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As security is essential in communications via electronic networks, there is a need to develop a structure that provides high levels of security. Public Key Infrastructure is a way to provide security by implementing the means of key pairs between two parties. Using a public key infrastructure and digital certificates can help you develop a secure network infrastructure for user access, eliminate threats, and keep data secure. let’s discuss more about digital or PKI certificates and types of digital certificates.


PKI provides a way to identify people, applications, and devices while offering robust encryption to keep the communication private between both parties. Besides identification and authentication, PKI offers digital certificates and signatures to generate unique credentials for the certificate holders and their validation.

In this article, we are going to start from what is PKI, What is PKI certificates, and cover different types of digital certificates with their usage. So let’s get started.

What Is PKI?

Public Key Infrastructure (PKI) is the technology behind digital certificates. These digital certificates prove identification and give certain allowances. PKI technology lets users encrypt data, sign documents digitally, and authenticate themselves using certification. It’s the most commonly used encryption technology, consisting of a public and a private key. Anyone with the public key can encode the message, and only the person with a private key can decode that message.

Users, apps, and devices use public and private keys for encryption and decryption. You can establish secure connections for web pages, internal Wi-Fi, VPN, and many other MFA-supported devices through this technology. Today, organizations depend on PKI technology for managing security via encryption. A common example of PKI security is the SSL certificate on websites. It is to ensure visitors that they are sending information to the intended recipient and their data will be encrypted. 

What Is A PKI Certificate?

It’s a digital certificate used to authenticate users, devices, and servers online. It is commonly used to sign documents, code, or email. PKI certificates help to encrypt data and communication while transmitting on an untrusted network. These certificates are issued by the Certificate Authorities and perform these primary operations.

● Verify the identity of the sender or receiver of an electronic message.

● Provide ways to encrypt and decrypt the message between the sender and the receiver.

Certificate Authorities or CA are the entities being trusted to issue digital certificates. There is a need for a trusted party for validating the users to which these certificates are issued. Everything breaks down if the CAs do not verify entities properly. 

A digital or PKI certificate has the following information.

  • It contains information about a host, doamin, or an individual.
  • Issued from a trusted third party.
  • Equivalent of a passport or a driver’s license.
  • Contains information proving its authenticity.
  • It is tamper-resistant.
  • Has an expiration date.
  • Can be presented to someone for validation.


See Also How to Fix CVE-2023-20864- A Critical Logs Deserialization Vulnerability in VMware Aria?

Common fields in the PKI certificates

Here are some of the most common fields in the PKI certificates. 

  • Serial Number: It is used to identify the certificate uniquely within a CA’s system. Specifically, it is used to track revocation information.
  • Issuer: It is the entity that validates the information and signs certificates.
  • Subject: It is the entity to which the certificate belongs, such as an individual, a machine, or an organization.
  • Not Before: The earliest date and time on which a certificate is valid. Generally set to a few hours or days before the certificate was issued for avoiding clock skew problems.
  • Not After: The date and time when the certificate is no longer valid.
  • Public Key: It’s a key that belongs to the certificate subject.
  • Key Usage: It’s the valid cryptographic use of the public key of the certificate. Common values include key encipherment, digital signature validation, and certificate signing.
  • Extended Key Usage: The applications in which the certificate may be used, such as email protection, TLS server authentication, and code signing.
  • Signature: The body of the certificate is hashed, and then the hash is encrypted with the issuer’s private key.
  • Signature Algorithm: It contains an encryption algorithm and a hashing algorithm. For example, “sha256RSA” where RSA is the encryption algorithm, and sha256 is the hashing algorithm.

What Are The Different Types Of PKI Certificates?

The common types of digital certificates are as follows.

#1. TLS/SSL Server Certificate

A TLS/ SSL server certificate is a PKI certificate issued to a server for two primary purposes_ to establish a secure communication channel with the user and to authenticate the identity of the server. A web server certificate or a server security certificate is more commonly known as TLS/SSL server certificate. These certificates have two main purposes.

  • It creates an encrypted communication channel between the client and the server.
  • It validates the server identity before authenticating


1.1. Domain Validation SSL

This type of PKI certificate needs the least stringent of all kinds of validation. With domain validation, the certificate authority verifies the applicant’s domain ownership rights. A Domain Validation (DV) is inexpensive and may be issued in minutes.

1.1.1. Single Domain SSL

A single-domain SSL certificate secures one fully-qualified domain name in a certificate.

1.1.2. Multi-Domain SSL Certificate

Multi-domain SSL certificates, also known as UCC/SAN certificates, cover various domains in one certificate. For example,,, and can be secure under a single certificate.

1.1.3. Wildcard SSL Certificate

A wildcard SSL certificate uses a single certificate to secure an unlimited number of first-level sub-domains. Let’s say a * can cover,, etc. If you want to secure second-level subdomains, you need to purchase a second wildcard or get the next type of certificate.

1.1.4. Multi-Domain Wildcards

It’s the most versatile of all types of digital certificates. A multi-domain wildcard certificate is a combination of a SAN certificate and wildcard SSL. It can secure up to 250 domains and different levels of accompanying subdomains in a single certificate.

