The simplest way would be to use an L bracket like is used for attaching table legs to the top or inside cabinet corners, or even a small shelf bracket. Secure the bracket to the blacktop with concrete anchors. (Expanding anchors that fit into a hole drilled into the pavement with a masonry bit) Wood screws can be used in the cedar.

Screws, anchors and brackets. The bracket is secured to the black top with anchors of some type. There are many on the market, most require a hole to be drilled in the blacktop. Since blacktop is soft compared to concrete, some long concrete nails and a big hammer may be the fastest and easiest. The screw the brackets to the bench legs. If this is a permanent installation that must be very secure, I suggest that concrete pads be installed to support the bench legs. At least 4" deep and 12" around each leg. This is much harder surface and will hold the bench in place. The blacktop is so soft that the bench will probably pull the anchors out of the surface in a very short time.