const box = nacl.box;

const newNonce = () => nacl.randomBytes(box.nonceLength);
const generateKeyPair = () => box.keyPair();

const json = {
  a: "test",
};

const encrypt = (secretOrSharedKey, json, key) => {
  const nonce = newNonce();
  const messageUint8 = new Uint8Array(
    Buffer.from(JSON.stringify(json), "utf-8")
  );
  const encrypted = key
    ? box(messageUint8, nonce, key, secretOrSharedKey)
    : box.after(messageUint8, nonce, secretOrSharedKey);

  const fullMessage = new Uint8Array(nonce.length + encrypted.length);
  fullMessage.set(nonce);
  fullMessage.set(encrypted, nonce.length);

  const base64FullMessage = Buffer.from(fullMessage).toString("base64");
  return base64FullMessage;
};

const decrypt = (secretOrSharedKey, messageWithNonce, key) => {
  const messageWithNonceAsUint8Array = new Uint8Array(
    new Buffer(messageWithNonce, "base64")
  );
  const nonce = messageWithNonceAsUint8Array.slice(0, box.nonceLength);
  const message = messageWithNonceAsUint8Array.slice(
    box.nonceLength,
    messageWithNonce.length
  );

  const decrypted = key
    ? box.open(message, nonce, key, secretOrSharedKey)
    : box.open.after(message, nonce, secretOrSharedKey);

  if (!decrypted) {
    throw new Error("Could not decrypt message");
  }

  const base64DecryptedMessage = Buffer.from(decrypted).toString("utf-8");
  return JSON.parse(base64DecryptedMessage);
};

const obj = { hello: "world" };
const pairA = generateKeyPair();
const pairB = generateKeyPair();
const sharedA = box.before(pairB.publicKey, pairA.secretKey);
const sharedB = box.before(pairA.publicKey, pairB.secretKey);
const encrypted = encrypt(sharedA, obj);
const decrypted = decrypt(sharedB, encrypted);
console.log(obj, encrypted, decrypted);

return <pre>{JSON.stringify({ obj, encrypted, decrypted }, undefined, 2)}</pre>;