encrypt-sharedkey.js

// Bob precomputes shared key with Alice (one time)
const sharedKey = nacl.box.before(alice.publicKey, bob.secretKey)

// generating one time nonce for encryption
const nonce = nacl.randomBytes(24)

// Bob encrypt message for Alice
const box = nacl.box.after(
  nacl.util.decodeUTF8('Hello Alice'),
  nonce,
  sharedKey // <-- using shared key
)
const message = {box, nonce}