Based on hashcat by Adam Back.
The proof-of-work involves scanning for a value that when hashed, such as with SHA-256, the hash begins with a number of zero bits. The average work required is exponential in the number of zero bits required and can be verified by executing a single hash. … Once the CPU effort has been expended to make it satisfy the proof-of-work, the block cannot be changed without redoing the work. [[bitcoin.pdf]]
[mining], [difficulty-target]