You're confusing a collision (find any x, y such that f(x) = f(y)) and a second preimage attack (find any x such that f(x) = y for some particular y). The latte

You're confusing a collision (find any x, y such that f(x) = f(y)) and a second preimage attack (find any x such that f(x) = y for some particular y). The latter is much more difficult, but the former can still be used - see "rogue CA".