How Do You Verify That the Client Certificate Is Trusted by the Web Server?


Scott Campbell

How Do You Verify That the Client Certificate Is Trusted by the Web Server?

When it comes to securing web applications, client certificates play a crucial role in establishing trust between the client and the server. These certificates are used to authenticate the identity of the client and ensure that only trusted clients can access sensitive resources.

But how do you verify that a client certificate is trusted by the web server? Let’s explore the steps involved in this process.

Step 1: Obtaining the Client Certificate

Before verifying the trustworthiness of a client certificate, it’s essential to obtain it first. Typically, client certificates are issued by a trusted certificate authority (CA).

The client needs to generate a key pair (public-private key) and submit a certificate signing request (CSR) to the CA. The CA then verifies the identity of the applicant and signs their public key with their own private key, issuing a digital certificate.

Step 2: Checking Certificate Authority Trust

The first step in verifying a client certificate is to check if its issuing CA is trusted by the web server. The web server maintains a list of trusted CAs or their root certificates.

This list ensures that only certificates signed by these trusted authorities are accepted. If the client’s certificate was issued by an untrusted or unknown CA, it won’t be considered valid.

Checking Root Certificates:

To check if an issuing authority’s root certificate is trusted, you can compare its details with those stored on your server using cryptographic algorithms. If there is a match, you can proceed with further verification.

Step 3: Certificate Revocation Check

Even if a client certificate is issued by a trusted CA, it’s crucial to check if the certificate has been revoked. Certificate revocation occurs when the CA determines that a certificate is no longer valid, for example, due to compromise or expiration.

To ensure the client certificate is still valid, the web server needs to verify its status against a certificate revocation list (CRL) or an online certificate status protocol (OCSP).

Using Certificate Revocation Lists (CRL):

A CRL contains a list of revoked certificates issued by a specific CA. The web server can periodically download and store CRLs from trusted authorities.

During client certificate validation, the server checks if the client’s certificate is included in any of the CRLs it possesses.

Using Online Certificate Status Protocol (OCSP):

OCSP provides real-time status information about certificates directly from the issuing CA. The web server can send a request to the CA’s OCSP server and receive an immediate response regarding the validity of the client’s certificate.

Step 4: Checking Certificate Attributes

Apart from verifying trust and revocation status, it’s important to check other attributes of a client certificate to ensure its authenticity. Some common attributes include:

  • Expiration Date: Verify that the client’s certificate is not expired.
  • Key Usage: Ensure that the key usage specified in the certificate aligns with your application’s requirements.
  • Certificate Policies: Check if any specific policies are associated with the client’s certificate.

Step 5: Chain of Trust Verification

A client certificate may be part of a chain of trust, where intermediate CAs sign the client’s certificate, which in turn is signed by a root CA. To ensure the entire chain is trusted, the web server needs to verify each certificate in the chain.

This includes checking the signature of each certificate and verifying its parent-child relationship.


Verifying that a client certificate is trusted by the web server involves several critical steps. By checking the issuing CA’s trust, revocation status, certificate attributes, and ensuring a valid chain of trust, you can establish confidence in the authenticity and integrity of client certificates.

Implementing these verification mechanisms strengthens the security of your web applications and protects sensitive data from unauthorized access.

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