate is suitable for websites with a single domain name.

    A small business with a single website, such as a restaurant or a local service provider, might use a single domain SSL/TLS certificate to secure their website.

  2. Wildcard: A wildcard SSL/TLS certificate secures a domain and all its subdomains at the first level. For instance, a wildcard certificate for *.example.com can protect www.example.com, blog.example.com, and shop.example.com. Wildcard certificates are cost-effective for websites with multiple subdomains under the same domain name.

    A company with multiple departments or services, such as a university with various schools and facilities (e.g., arts.university.edu, science.university.edu, library.university.edu), could benefit from using a wildcard SSL/TLS certificate.

  3. Multi-Domain (also known as Subject Alternative Name or SAN): A multi-domain SSL/TLS certificate allows you to secure multiple domains or subdomains with a single certificate. For example, a multi-domain certificate can protect www.example.com, www.example.net, and blog.example.com. This type of certificate is useful for organizations with multiple websites or subdomains that require SSL/TLS protection.

    A company that owns several brands or operates in different countries with localized domain names (e.g., www.brand.com, www.brand.co.uk, www.brand.de) could use a multi-domain SSL/TLS certificate to secure all their websites with a single certificate.

Here's a comparison table of the three domain types:

Domain Type Number of Domains Subdomains Example
Single Domain One One www.example.com
Wildcard One Unlimited *.example.com
Multi-Domain (SAN) Multiple Multiple www.example.com, www.example.net, blog.example.com

Certificate Authorities and Validity

Certificate Authorities (CAs) are organizations that issue SSL/TLS certificates after checking the domain and owner details. They help build trust between websites and users by verifying the identity of the certificate applicant.

When a website owner applies for an SSL/TLS certificate, the CA checks the applicant's domain ownership and, depending on the validation level, may also check their organization's identity. This process makes sure that the certificate is issued to the rightful owner of the domain.

To be trusted by operating systems and web browsers, CAs must follow requirements set by the CA/Browser Forum, an industry group that sets standards for certificate issuance. These requirements include:

  • Using secure certificate issuance and management practices
  • Regularly auditing their systems and processes
  • Maintaining a high level of security to protect their root certificates

Well-known Certificate Authorities

When you visit a website like https://www.example.com, your browser checks the SSL/TLS certificate and traces it back to the issuing CA. If the CA is trusted, and the certificate is valid, your browser establishes a secure connection to the website.

Certificate Validity Periods

SSL/TLS certificates have a limited validity period to reduce security risks. The CA/Browser Forum has gradually reduced the maximum validity period to improve security.

Effective Date Maximum Validity Period
Before March 2018 3 years (1095 days)
March 2018 2 years (825 days)
September 2020 13 months (397 days)

Shorter validity periods help reduce the risk of:

  • Misuse of compromised or stolen certificates
  • Outdated encryption methods being used for a long time
  • Unauthorized domain transfers or ownership changes

Website owners must renew their SSL/TLS certificates before they expire to maintain secure connections. Failing to renew a certificate before its expiration date can lead to:

  • Browser warnings saying that the website is not secure
  • Loss of user trust and potential loss of business
  • Lower search engine rankings, as search engines may flag the website as unsafe

Best Practices for Certificate Management

To maintain uninterrupted security, website owner should:

  • Monitor their SSL/TLS certificates' expiration dates
  • Set up reminders to renew certificates well before they expire
  • Use automated certificate management tools to simplify the renewal process

For example, if you have an e-commerce website that processes sensitive customer information, you must make sure that your SSL/TLS certificate is always valid to maintain trust and secure transactions. You can use a certificate monitoring tool that sends you reminders 30 days, 15 days, and 7 days before your certificate expires, giving you enough time to renew it without disrupting your website's security.