def zero_pad(n, message)
      len = message.bytesize
      if len == n
        message
      elsif len > n
        raise LengthError, "String too long for zero-padding to #{n} bytes"
      else
        message + zeros(n - len)
      end
    end
def zero_pad(n, message)
      len = message.bytesize
      if len == n
        message
      elsif len > n
        raise LengthError, "String too long for zero-padding to #{n} bytes"
      else
        message + zeros(n - len)
      end
    end
Pad a string out to n characters with zeros

 @param [Integer] n The length of the resulting string
 @param [String]  message the message to be padded

 @raise [RbNaCl::LengthError] If the string is too long

 @return [String] A string, n bytes long