What is Certificate-Based Authentication?

What is Certificate-Based Authentication?
What is Certificate-Based Authentication?

Authentication is the process of authenticating a user's or device's identity before allowing them to access a system or network. Various methods of authentication have been used over the years, such as passwords, multi-factor authentication with tokens, and biometrics.

However, these methods have weaknesses that make them vulnerable to attacks. Passwords can be guessed through brute force attacks or phishing schemes.

Hardware tokens can be lost or stolen. And biometrics have accuracy limitations.

What is Certificate-Based Authentication?

Certificate-based authentication provides enhanced security and usability compared to these traditional methods. A digital certificate contains unique encrypted data that can authenticate a user or device without exposing a password or other factors to attackers.

Certificate-based authentication utilises public key infrastructure (PKI) to allow secure authentication between two parties – a client and a server. The client holds a private key corresponding to the public key in their digital certificate authentication issued by a trusted certificate authority (CA). The server can verify the authenticity of this certificate through signature validation and certificate path discovery.

If the certificate is validated successfully, the client then proves ownership of the private key by using it to sign a challenge issued by the server digitally. This eliminates the risks associated with transmitting static passwords over networks. The mutual authentication process creates a secure channel for communication and access to applications and resources.

Certificate-based authentication brings important advantages like phishing resistance, ease of use, extensibility to third parties, and support for a zero-trust model. Organisations of all sizes can now leverage PKI and digital certificates to enhance their cybersecurity defences through certificate-based multi-factor authentication and other use cases.

What Is A Digital Certificate?

An electronic document that identifies and authenticates an entity like a user, device, or service on a network is A digital certificate. Digital certificates serve as digital identities and contain information about the certificate subject and issuer.

A digital certificate contains data fields with information that the certificate's issuer has digitally signed. This allows any entity receiving the certificate to verify its authenticity and validity. Digital certificates utilise public key cryptography, which uses key pairs that include public and private keys.


Digital certificates typically contain the following components:

  • Subject: The identity of the entity being certified, which can be a user, device, server, or other item. This includes fields like common name, organisation, country, etc.
  • Issuer: The certificate authority (CA) that verified the subject's identity, generated their crucial pair and issued the digital certificate.
  • Public Key: The public key belonging to the subject corresponds to the private key stored separately. Others can use this key to encrypt data destined for the subject.
  • Validity Period: Date stamps indicating the digital certificate's validity period.
  • Signature Algorithm: The encryption algorithm used by the CA to digitally sign and validate the authenticity of the certificate.

Different Types Of Certificates

Some common types of digital certificates include:

  • SSL/TLS Certificates: Enables secure HTTPS websites and web traffic encryption.
  • Client Certificates: Authenticates users and devices to servers/networks.
  • Code Signing Certificates: Verifies authenticity and integrity of software code.
  • Email Certificates: Used for encrypted email and proving identity.
  • Device Certificates: Authenticates IoT and other connected devices.

The structure and contents of digital certificates may vary slightly based on their intended function and the standards they adhere to.

How Certificate-Based Authentication Works

By leveraging digital certificates, certificate-based authentication provides enhanced security between a client (user, device, etc.) and a server.

The Client Requests Access to the Server.

The authentication process is initiated when a client attempts to access a server's protected resource, application, or network. This access attempt triggers an authentication request from the server to the client. The client then sends its unique identifier, typically a pre-registered username, to the server in response.

Server Presents Certificate

Upon receiving the client's identifier, the server presents its public key certificate to the client. This digital certificate contains identifying information about the server validated by the certificate authority (CA), including the issuing CA, server credentials, the public key used for encryption, and more. The certificate also contains a CA digital signature, which allows the client to verify its authenticity and that it has not been tampered with.

Client Validates Certificate

The client receives the public key certificate presented by the server and performs validation checks on it before continuing the authentication process. These checks typically include verifying the CA digital signature to confirm it was signed by a trusted CA, checking the certificate path back to a trusted root CA, and ensuring the certificate has not expired or been revoked.

Secure SSL/TLS Channel Established.

After the server's certificate has been validated, the client and server negotiate a secure encrypted session using encryption protocols like SSL (Secure Sockets Layer) or TLS (Transport Layer Security). Establishing this encrypted session protects the confidentiality and integrity of all further communications between the client and server throughout the authentication process.

Mutual Authentication Occurs

With a secure channel now established, the server requests client authentication to verify the client's identity. In response, the client presents its client certificate to the server. This certificate contains identifying details about the client and its public key, digitally signed by a CA.

Leveraging public key cryptography, the client proves ownership of the private key associated with the certificate by digitally signing a server-issued challenge. After the client's certificate ownership is verified, the client is successfully authenticated to the server.

This mutual authentication process assures both the client and server that they have cryptographically proven their identities to each other before being granted access to protected resources.

