# Proof-of-Work and HASH-256

#### What is Proof-of-Work?

As described in the **Bitcoin white paper (that BSV is based on)**, a **proof-of-work (PoW) system** is a **signaling mechanism**. It requires participants to prove they have performed a **necessary amount of computational work** before they can carry out a desired action.

In BSV:

* Nodes apply **HASH-256** (double SHA-256) to the **serialized block header**.
* They repeat this process until they find a **hash value lower than the current difficulty target**.

The properties of **SHA-256** — **preimage resistance, second preimage resistance, and collision resistance** — ensure there is **no shortcut**. The only way to succeed is by **brute force hashing**, cycling through nonce values until a valid digest is discovered.

👉 **Key takeaway:** Proof-of-work ensures fairness by making block discovery dependent on **computational effort**, not trust.

***

#### How Proof-of-Work is Used in BSV

1. **Nodes collect transactions** and organize them into a **Merkle Tree**.
2. They construct a **block header template**, including the Merkle Root and timestamp.
3. This template becomes a **mining candidate**.
4. **Hashing pools** request candidates using a **Get Mining Candidate (GMC)** call.
5. Mining candidates are updated about **10 times per second**; pools typically poll nodes every **30 seconds**.

Nodes and miners then iterate through **nonce values**, hashing the block header repeatedly, searching for a proof-of-work solution.

***

#### Difficulty and Block Discovery

To maintain a block discovery interval of **\~10 minutes**:

* The **difficulty target** is adjusted by changing the number of **leading zeroes** required in the hash.
* Increasing leading zeros = **harder difficulty**.
* Reducing leading zeros = **easier difficulty**.

This balance keeps the network stable:

* **Fast enough** to keep miners competitive.
* **Slow enough** to minimize orphan races (conflicting block discoveries).

***

#### Evolution of Mining

* **Early Bitcoin:** Difficulty was low enough that proof-of-work could be solved with a **CPU**.
* As difficulty rose, miners shifted to **GPUs**.
* Today, **ASICs (Application-Specific Integrated Circuits)** purpose-built for SHA-256 dominate mining.

Originally, difficulty was recalculated every **2,016 blocks (\~2 weeks)**. In 2017, it shifted to **per-block adjustments**. BSV node operators plan to restore the **2,016-block cycle**.

***

#### Consensus and Incentives

A block is only considered **“won”** when:

* The majority of nodes begin using its **hash as the previous block hash** to mine the next block.
* This action — building on top of the block — is the **only consensus mechanism in BSV**.

👉 For a participant to be recognized as a **node**, they must:

1. Accept valid blocks.
2. Successfully find proof-of-work solutions for new blocks.

This design creates strong **economic incentives** for nodes to be:

* **Efficient** in hashing.
* **Interconnected** for rapid block propagation.

✅ **Result:** The network can **scale transaction throughput and block sizes** to meet **any level of growth — even exponential expansion**.