It wouldn't be just an equality check on the bits anyway, rather some kind of theoretical function over the bits. For example, is this JPG image a copyrighted i

It wouldn't be just an equality check on the bits anyway, rather some kind of theoretical function over the bits. For example, is this JPG image a copyrighted image? Perhaps my function would be: - take the source image, reduce it to a 10px by 10px grid - take the image in question, reduce it to a 10px by 10px grid - for each pixel, calculate the deviation between the source pixel and the candidate pixel - if the sum of the squares of the deviation is smaller than some threshold, output true. The point is that no matter how clever a function you make, it cannot capture the notion of where the bits came from and can only capture information about what the bits are. Computers care about the latter. All these clever schemes care about the latter. Copyright law cares about the former. From a copyright law perspective, the notion that you might have two files on your computer whose bits are completely identical in every way, but only one of which you are legally permitted to copy, is perfectly reasonable.