code: func (c *Client) SignMessage(address btcutil.Address, message string) (string, error) {
  	return c.SignMessageAsync(address, message).Receive()
  }
docs: // SignMessage signs a message with the private key of the specified address.
  //
  // NOTE: This function requires to the wallet to be unlocked.  See the
  // WalletPassphrase function for more details.