MTLS

Mutual TLS (mTLS) is a variant of Transport Layer Security in which both the client and the server authenticate to each other using X.509 certificates during the TLS handshake.

In an mTLS handshake, each party presents its certificate, and each certificate is validated against a trusted certificate authority (CA) or a configured trust store. If both certificates are valid, the handshake completes and a secure channel is established. Certificates may be issued by an internal enterprise CA or by a public CA, and may be designed with short lifetimes to limit risk. Revocation can be handled via CRLs or OCSP.

Typical use cases include service-to-service authentication in microservice architectures and API security, zero-trust networks, IoT devices communicating with backends, and internal applications where strong identity verification is important.

Deployment considerations include the need for a PKI infrastructure to issue, renew, and revoke certificates, and trust stores on both clients and servers. mTLS can be configured at the application level or enforced by network proxies, load balancers, or service meshes (for example Istio, Linkerd). Automation tools such as cert-manager and SPIRE can help issue and manage certificates.

Limitations and challenges encompass increased operational complexity, certificate management overhead, potential performance impact, and interoperability concerns with legacy clients or intermediary proxies. In practice, many deployments use a service mesh or API gateway to centralize mTLS termination and management, while allowing end-to-end encryption to be preserved across trusted segments.