EXERCISE 1 — mTLS vs normal TLS
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WHO AUTHENTICATES WHOM:
  - NORMAL TLS: only the SERVER proves its identity (via its certificate).
    The CLIENT is anonymous at the TLS layer -- it authenticates separately
    and later (password, token, cookie) at the application layer.
  - MUTUAL TLS (mTLS): BOTH sides present certificates. The server proves
    identity to the client AND the client proves identity to the server,
    each verifying the other's cert the same way (chain to a trusted CA,
    validity, identity match). Both ends are cryptographically
    authenticated before any application data flows.

THE ONE EXTRA HANDSHAKE STEP (tie to Chapter 6):
  - It's the same handshake from Chapter 6 with one addition: after sending
    its own Certificate, the server sends a CertificateRequest asking the
    client for a cert. The client then sends its Certificate plus a
    CertificateVerify (a signature with its private key proving it owns the
    cert). The server verifies that before completing the handshake.
  - So: normal TLS authenticates one direction; mTLS adds the mirror-image
    client-side certificate + verify.

TWO SCENARIOS WHERE mTLS IS APPROPRIATE:
  1. Service-to-service / microservices: each service holds a client cert,
    so service A and service B mutually authenticate (e.g. a service mesh
    like Istio issuing certs to every pod). Zero-trust: even inside the
    network, every connection proves identity.
  2. Machine API clients / partners: a payment gateway or B2B API requires
    each client to present an issued cert, so only provisioned clients can
    connect -- stronger than a shared API key, and the key can't be
    phished/replayed as easily.
  (Also: VPNs, IoT device fleets, admin access to sensitive infra.)

ONE SCENARIO WHERE mTLS IS NOT APPROPRIATE:
  - The PUBLIC WEB / a consumer website. You can't issue and manage a
    client certificate for every anonymous visitor, and users have no cert.
    Normal server-only TLS + application login (password/passkey) is the
    right model there. mTLS is for closed sets of known clients, not the
    open web.

SUMMARY:
  mTLS = TLS where the client is also a first-class authenticated party.
  Great for known machine-to-machine clients; impractical for the open web.