TLS, VPNs, and secure protocols
Cryptographic building blocks come together in protocols such as TLS, which protects the web, and VPNs, which protect whole networks of traffic.
TLS
Transport Layer Security (TLS), successor to Netscape's SSL, secures connections for HTTPS, email, and many other applications. TLS 1.3, published as RFC 8446 in 2018, simplified the protocol, removed weak algorithms, and requires forward secrecy. Older versions SSL 2.0, SSL 3.0, TLS 1.0, and TLS 1.1 are deprecated and should not be used.
The TLS 1.3 handshake
In TLS 1.3, the client's first message offers supported cipher suites and an ephemeral key share; the server replies with its key share, its certificate, a signature proving it holds the certificate's private key, and a finished message. Both derive symmetric session keys from the Diffie-Hellman exchange. The handshake takes one round trip, and resumed sessions can even send data immediately (0-RTT), with some replay risk.
Forward secrecy
Because TLS 1.3 derives session keys from ephemeral Diffie-Hellman values discarded after use, recording today's traffic and later stealing the server's long-term private key does not reveal past sessions. This forward secrecy limits the damage of key compromise and defeats 'harvest now, decrypt later' attacks against classical key exchange, though not against future quantum computers, which is why post-quantum hybrids are being added.
Certificates in practice
When a browser connects to example.com, it checks that the server's certificate names example.com, is within its validity dates, has not been revoked, and chains up to a trusted root CA. Let's Encrypt, launched in 2015, issues free, automated certificates and helped push the share of web pages loaded over HTTPS above 80% in major browsers. Certificate Transparency logs publicly record issued certificates so misissuance can be spotted.
IPsec and VPNs
A virtual private network (VPN) creates an encrypted tunnel over a public network, making remote devices or sites appear to be on a private network. IPsec secures traffic at the network layer, protecting every IP packet between two gateways or a gateway and a host, using protocols such as ESP for encryption and IKE for key exchange. WireGuard is a newer, much simpler VPN protocol now built into Linux.
Securing other protocols
SSH secures remote login and file transfer; DNSSEC authenticates DNS answers; DNS over HTTPS encrypts lookups; WPA3 protects Wi-Fi; and STARTTLS and MTA-STS upgrade email transfer to TLS. Secure email end to end requires additional tools such as S/MIME or PGP, because TLS protects only each hop between mail servers, not the stored message.
Zero trust
Traditional network security trusted anything inside the corporate perimeter. Zero trust architectures assume no implicit trust based on network location: every request is authenticated, authorised, and encrypted, using strong identity, device checks, and least privilege, whether it comes from the office or a coffee shop. Remote work and cloud services have made this approach widespread.
Worked example
Visiting https://bank.example: DNS returns the server's address; TCP connects (one RTT); the TLS 1.3 client hello and server response complete the handshake (one more RTT). The browser verifies the certificate chain to a trusted root and that the name matches bank.example. HTTP requests then travel encrypted with AES-GCM or ChaCha20-Poly1305 keys unique to this session. With HTTP/3 over QUIC, the transport and TLS handshakes combine into a single round trip.
Key takeaways
TLS secures HTTPS and many protocols; TLS 1.3 (RFC 8446, 2018) removed weak options, and SSL and TLS 1.0 and 1.1 are deprecated. Its one-round-trip handshake exchanges ephemeral key shares and a certificate signature, with optional 0-RTT resumption. Ephemeral keys give forward secrecy. Browsers validate names, dates, revocation, and chains to trusted roots; Let's Encrypt (2015) made certificates free, and Certificate Transparency exposes misissuance. VPNs tunnel traffic using IPsec or WireGuard. SSH, DNSSEC, DoH, WPA3, and email TLS secure other protocols; end-to-end email needs S/MIME or PGP. Zero trust authenticates every request regardless of location.
Flashcard
Transport Layer Security
Check your understanding
An attacker records a TLS 1.3 session today and steals the server's private certificate key next year. Why can't they decrypt the recording?