After successful mutual authentication, the client can now securely access authorised applications, networks, and data on the server. This authentication process increases security and trust while enabling verified access.

Benefits of Certificate-Based Authentication

There are several notable benefits of employing certificate-based authentication instead of traditional username and password systems:

  • Enhanced security

Certificate-based authentication greatly enhances security by eliminating the use of static passwords. Private keys are stored securely and not transmitted or exposed even to the user. Mutual authentication also ensures that the client and server are verified before access is granted.

  • Non-repudiation

By cryptographically proving identity through private key signatures, certificates provide strong non-repudiation assurances. It confirms that the entity that accessed resources or performed a transaction is the one bound to the certificate.

  • Flexible deployment options

Certificate-based solutions can be deployed on-premises or in the cloud as a managed service. They integrate well with existing infrastructures and scale as needed to grow with the organisation. Automated deployment and configuration further add to the flexibility.

  • Scalability

Certificates scale seamlessly alongside growth in users, devices, applications, and other resources being secured. Certificate lifetimes can be adapted as needed, and automated certificate lifecycle management further enables scaling without compromising security or increasing overhead for IT teams.

Implementation Considerations

Implementing certificate-based authentication requires upfront planning and configuration across servers, clients, networks, policies, and security controls to ensure smooth and secure operations long-term.

Some key implementation considerations include:

Obtaining and Managing Certificates

A reliable and efficient process must be established for requesting, validating, issuing, distributing, renewing, replacing, and revoking certificates. Utilising a dedicated Certificate Management System (CMS) can help greatly automate many crucial certificate lifecycle functions at scale.

Carefully vetted and globally trusted Certificate Authorities (CAs) should be used to issue digital certificates to maximise trust across all relying parties.

Configuring Servers and Clients

Servers need to be configured to validate client certificates. Map certificate attributes to local user accounts, enable client authentication on access-controlled applications and services, integrate with LDAP or AD backends to obtain additional user data, and more.

Client desktops, laptops, mobiles, devices, etc., need user certificates installed and properly configured to enable authentication to servers protected by certificate-based access controls.

Revocation Checking

All servers validating certificates must frequently check Certificate Authority revocation lists (CRL) and/or use OCSP (Online Certificate Status Protocol) to check the real-time status of a presented certificate to ensure it has not been revoked.

This prevents users and devices with revoked certificates from accessing protected resources.

Root Certificate Authorities

The public key infrastructure needs vetted and globally trusted root Certificate Authorities to serve as anchors that create the foundation of trust.

These root CAs then issue subordinate intermediate CAs who issue end-entity certificates to users and devices. Certificates issued ultimately by trusted roots are critical in enabling inherent trust in all other components of the infrastructure.

Key Storage and Recovery

Private keys underlying issued certificates should be safely stored and recovered from secure locations like smartcards and hardware security modules (HSMs) to prevent extraction or duplication.

However, backup and key escrow mechanisms need to be established to protect against the loss of keys due to forgotten PINs or damaged tokens.

Certificate-Based Authentication Pros and Cons

While certificate-based authentication delivers substantial security and usability advantages over passwords and other legacy authentication methods, it is an advanced approach that may not suit every organisation's risk tolerance, capabilities, or resources.


  • Enables secure virtual deployment: Certificate infrastructure and management platforms can now be deployed entirely through cloud services, reducing hardware requirements down to the trust anchors embedded in devices like smart cards or TPM chips. This is far more cost-effective than traditional physical smart card readers and terminals.
  • Convenience via single sign-on: By issuing user and device certificates and configuring applications to leverage them for authentication, users can seamlessly authenticate across services without managing multiple passwords. This offers tremendous convenience.
  • Resilient to phishing: With no static passwords that can be phished through deception, certificate-based authentication shuts down this very common attack vector that easily circumvents passwords and two-factor methods relying on SMS or emailed one-time codes.
  • Provides mutual authentication: Both user and device clients, as well as servers, authenticate via public key cryptography mechanisms, providing mutual assurance and allowing suspicious anomalies to be flagged through deviations from known trusted identities and patterns.
  • Easy to provision for 3rd parties: Partners, vendors, and contractors can be seamlessly issued appropriately scoped certificates to enable authentication to internal resources. No need to handle credentials externally.
  • Built-in large-scale administration: Certificate management systems allow all facets of credentials, like policy-based automated issuance, renewal, and revocation, to be handled through a centralised administration console designed expressly for scale.


  • Significant startup and licensing costs: While cloud services reduce hardware costs, the PKI software, certificate management systems, and adequate staffing administration require steep upfront and ongoing investments that keep costs high.
  • Management overhead persists: Ongoing certificate lifecycle management functions for a large installation still demand skilled PKI administrators to craft policies, handle exceptions, implement extensions like SSO integrations, and maintain performance. This recurring cost can be substantial.
  • Overkill for lower-risk scenarios: Password managers provide sufficient defences for lower-risk use cases. Opting for a more advanced model like certificate-based authentication should depend on assessed risk levels and threat profiles.
  • No user anonymity: All authentication certificates depend fundamentally on binding identifiable attributes to individuals or devices.