1.2. Organization Validation SSL

Organization validation SSL certificates are designed for enterprise infrastructure. For organization validation, the Certificate Authorities not only assess the user rights to the domain name but also perform additional investigations to regulate the organization’s legitimacy at a basic level. It will take a day or two to issue an OV certificate.


1.3. Extended Validation SSL

As the name suggests, extended validation SSL certificates entail a more robust vetting process. The certificate authority conducts an intense validation where the user is required to submit authentic documents to prove that they run a genuine business. Generally, it takes time to issue an EV SSL certificate because the company needs to be thoroughly vetted.

#2. TLS/SSL Client Certificate

TLS/SSL client certificates are not as common as server certificates and are used to validate the client connecting to a TLS service. For example, to provide access control. As most services give access to users instead of devices, most client certificates comprise the username rather than a hostname and an email address. 

However, authentication is generally managed by the service provider TLS/SSL client certificates are not issued by a public CA, providing server certificates. A service operator requiring client certificates will operate their internal CA to issue them. These certificates are supported by many web browsers. However, many services use cookies and passwords to authenticate users rather than client certificates.

2.1. User Certificate

This client authentication certificate is used to authenticate users during an SSL handshake. It validates users accessing a server by exchanging the user authentication certificate.

2.2. Machine Certificate

Machine certificates, also known as computer certificates, give the ability to machines to do something instead of the users. The purpose of machine certificates is authentication. Certificates are the source through which public keys are exchanged in a PKI. The certificates involved with these exchanges are generally overseen by the operating system. 

#3. Email Certificate or S/MIME Certificate

An email certificate, also known as the S/MIME certificate, is used to establish secure communication between email clients. It secures data while in transit or at rest by encrypting the email data before it leaves your email client. This type of digital certificate increases email security by letting you sign your emails digitally and encrypt them. It ensures the confidentiality and integrity of the email content, gives assurance about the identity of the sender, and provides prevention from eavesdropping.

#4. EMV Certificate

The EMV certification ensures that merchants accept cards with chip technology. It adds an extra security layer during card-present transactions. EMV lets issuers assert that they are processing the user’s card instead of a copied version. Issuers can enforce a liability shift with this confidence and take on any risk while processing a transaction that is EMV compliant. The EMS CA certificates are loaded on POS cad terminals, or ATM is used to validate the card issuer certificate.

#5. Code-Signing Certificate

A Code-signing certificate is a digital certificate that ensures the code authenticity used to develop software applications. Code signing authenticates the software author and proves that the signed code has not been tampered with or altered after it was signed. These types of PKI certificates are used to secure downloadable software, such as applications, device drivers, scripts, and executables.

#6. Qualified Certificate

A qualified certificate is a PKI certificate issued by a qualified trust service provider ensuring the data integrity and authenticity of an electronic signature and its attached data or accompanying message. eIDAS describes multiple tiers of electronic signatures that can be used to conduct public and private transactions within and across the EU member states’ borders. A qualified digital certificate is required to elevate the electronic signature status to that being considered a qualified electronic signature. 


#7. Chain Certificate

When you visit a website using a secure connection, it sends a digital certificate to the browser. The browser then compares the issuer with a list of trusted CAs. If there is no match, the client browser checks whether a trusted root CA signs the issuing certificate. The chaining engine of the browser continues to verify each certificate’s issuer until it finds a trusted root. The chain of trust certifications proves that a particular certificate originates from a trusted source. 

7.1. Root Certificate

It’s a self-signed certificate following the X.509 certificate standards. A multi-level hierarchical chain of trust allows web applications and clients to verify a trusted source has authenticated the identity of the end-entity. A certificate authority can issue several certificates in a tree structure form. A root certificate is a top-most certificate whose private key is used to sign other certificates. And if this private key is compromised, the certificates signed under this key will be compromised. It will lead to the re-issue of the new certificates to every intermediate CA and end-entity in the certificate chain. 

7.2. Intermediate Or Subordinate Certificate

At least one intermediate certificate is always present in an SSL certificate chain. It provides an essential link to enable Root CA to extend the trustworthy reputation to untrustworthy end-entities. An intermediate or subordinate certificate must be signed by a root certificate or another intermediate certificate.

#8. Self-Signed Certificate

A self-signed certificate is the type of PKI certificate that is not signed by the Certificate Authority (CA). These certificates do not cost money and are easy to make. However, they do not give all security properties that are provided by the certificates signed by a CA. For example, when a site owner used a self-signed certificate to give HTTPS services, users visiting that website will see a warning. And visitors bypassing these warnings become vulnerable to a risk that a third party could stop traffic to the site using their own self-signed certificate. However, website visitors that used a CA-signed certificate do not see these warnings. 


PKI technology is a revolution in electronic communication. Organizations or individuals need to use an appropriate PKI certificate to protect themselves and their users from various potential cyberattacks, such as email spoofing, man-in-the-middle, distributed-denial-of-service (DDoS) attacks, phishing attacks, and session hijacking. In this article, we have discussed different types of PKI certificates. You can get one based on your requirements. Otherwise, one potential cyberattack is enough for an organization to lose its data and reputation and millions of dollars in non-compliance penalties.

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