Puzzle friendliness

In cryptography, puzzle friendliness is a property of cryptographic hash functions. Not all cryptographic hash functions have this property. SHA-256 is a cryptographic hash function that is thought to have this property. Informally, a hash function is puzzle friendly if no solution exists which is better than just making random guesses, and the only way to find a solution is the brute force method. Although the property is very general, it is of particular importance to proof-of-work, such as in Bitcoin mining.