Part 4 · 1 chapters · ~8 min

Key Exchange

Diffie-Hellman and elliptic curve Diffie-Hellman (X25519), key derivation with HKDF, why exchange needs authentication, man-in-the-middle attacks, forward secrecy, and hybrid post-quantum key exchange.

Agreeing on a secret in public

code

import { generateKeyPairSync, diffieHellman, hkdfSync } from 'node:crypto';
const ada = generateKeyPairSync('x25519'), bank = generateKeyPairSync('x25519');
const s1 = diffieHellman({ privateKey: ada.privateKey, publicKey: bank.publicKey });
const s2 = diffieHellman({ privateKey: bank.privateKey, publicKey: ada.publicKey });
s1.equals(s2);                                                        // true
const key = Buffer.from(hkdfSync('sha256', s1, 'salt', 'session v1', 32));   // derive an AES-256 key

DIFFIE-HELLMAN KEY EXCHANGE

two parties agree on a shared secret over a public channel

swipe the figure sideways, or tap expand for full screen

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ephemeral secrets

Each side picks a random private value and computes a public value from it (X25519: a point on Curve25519).

random private values, public pointsX25519 is the common choice