Unsolved math problem which could be worth a billion dollars.

No one on Earth knows how to reverse one of the most popular computer algorithms. Yet, it's really easy to compute one way. If you solve this mathematical problem, you can make billions of dollars, which is calculated 500 million times per second in the Bitcoin mining competition. SHA256 has some amazing features, can be used for digital signatures, cryptography, identity verification, and is a core part of the Bitcoin protocol.