clientcertificaten

Client certificates are digital credentials used to authenticate clients to servers in TLS communications, forming the core of mutual TLS (mTLS). An X.509 certificate issued to a client binds a public key to an identity and is issued by a trusted certificate authority. The certificate contains identifying information, a public key, a validity period, and usage attributes that designate it for client authentication.

During a TLS handshake configured for mTLS, the server requests a client certificate. The client sends a certificate chain that leads to a trusted root, and the server verifies the chain and the certificate’s validity. The client proves possession of the corresponding private key, typically by signing handshake data. If verification succeeds, the client is authenticated and a secure channel is established with both sides optionally authenticating each other.

Client certificates are issued by public or private PKI and are commonly stored on client devices in software keystores or on hardware tokens, such as smart cards or USB tokens. Revocation is supported via Certificate Revocation Lists (CRLs) or Online Certificate Status Protocol (OCSP). Regular renewal, secure key protection, and trustworthy certificate authorities are essential for a functioning PKI-based authentication system.

Use cases include enterprise VPNs and web services APIs that require strong client authentication, IoT devices that need identity, and service-to-service communication within secured networks. While offering strong security guarantees, client certificates introduce management complexity and require careful provisioning, storage, and revocation processes to maintain trust.