Time Locked Puzzles and Proof of Work

So how might we a lock down a file for a given amount of time or make sure that someone does not get their present until their birthday? Well one way is to get others to do some work, that you know will be done within a certain amount of time. If I know it will take you one hour to cut the grass in your garden, before I give you your reward, I will hope you will not be able to do it quicker than one hour.

Hashing

This method is defined here.

One method is to get the receiver to perform some work to find the encryption key. For example if we wanted to lock it down for one hour we could take a seed value, and then continually hash it for a given amount of time that we thing the recipient would required to generate the same key. This would then generate the same key, if the sender and recipient share the number of iterations of the hash used. We can then tell the receiver the seed value and the number of iterations they must use in order to compute the key, along with the encrypted message. The receiver must then compute the key and prove their work.

The method, though, is not perfect as the cost of the operation will vary with the clock speed of the processor. We will not be able to compute in parallel as the hashing operations must be done one at a time. The other side will still have a cost in the…