Client Certificates vs. Server Certificates: Key Differences

What Is an X.509 Certificate?

X.509 is a standard format for digital certificates used in Public Key Infrastructure (PKI). An X.509 certificate binds a public key to an identity and contains information such as:

• Subject – the identity associated with the certificate.
• Issuer – the Certificate Authority (CA) that issued it.
• Public key – the public key associated with the certificate holder.
• Validity period – the dates during which the certificate is valid.
• Extensions – additional information defining permitted uses and certificate constraints.

The certificate contains the public key, while the corresponding private key is stored separately and must be protected by its owner.

X.509 certificates are used for different purposes, including TLS server authentication, TLS client authentication, S/MIME email protection, and code signing.

What Is a Server Certificate?

A server certificate is an X.509 certificate used to authenticate a server to a connecting client, such as a web browser or another application.

For example, when a user visits an HTTPS website, the server presents its TLS certificate during the handshake. The browser checks whether:

• The certificate is valid and within its validity period.
• The certificate chains to a trusted Certificate Authority.
• The requested hostname matches the certificate's Subject Alternative Name (SAN).
• The certificate is permitted for server authentication.

The corresponding Extended Key Usage (EKU) identifier is:

1.3.6.1.5.5.7.3.1 – Server Authentication (serverAuth).

After successful authentication, the TLS handshake establishes shared session keys used to encrypt and protect the connection.

What Is a Client Certificate?

A client certificate is an X.509 certificate used to authenticate a client to a server. The client may be a user, device, application, or another server.

Client certificates are commonly used in mutual TLS (mTLS), where both sides authenticate each other before exchanging application data.

During an mTLS handshake:

1. The client connects to the server.
2. The server presents its certificate, which the client validates.
3. The server requests a client certificate.
4. The client presents its certificate and proves possession of the corresponding private key.
5. The server validates the client certificate against its configured trust store and authentication policies.
6. If authentication succeeds, the TLS connection is established.

The corresponding EKU identifier is:

1.3.6.1.5.5.7.3.2 – Client Authentication (clientAuth).

Client certificates are commonly used for:

• API and application authentication.
• Server-to-server communication.
• Device and machine identity.
• Financial services and payment systems.
• Secure connections between business partners.

Unlike a password-based login, certificate-based authentication uses cryptographic proof that the client possesses the private key associated with its certificate.

What Is Extended Key Usage (EKU)?

Extended Key Usage (EKU) is an X.509 certificate extension that identifies the purposes for which a certificate may be used.

Common EKU identifiers include:

A certificate may contain one or more EKU purposes, depending on its issuance profile. If both Key Usage and EKU extensions are present, applications must evaluate them together.

The anyExtendedKeyUsage identifier does not guarantee that a certificate will be accepted for every purpose. Applications may require a specific EKU.

How to Check a Certificate's EKU?

You can inspect the EKU extension using OpenSSL:

openssl x509 -in certificate.crt -text -noout

Look for the X509v3 Extended Key Usage section. It will show the purposes permitted by the certificate, such as TLS Web Server Authentication or TLS Web Client Authentication.

You can also view certificate details in your operating system or browser certificate viewer.

Client Certificate vs. Server Certificate: Key Differences

Although both certificates use the X.509 format, they serve different authentication roles.

Neither certificate replaces the other. They authenticate different parties and are used together in mTLS when mutual authentication is required.

How Are Client and Server Certificates Used Together?

The most common example is mutual TLS (mTLS).

In standard HTTPS, the server authenticates itself to the browser using a server certificate. The browser does not normally need to present a client certificate.

With mTLS, the server also requests a client certificate. Both parties verify each other's identities before the application connection proceeds.

For example, a business may use mTLS to secure communication between its API and an external payment provider. The API presents its server certificate, while the payment provider presents a client certificate to prove its identity.

This provides authentication in both directions and helps prevent unauthorized systems from accessing protected services.

Changes to Client Authentication in Public TLS Certificates

Public TLS certificates have traditionally been issued with both Server Authentication and Client Authentication EKUs in some cases. However, this is changing.

DigiCert will remove Client Authentication EKU from its public TLS certificate issuance process on March 1, 2027. After that date, new certificates, renewals, reissues, and duplicates will no longer include this EKU.

This change primarily affects organizations that use public TLS certificates for mTLS or other client authentication scenarios. Standard HTTPS websites that use certificates only for server authentication are not affected.

Organizations that require client authentication should review their certificate configurations and consider alternatives such as X9 PKI for TLS or Private PKI.

Conclusion

Client certificates and server certificates are both essential components of certificate-based authentication, but they perform different roles.

Server certificates authenticate servers to clients and are fundamental to HTTPS. Client certificates authenticate users, devices, or applications to servers and are commonly used in mTLS.

Understanding their differences, EKU extensions, and trust requirements helps organizations choose the right certificate type and configure secure connections correctly.