function hashMessage(timestamp, account, method, params, nonce) {
  const first = createHash('sha256');
  first.update(timestamp);
  first.update(account);
  first.update(method);
  first.update(params);

  const second = createHash('sha256');
  second.update(K);
  second.update(first.digest());
  second.update(nonce);

  return second.digest();
}
Create request hash to be signed.

@param timestamp  ISO8601 formatted date e.g. `2017-11-14T19:40:29.077Z`.
@param account    Steem account name that is the signer.
@param method     RPC request method.
@param params     Base64 encoded JSON string containing request params.
@param nonce      8 bytes of random data.

@returns bytes to be signed or validated.