CoreGraphics Quartz drawing shadow on transparent alpha path

Searching the web for about 4 hours not getting an answer so:
How to draw a shadow on a path which has transparency?

- (void)drawRect:(CGRect)rect
{
    CGContextRef c = UIGraphicsGetCurrentContext();
    CGContextSetLineWidth(c, 2);
    CGContextSetStrokeColorWithColor(c, [[UIColor whiteColor] CGColor]);
    CGContextSetShadowWithColor(c, CGSizeMake(0, 5), 5.0, [[UIColor blackColor]CGColor]);
    CGContextSetFillColorWithColor(c, [[UIColor colorWithWhite:1.0 alpha:0.8] CGColor]);

    // Sample Path
    CGContextMoveToPoint(c, 20.0, 10.0);
    CGContextAddLineToPoint(c, 100.0, 40.0);
    CGContextAddLineToPoint(c, 40.0, 70.0);
    CGContextClosePath(c);

    CGContextDrawPath(c, kCGPathFillStroke);
}

The first thing I notice, the shadow is only around the stroke. But that isn't the problem so far. The shadow behind the path/rect is still visible, which means: the shadow color is effecting the fill color of my path. The fill color should be white but instead its grey. How to solve this issue?