Understanding SSL/TLS Certificates: Types and Validation

An SSL/TLS certificate associates with various services, such as web and mail servers, to guarantee data integrity and secure communications. These certificates verify the identity of the origin server, preventing malicious actors from impersonating it to steal user data. Each certificate contains a pair of keys — a public key and a private key. The public key encrypts the transmitted information, and only the private key on the server can decrypt it, maintaining confidentiality throughout the data transfer.

Transport Layer Security (TLS) is the modern successor of Secure Socket Layer (SSL) technology. Although SSL is the older protocol, the term “SSL certificate” is still commonly used to refer to TLS certificates today.

This guide provides an overview of SSL certificate types, their validation methods, and a comparison between commercial, Let’s Encrypt, and self-signed certificates.

Types of SSL Certificates

There are multiple kinds of SSL certificates designed for different use cases and coverage levels. A basic SSL certificate secures one domain name, but some environments require protection for several domains, subdomains, or IP addresses.

The main categories of SSL certificates include:

• Single Domain Certificate
• Multiple Domain Certificate
• Wildcard Certificate
• IP Address Certificate

A single domain certificate secures one Fully Qualified Domain Name (FQDN) and all of its associated pages. For instance, a certificate for example.com secures both example.com and all its internal pages.

A multiple domain certificate covers several FQDNs under one certificate. Unlike a single domain certificate, it can protect multiple unique domain names, such as example.com and example.net, simultaneously.

A wildcard certificate secures all subdomains of a specific domain. For example, a wildcard certificate for example.com secures www.example.com, app.example.com, and any other subdomain under that domain.

An IP address certificate secures services that operate directly via an IP address rather than a hostname. This type functions similarly to a single domain certificate but uses an IP address instead of a domain name.

SSL Validation Levels

A Certification Authority (CA) verifies Certificate Signing Requests (CSRs) and issues certificates for specific subjects, such as domains or IPs. Self-signed certificates typically do not include any validation level since they are not verified by a CA.

The three primary SSL validation levels are:

• Domain Validation (DV)
• Organization Validation (OV)
• Extended Validation (EV)

Domain Validation (DV) is the most basic validation type. Let’s Encrypt only issues DV certificates. Here, the CA confirms ownership of the domain through an email, DNS TXT record, or a verification file in the site’s directory. DV certificates do not apply to IP addresses.

Organization Validation (OV) adds another layer of verification by confirming the legitimacy of the organization requesting the certificate. OV certificates provide higher user trust and are commonly used by businesses, government agencies, and institutions.

Extended Validation (EV) offers the highest level of verification. In addition to validating the organization, the CA confirms the entity’s physical presence, legal status, and exclusive rights. EV certificates provide the strongest assurance and are ideal for banks, enterprises, and high-trust websites.

Self-Signed SSL Certificates

Self-signed certificates are generated and signed using their private key without third-party validation. These certificates are often used in internal testing or development environments. Because they lack a CA signature, browsers mark them as untrusted by default — though they can be manually trusted on systems or browsers.

Validity: Custom

Available Types: Single Domain, Multiple Domain, Wildcard, Any IP Address

You can create a self-signed certificate with the openssl command-line utility available on most systems. It generates public and private keys to secure SSL/TLS services. Refer to the official openssl-req documentation for detailed instructions.

Self-signed certificates are completely free and suitable for non-public environments. They can secure any subject, including private IP addresses, and have no expiration limits. Additionally, multiple self-signed certificates can form a private CA (Certificate Authority) that can issue trusted certificates internally.

Let’s Encrypt SSL Certificates

Let’s Encrypt provides free SSL/TLS certificates validated at the DV level. These certificates have short validity periods to prevent misuse from compromised servers. Let’s Encrypt uses the Automated Certificate Management Environment (ACME) protocol to automate certificate issuance and renewal.

Validity: 90 days

Available Types: Single Domain, Multiple Domain, Wildcard

You can create and manage Let’s Encrypt certificates using the certbot ACME client. The HTTP-01 challenge verifies domain ownership for publicly accessible websites, while the DNS-01 challenge is used for internal or non-web environments. For more information, refer to the official Certbot documentation.

Note: Let’s Encrypt enforces rate limits on certificate issuance. If you’re testing, use the staging ACME server to avoid restrictions.

Let’s Encrypt certificates are trusted by all major browsers and are ideal for small websites, blogs, and applications. However, these certificates cannot be issued for IP addresses. Each certificate can include up to 100 hostnames or be configured as a wildcard certificate for subdomains. Due to ACME automation, Let’s Encrypt works perfectly for containerized environments such as Docker and Kubernetes.

Commercial SSL Certificates

Commercial certificates are issued by third-party CAs after validating subjects at the DV, OV, or EV levels. These certificates are universally trusted by browsers and provide different tiers of assurance depending on the validation type. Unlike Let’s Encrypt, commercial certificates are paid.

Validity: 1 year

Available Types: Single Domain, Multiple Domain, Wildcard, Public IP Address

To obtain a commercial certificate, you must purchase it from a vendor, submit a CSR, and select your desired validation method. Though the standard validity is one year, some vendors offer multi-year bundles with annual reissuance requirements.

Commercial certificates provide brand reputation and trust, particularly at higher validation levels (OV and EV). They are ideal for organizations, e-commerce platforms, and financial institutions that require strong verification. Moreover, they support IP address validation (for public IPs only) and are compatible with services not supporting ACME automation.

Prices vary depending on the certificate type and validation level, ranging from around $5 to several thousand dollars. DV-level certificates are the most affordable, while OV and EV certificates cost more due to additional verification procedures.

Conclusion

In summary, SSL/TLS certificates come in different types and validation levels, each serving distinct use cases. Internal or private services can rely on self-signed certificates, while public-facing systems should use CA-signed certificates for browser trust. Commercial SSL options provide the highest credibility and protection for organizations handling sensitive data, such as financial or e-commerce platforms.