SVG - make viewBox(0, 0, 100%, 100%) with percentages

How can I make SVG viewBox user coordinate system the same as the viewport coordinates system provided by SVG itself (height="100%" and width="100%")?

I need this special case for a project I'm doing, SVG element should be responsive, but still we need to keep height and width 100% on the SVG itself.

So, I need something like this:

<svg height="100%" width="100%" viewBox="0, 0, 100%, 100%">
  <circle cx="25" cy="25" r="20" stroke="black" strokeWidth="1" fill="black" />
</svg>

.. but the viewBox attribute doesn't accept percentages.