HTTPS Transport Layer Security


HTTP is an application-level protocol (see TCP/IP model)

  • On top of the transport layer (TCP),
  • Data is sent as text, no cryptographic protection,
  • Middle boxes can intercept, read, block, alter data:
    • Wiretapping on a local network,
    • Sniffing/injecting wifi packets,
    • Wiretapping internet backbones (e.g., transatlantic cables).

Transport Layer Security (TLS)

  • Adds cryptographic protections above TCP layer: Confidentiality, Integrity, Authenticity, Forward secrecy.
  • First standard: SSL 1.0, 1995 (Netscape Communications),
  • Latest version: TLS 1.3, March 2018.


From Greek κρυπτoς + γραφειν (hidden writing)

Used for: secret messages, proving identity, signing digital data, electronic voting, digital cash, …


  • Based on mathematics:
    • Based on widely believed mathematical assumptions, most of the time;
    • A few constructions based on theorems.
  • Based on computational hardness:
    • Assumes adversary has bounded computational capacity / bounded success probability;
    • Usually bounds are taken much larger than physically possible (e.g.: more computation time than the age of the universe / smaller probability than presence of active substance in homeopathic dilution).
  • Based on secrets:
    • keys, passwords must be securely stored.

Moral of the story

  1. Security is strong, but must be constantly reevaluated.
  2. Theft, operational security failures, law enforcement, social engineering are real threats!

Symmetric cryptography

  • Known since the ancient times: YBH’IR QBAR VG NF N XVQ.
  • Uses one shared key, known to all partners.
  • Good for:
    • secret messages (encryption),
    • signing messages (message authentication code).

Asymmetric cryptography

  • Known since the ’70s.
  • Two keys:
    • one secret key held by one party,
    • associated public key known to everyone.
  • Good for:
    • secret messages (public-key encryption),
    • proving identity (identification),
    • signing messages (signatures),
    • establishing shared keys (key agreement or key exchange).

Cryptographic guarantees in TLS


  • Messages are only intelligible by client and server;
  • Achieved using symmetric encryption (e.g., AES).


  • Messages cannot be altered by third party;
  • Achieved using symmetric encryption (e.g., HMAC, GCM).


  • Client verifies identity of the server;
  • Optionally: server verifies identity of the client;
  • Achieved using asymmetric identification (e.g, RSA, DSA, ECDSA).

Forward secrecy

  • Confidentiality maintained even if server/client keys are later compromised (e.g., stolen, or obtained by law enforcement);
  • Achieved using asymmetric key exchange (e.g., DH, ECDH);
  • Only mandatory in TLS ≥1.3.

How cryptograpy is used in TLS

Identification (authenticity)

  • Server sends a certificate, containing its public key.
  • Clients validate the certificate before starting conversation.
  • Optionally, clients can send certificates too (rare).

Key exchange (forward secrecy)

  • Client and server do a key exchange to establish a session key.

Symmetric encryption (confidentiality, integrity)

  • Client and server use session key to exchange secret messages.


  • Certificates prove ownership of a domain name:

    I’m talking to someone who knows the secret key for

  • Certificates are signed by certificate authorities:

    This has fact has been signed for by DigiCert Inc.

  • Root certificate authorities are stored in the browser:

    My browser tells me to trust DigiCert.

TLS 1.3: (simplified) technical overview

  1. Client sends ClientHello to server:

    Hi, I speak TLS 1.2 and TLS 1.3. Here’s my ephemeral public key.

  2. Server computes session key and responds with ServerHello:

    Hi, I want to speak TLS 1.3 with you. Here’s my ephemeral public key, encrypted certificate and signature.

  3. Client

    • Computes session key,
    • validates certificate,
    • validates signature.
  4. Client and server can start encrypted HTTP exchange using the session key.

Obtaining a TLS certificate

Certificate authorities verify that:

  • You own the domain (you have write access to the site hosted at that domain),
  • You know the secret key for your certificate,
  • Optionally: you are who you claim to be in real life (address, phone number, etc.)

If all checks pass, they sign your certificate.

Free certificates

  • Domain validation only (no extended validation),
  • Free for everyone,
  • Automated setup,
  • Supports wildcard certificates.
  • Goal: 100% encrypted Web!
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