Demystifying public certificates vs. private certificates: Key pillars of digital security

What are public certificates?

Public certificates, also called public key certificates or digital certificates, prove the ownership of a public key in the context of a public key structure (PKI). These types of certificates include data about the key, the identity of the key owner (the subject), and the digital signature of the entity confirming the certificate’s contents (the issuer).

The credibility of the public key is fundamentally anchored to the certificate issuer’s trustworthiness. If the issuer is deemed trustworthy, users can confidently determine that the public key is the rightful property of the associated entity.

The most common type of public certificate is the SSL/TLS certificate used for secure web communications. While SSL is still a widely used term and was once standard for secure connections, it is now considered deprecated due to inherent vulnerabilities. The more secure TLS protocol replaces it. This certificate contains the website’s public key plus information about the website’s owner. It also includes the certificate authority (CA) signature that the browser trusts. 

When a web browser connects to a website, the site sends its SSL certificate. The browser checks this certificate to ensure that a trusted CA issued it and matches the website it connects to.

Some other common use cases for public certificates include:

• Secure web browsing (HTTPS) : Websites use SSL/TLS certificates to establish secure connections with browsers and confirm their identities. For example, browsers use the site's digital certificate to establish an encrypted connection when users visit an online banking portal.

• Email signing and encryption (S/MIME) : S/MIME leverages digital certificates to sign and encrypt email. The email sender uses their private key to sign the email, and the recipient uses the sender’s public key (obtained from the public certificate) to verify the signature. For encryption, the sender uses the recipient’s public key to encrypt the email, and the recipient uses their private key to decrypt it.
• Code signing : Software developers often use digital certificates to sign software applications. They verify to users that the software is from the stated developer and hasn’t been tampered with since signing. They offer assurance for end users that the software is genuine and safe.

What are private certificates?

Private certificates are digital documents issued by an internal or private CA. They primarily function within a restricted environment, often confined to a single organization or group of known entities. 

While globally trusted CAs recognize public certificates, private certificates are not inherently trusted by browsers or devices. As a result, they must be manually configured within the specific devices or software intended to recognize them. They are most commonly used in an intranet or private network to secure internal communications, authenticate servers and clients, and establish secure connections within the organization.

A distinct characteristic of private certificates is the level of control it gives the issuing entity. An organization employing private certificates has complete autonomy over its certificate policies, including issuance, revocation, and renewal.

In addition, private certificates are critical in establishing and maintaining secure communication channels within the boundaries of an organization or known group. They enhance the overall security posture of an organization by enabling secure internal connections. Their applications are diverse and often mirror those of public certificates but within a defined scope. 

Private certificate deployment also offers potential cost reductions. Since organizations can issue them, they can avoid purchasing certificates from public CAs.

Some common use cases for private certificates include:

• Internal websites and applications. Private certificates can secure communications within internal websites and applications. For example, a company might leverage private certificates to enable HTTPS on an internal employee portal.

• VPNs. Private certificates can be used for client and server authentication in a VPN scenario. This ensures that only trusted devices with the appropriate private certificate can connect to the corporate VPN.
• Inter-organizational communication. Partner companies can manually configure their systems to accept each other’s private certificates.

Public certificates vs. Private certificates: How are they different?

Public and private certificates differ in their specific purposes and scope of use. 

Some of their most significant similarities are:

• PKI-based. Both are based on PKI. They use asymmetric cryptography, consisting of a public and private key.
• Contained information. Public and private certificates contain similar information, including the owner’s name, the certificate’s serial number, expiration date, a copy of the owner’s public key, and the issuer's digital signature.
• Purpose. They both have the same fundamental purpose: to authenticate the entity identity (whether a person, server, company, etc.) and to facilitate encryption and digital information signing.

However, they do have critical differences:

• Issuer. Publicly trusted CAs issue public certificates, whereas private certificates are issued by internal CAs within organizations or a private CA designated by the company.

• Trust level. Web browsers and operating systems inherently trust public certificates because trusted CAs issue them. However, private certificates are not automatically trusted and need to be manually installed or configured on the devices or systems that should trust them.
• Scope. Public certificates are used on public servers and intended to be trusted by the general public (like a website on the internet). Private certificates are often used within a single organization or between known parties.
• Cost and control. Most public certificates require a subscription fee, and the issuing CA controls the certificate policies. Organizations can issue private certificates at no cost (beyond the infrastructure and management costs) and maintain complete control over certificate policies.

While they have a similar structure and purpose, the key difference between public and private certificates lies in their level of trust and where they’re used. Public certificates are designed for broader, public-facing applications, whereas private certificates are designed for a more controlled, private environment.

Finding the right certificates for your business

Public and private certificates play crucial roles in today’s digital landscape. While public certificates establish trust on the internet and secure interactions with public-facing servers, private certificates provide a cost-effective security solution for internal, inter-organizational, or specific environment systems. Understanding the nuances of both is essential for identifying the right strategy to meet your organizational security needs.

Ready to implement or upgrade your certificate management strategy? Sectigo's team of experts can provide your organization with the guidance and tools to effectively use public and private certificates in your environment, ensuring robust and reliable security. Contact us today to know more.