42. Signing and verifying data with crypto

`node:crypto` supports symmetric HMACs (shared-secret message authentication) and asymmetric key pairs (sign with a private key, verify with the public key) — the same pattern that underlies transaction signing on most blockchains, even though production wallet code typically uses a dedicated elliptic-curve library (secp256k1) rather than Node's generic crypto module.

js
import { createHmac, generateKeyPairSync, sign, verify } from "node:crypto";

// symmetric: both sides share a secret
const mac = createHmac("sha256", "shared-secret").update("payload").digest("hex");

// asymmetric: sign with private key, verify with public key
const { privateKey, publicKey } = generateKeyPairSync("ed25519");
const signature = sign(null, Buffer.from("payload"), privateKey);
const isValid = verify(null, Buffer.from("payload"), publicKey, signature);

The core idea to internalize: a signature proves the holder of a private key authorized a specific message, and anyone with the corresponding public key can verify that without ever seeing the private key.

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