**Proof of work**is the concept which makes things difficult for the fraudulent note to catch up with the Ow-nest notes.

**Ideally the purpose of Proof-of-work**

- Produce a challenge to the user / computer
- The user / computer has to produce the challenge result to submit some value.
- Essentially showing the Proof of Some Work Done.
- And eliminate entities which are slow or not capable enough to generate the proof of work

**The idea behind Proof of Work is:**

- Find or generate a Value which is:

- And yet easily verifiable.

**BlockChain Solve Problem**

For example, proof Of Work in Blockchain

- Proof of Work : Block Should have n leading 0.

For example: n = 4, hash should have 4 leading zeros.

0000AB23EF23DD223432FC0390040438DF.....

**How to Get Such Proof of Work ?**

Generate Nonce, (may be by Bruteforce) such that :

- Combination of Nonce and Block Data generates the leading zeros.
- As per the difficulty of the challenge (Value of n)
- More the value of n more the difficulty.

**Significance of Proof of Work**

- Signifies that the computation required is exponential to the number of leading zeros required.
- Once the CPU effort has been expended to make satisfy proof of work the block cannot be changed.
- Until redoing the proof of work.
- And as the blocks are chained, redoing will require entire chain to be redone.

**A Validity of Proof-of-Work**

- Even if Attacker tries to catch up and produce the Proof-of-Work faster, the added block will be assessed again when new block is added
- Chain with longest proof of work will be accepted.
- Parallel chains are formed.
- Other Block will be rejected (Called the Dangling)

- If majority of the nodes are honest, they will grow and outpace the any competing chains.
- To modify any block

- The attacker would have to redo all the proof of work for all the blocks from that block till the last block.

- And catch up and surpass work of the honest nodes

- As per the original paper of Satashi Nakamoto:

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