Common Use Cases

User Authentication

Logging into corporate applications and networks often requires cumbersome passwords that are difficult to remember and prone to security vulnerabilities. This frustrates employees and creates opportunities for hackers.

Certificate-based authentication provides a seamless log-in process for users by verifying their identity based on the certificate stored on their device. This eliminates the need to enter passwords while still restricting access to authorised users and devices. Employees can access everything they need to begin their day.

Mobile Device Authentication

Allowing employees to bring their mobile devices can help increase productivity. However, securing the corporate network and data from unauthorised personal devices is challenging.

Certificate-based authentication assigns a unique digital ID to each employee-owned device, which must be verified each time it attempts to connect to the corporate Wi-Fi or access cloud apps. This ensures that only approved personal devices can access sensitive company information.

Machine Authentication

Public-facing devices like payment kiosks present security risks due to their connectivity to backend systems. If compromised, hackers could steal customer data or interrupt service.

By requiring certificate-based authentication for all machine-to-machine communications, organisations can ensure that only approved devices are able to access the network and transmit sensitive information. This reduces the risk of data breaches or service disruptions.

Alternatives to Certificate-Based Authentication

While certificate-based authentication has many security and usability benefits, alternative authentication approaches do exist that may be better suited for some use cases:

Compared to Passwords, Tokens, Biometrics, etc.

Passwords have been the dominant authentication mechanism for decades owing to their universal familiarity and convenience. However, longstanding security issues like vulnerability to guessing, brute force attacks, and phishing make them less than ideal, especially for high-risk scenarios.

Multi-factor authentication token methods like one-time passwords (OTPs) and smart cards add an additional layer of security needing to physically possess the token. However, distribution, management, and replacement of hardware tokens add cost and operational overhead.

Biometrics like fingerprints and facial recognition provide uniqueness tied to individual users. However, they come with accuracy constraints in real-world conditions and a lack of secrecy since physical attributes cannot be revoked if compromised.

Certificate-based authentication offers superior security against modern threats like phishing without introducing major usability barriers for end users compared to these alternatives when implemented properly.

Use Cases Where Certificates May Not Be Suitable

For protecting low to medium-risk scenarios with very limited users and devices, sole reliance on complex passwords may still suffice, especially when convenience outweighs enhanced security.

Disposable one-time passcodes and similar methods work reasonably well for securely authenticating external users who lack device control, like customers and remote contractors.

In some niche cases centred around ensuring complete user anonymity, certificate-based authentication may be eschewed in favour of less accountable mechanisms.

Some notable trends shaping the future of certificate-based authentication include:

Increasing Adoption of TLS and SSL

The rising prioritisation of privacy protections for websites and applications is driving increasing Transport Layer Security (TLS) and Secure Sockets Layer (SSL) usage, which rely on certificates. Moreover, growth in connected devices and mobility generating exploding network traffic has reinforced the need for encryption.

Growth in IoT Devices and Transient Environments

Provisioning and de-provisioning authentication credentials efficiently are critical with the rapid emergence of transient IoT devices, short-lived compute instances and ephemeral authentication use cases. Just in time, automated certificate issuance and revocation will grow in importance.

The Potential Impact of Quantum Computing

Looking ahead, quantum computers may one day possess the potential to break widely used RSA and ECC-based cryptographic protocols. Future-proof schemes like lattice-based cryptography are already being assessed to maintain security in an eventual post-quantum world.


Certificate-based authentication enables organisations to securely identify users and devices attempting to access corporate networks and data. By issuing digital certificates and requiring them for authentication, companies can allow access to only approved users and machines.

InstaSafe seamlessly integrates with existing systems to verify user identities and stop attackers in their tracks.

Utilising Instasafe’s multi-factor authentication alongside certificate-based authentication provides robust security for all users and devices.

Frequently Asked Questions (FAQs)

  • What is an example of a certificate authentication?

An example is using a digital certificate stored on a device to authenticate a user logging into a corporate application rather than entering a password.

  • What is the purpose of certificate client authentication?

The purpose is to authenticate client devices and applications to servers/services using certificates rather than less secure methods like passwords. This proves the client's identity and enables secure encrypted communication.

  • Is certificate-based authentication considered MFA?

Certificate authentication relies on a single factor - the digital certificate stored on the user's device - to validate their identity. In contrast, multi-factor authentication (MFA) requires at least two factors, such as a password AND a security token. While certificate authentication can be combined with an additional factor like a password to enable two-factor authentication, on its own, it only utilises one factor.