Mutual TLS authenticates both ends
Ordinary TLS checks the server. Mutual TLS makes the client prove itself too.
That is unnecessary for a public website, where the whole point is that anybody may visit, and valuable between systems, where only a known set of callers should ever connect. It replaces a shared secret in a config file with a certificate the workload holds, which is both stronger and easier to rotate.
More on TLS and PKI
- TLS encrypts a connection, not endpoint intentionsA sealed pipe to a stranger
- Certificates bind keys to names through trust chainsHeld together by a chain of seals
- Certificate expiry creates operational pressureEvery one of them runs out
- Private-key compromise survives a valid certificateThe certificate is fine
- HSTS removes the insecure choice after a browser has learned the policy, while preload can protect the first visit tooThe turning that stops existing
- OCSP stapling moves status evidence closerThe proof comes stapled on