# Scanning Random Space

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*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.*

\- Satoshi Nakamoto, Bitcoin Whitepaper

The proof-of-work process involves:

* Taking a block header which is arranged in a pre-defined format and using it **plus a ‘nonce’ value** (a nonce is an arbitrary number that can be used just once in cryptographic communication) as the message to be hashed.
* The node passes out block headers containing a hashed record of all the transactions the block includes plus a timestamp and a unique identifier to hash machines whose hashing power is being applied to solving its block.

Critically, **validation of the Proof-of-work** is as simple as hashing the block header including the winning nonce value. Nodes do this before verifying the block’s contents to ensure they are working on a block solution which has been solved by a capable node.


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