First, break stuff. That's what the Cryptopals exercises are doing, but because in our actual world there's lot of good stuff out there now these exercises provide bad examples† you can break. If, in your hypothetical, there is no good crypto, this will be very easy to do just by taking anything you find in the real world, right?

Now, use what you learned (from breaking stuff) to make something which resists the attacks you learned. Congratulations, you have improved the state of the art. This is how actual experts (not sure any of them are or should be "exalted") did it.

† Bad, but, in many cases, very real. Because people stubbornly will not learn this lesson and keep rolling their own we are still finding broken garbage in the real world it's just becoming gradually rarer.