sci-fi short story about an alien box/puzzle in the woods

Please help me find the title / author of a great story I read a few years ago. It is modern (last 50 years at least). The synopsis is: Guy finds a grafitti-covered box (like a telephone cable box- bolted to the ground) in the woods. He fools with it and realizes it is a puzzle. He checks with local utilities, Forest Service, etc and nobody claims it. After repeated trips into the woods to work on the puzzle, he gets to a certian level and can go no further because it is too difficult. The last grafitti he sees is of somebody famous (Ben Franklin, I think). It turns out it is a test left by aliens to judge when they need to make first contact. Any help would be much appreciated! Thanks BH