# Hash Function that is Collision Resistant but not Puzzle Friendly

Is there any example of a Hash function that is collision-resistant but may not be puzzle-friendly, similarly example of a hash function that is puzzle-friendly but not collision-resistant?

Puzzle friendliness: A hash function $$H$$ is said to be puzzle‐friendly if for every possible $$n$$‐bit output value $$y$$, if $$k$$ is chosen from a distribution with high min‐entropy, then it is infeasible to find $$x$$ such that $$H(k|| x) = y$$ in time significantly less than $$2^n$$ ($$k$$ and $$y$$ is given).

• I am thinking of an example in which a Hash function is collision-resistant but not puzzle-friendly, and another example in which function is puzzle-friendly but not collision-resistant. – mohit nihalani Mar 3 at 20:37
• Puzzle friendliness. A hash function H is said to be puzzle‐friendly if for every possible n‐bit output value y, if k is chosen from a distribution with high min‐entropy, then it is infeasible to find x such that H(k ‖ x) = y in time significantly less than 2^n. – mohit nihalani Mar 3 at 20:43
• I don't see the relation with collision. This is rather finding a suffix to attack. – kelalaka Mar 3 at 20:49
• stackoverflow.com/questions/42042840/… It seems to be something from the cryptocurrency community, rather than a common cryptographic definition. – SAI Peregrinus Mar 3 at 20:50
• Consider $SHAx(k||x) = SHA256(k||x) || k||x$ collision resistant but no puzzle-friendly and no pre-image resistance. – kelalaka Mar 3 at 21:14