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 has 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. [1]
Here is the formal technical definition of the puzzle friendliness property. [2] [1]
In the above definition, the distribution has high min-entropy means that the distribution from which k is chosen is hugely distributed so that choosing some particular random value from the distribution has only a negligible probability.
Let H be a cryptographic hash function and let an output y be given. Let it be required to find z such that H( z ) = y. Let us also assume that a part of the string z, say k, is known. Then, the problem of determining z boils down to finding x that should be concatenated with k to get z. The problem of determining x can be thought of a puzzle. It is really a puzzle only if the task of finding x is nontrivial and is nearly infeasible. Thus the puzzle friendliness property of a cryptographic hash function makes the problem of finding x closer to being a real puzzle.
The puzzle friendliness property of cryptographic hash functions is used in Bitcoin mining.
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