Network & Transport Security

The internet was designed for resilience, not for trust. Every layer above IP — DNS, TCP, the first decade of HTTP — assumed that anyone who could see the bits was at worst a curious researcher. We now operate the same protocols against nation-state adversaries with line-rate hardware. Transport security is the bandage we have all wrapped around that mismatch, and Zero Trust is what's replacing the bandage.

TLS 1.3 — What's Really Happening

You saw the four-message sketch on Day 2. Now the engineering details.

Why That's a Big Deal

• 1-RTT — half the connection latency of TLS 1.2 (and 0-RTT resumption is possible, with replay caveats).
• Forward secrecy mandatory — every session uses an ephemeral ECDHE key. Compromising the cert later does not retroactively decrypt today's traffic.
• Five cipher suites, all AEAD: TLS_AES_128_GCM_SHA256, TLS_AES_256_GCM_SHA384, TLS_CHACHA20_POLY1305_SHA256, plus two for legacy. No more downgrade-prone catalog.
• Encrypted Extensions — hides ALPN and other negotiation from passive observers (SNI is still cleartext until ECH lands universally).

Certificates — What the Server Is Actually Sending

An X.509 certificate binds a public key to a name via the signature of a certificate authority. The browser trusts the public-key bag of trusted roots in its trust store; everything else chains to one of them.

# the leaf certificate
Subject:      CN=api.example.com
SAN:          DNS:api.example.com, DNS:*.api.example.com
Issuer:       CN=R3, O=Let's Encrypt
Not Before:   2026-01-12   Not After: 2026-04-12
Public Key:   ECDSA P-256
SCT:          embedded (Certificate Transparency)
OCSP:         http://r3.o.lencr.org

Five things the client checks on every handshake:

1. Name — the Subject Alternative Name matches the host being connected to. (CN-only matching is dead — modern clients ignore CN.)
2. Validity period — Not Before < now < Not After.
3. Chain — leaf → intermediate(s) → trusted root.
4. Revocation — OCSP stapling preferred; CRLs fall back; some browsers use CRLite.
5. Certificate Transparency — embedded SCTs prove the cert was published to public logs (defends against rogue/CA-misissued certs).

HSTS — The Critical Header

Even with TLS, the very first request to a site might be HTTP. HSTS tells the browser: for this domain, never speak HTTP again — for the next N seconds.

Strict-Transport-Security: max-age=63072000; includeSubDomains; preload

Submitting your domain to the HSTS preload list means even the first request from a fresh browser is HTTPS-only.

mTLS — Mutual Authentication for Service-to-Service

In one-way TLS the server proves who it is; the client is anonymous (and authenticated above the protocol, e.g. by JWT). mTLS requires both ends to present a certificate. It is the workhorse of service mesh authentication.

Standards / tools to know:

• SPIFFE / SPIRE — workload identity standard (spiffe:// URIs in cert SAN).
• Service mesh — Istio, Linkerd, Consul Connect issue and rotate certs sidecar-style.
• Cloud-native equivalents — AWS IAM Roles Anywhere, GCP Workload Identity Federation.
• Short-lived certs — measured in hours, not years. Rotation is the answer to revocation.

Certificate Pinning — Use With Care

Pinning means the client refuses any cert outside a predetermined set, ignoring even valid CA-issued certs. Once standard for high-assurance mobile apps; now usually overkill thanks to Certificate Transparency and revocation tooling. The pitfall: a botched rotation can brick every installed client.

Good fits: dedicated mobile/IoT apps you ship updates to. Bad fits: browsers (Chrome and Firefox have removed HPKP for exactly the brick risk).

Zero Trust — The End of Network Position

The classical model: trust the inside, distrust the outside, watch the perimeter. Zero Trust says: network position implies nothing. Every request must authenticate, every authorization must be explicit, every session must be observable.

Concretely, in 2025 Zero Trust shows up as:

• Identity-aware proxies at the edge (Cloudflare Access, Google IAP, Tailscale ACLs).
• Service mesh mTLS with policy engines (OPA, Cedar, AuthZed) deciding per-call.
• Conditional access — login allowed only from managed devices with current OS patch level.
• Privileged access management — short-lived JIT credentials for admin tasks.

Network Hardening Checklist

• TLS 1.2 or 1.3 only; no 1.0/1.1, no SSLv3. Disable weak suites (no RC4, 3DES, CBC-without-AEAD).
• HSTS preloaded for every public hostname.
• Certificate Transparency monitoring — alert on any cert issued for your domains you didn't request.
• Egress filtering — deny by default, allow specific destinations. Defeats data exfiltration.
• Network segmentation per workload class (PCI separate from analytics from prod-DB).
• DNS security: DNSSEC for authoritative; DoH/DoT for resolvers; no plaintext UDP/53 to the internet.
• Regular external attack-surface review (Shodan, Nuclei templates, your own).