[auganov]

Pick a sufficient number of iterations. Recursively hash a random input that many times and use the resulting hash as the encryption secret for the message. Publish the encrypted message, the seed[s], the number of iterations and detailed decryption instructions.

You cannot easily parallelize the solving, but using multiple chain encrypted seeds we can parallelize the generation.

You could mix different hashing functions with different characteristics to hedge against different kinds of computational speedups. Though speedups could also work in our favor, since nobody may want to start solving the challenge today.

For redundancy you could try different less reliable methods to deliver solved seeds at the right time. Say you try to deliver the seed that should have taken 100 years to compute after 100 years somehow.

I don't think too many people have made a lot of fanfare about doing this, so you'd have an advantage in being one of the first.