TLS Handshaking

Introduction

This page is just to remind me how this works

TLS Handshake and Ephemeral Keys

Overview

Transport Layer Security (TLS) is the protocol that secures communication between a client and a server. It provides encryption, authentication, and integrity. The handshake process establishes a shared session key used for encrypting all subsequent traffic.

TLS Handshake (TLS 1.3)

1. Client Hello: The client initiates the handshake, sending supported TLS versions, cipher suites, and a random value.
2. Server Hello + Certificate: The server chooses a cipher suite, sends its certificate (public key + identity), and its own random value.
3. Ephemeral Key Exchange (ECDHE): Client and server exchange ephemeral public keys. Each side uses its private ephemeral key and the other’s public ephemeral key to compute the same shared secret.
4. Session Key Derivation: Both sides derive a symmetric session key from the shared secret and random values. This key is never sent over the network.
5. Secure Communication: All application data is encrypted with the symmetric session key. Only the client and server can decrypt it.

Ephemeral Keys

• Definition: Ephemeral means temporary, single-use keys created only for the duration of one TLS session.
• Purpose: They provide forward secrecy. Even if a server’s long-term private key is stolen later, past traffic cannot be decrypted.
• Lifecycle: Generated at the start of the handshake, used to derive the session key, and then discarded.

Diffie-Hellman Key Exchange

• Goal: Allow two parties to compute the same shared secret without sending the secret itself over the network.
• Process:
  • Each side generates a private ephemeral key.
  • Each side computes a public ephemeral key from its private key.
  • They exchange public ephemeral keys.
  • Each side combines its private key with the other’s public key to compute the same shared secret.
• Security: An eavesdropper cannot compute the shared secret without knowing one of the private ephemeral keys.

So to put it into English, the maths dictates when one side shares a public value and combines it with their public value and private value the number will be the same.

Elliptic Curve Diffie-Hellman (ECDHE)

• ECDHE is a modern variant of Diffie-Hellman that uses elliptic curves.
• Advantages:
  • Stronger security per bit compared to classic DH.
  • Faster computations, making it efficient for TLS 1.3.
• Usage: TLS 1.3 almost always uses ECDHE for its key exchange.

Session Key

• Derived from the shared secret and random values exchanged during the handshake.
• Used for symmetric encryption (e.g., AES, ChaCha20).
• Never transmitted; both client and server compute it independently.
• Ensures that only the client and server can encrypt and decrypt the traffic.

Identity Verification

• The server’s certificate allows the client to confirm the server’s identity.
• With SslMode=VerifyFull (in Npgsql), the client checks:
  • The certificate is signed by a trusted CA (RootCertificate).
  • The hostname matches the certificate’s Subject Alternative Name (SAN).
• If verification fails, the connection is refused.