In a proof-of-work blockchain, what is the purpose of a nonce?

• A. To serve as a random seed for encryption
• B. To act as a number that, when hashed, meets difficulty criteria
• C. To identify the miner
• D. To encrypt the block contents

Answer: (B)

The main idea is that the nonce is the adjustable value miners vary to produce a hash that meets the network’s difficulty. In proof-of-work, the block header (which includes things like the previous block hash and the Merkle root) is hashed together with a candidate nonce. Because cryptographic hashes behave like random functions, most nonces won’t produce a hash below the difficulty target, but a tiny fraction will. Miners keep trying different nonce values until the hash falls under the required threshold, proving that a certain amount of computational work was done. Once such a nonce is found, the block is considered mined and broadcast.

It isn’t used as an encryption seed, it doesn’t identify the miner, and it doesn’t encrypt or alter the block contents. Those roles are separate; the nonce’s sole purpose is to enable the proof-of-work condition by providing the variable value that can be hashed to meet the difficulty.