pow_sha256/README.md

# PoW_SHA256

Rust crate which generates SHA256 Proofs of Work on serializable datatypes. 

Whether for blockchain-related projects or Hashcash-like schemes, this crate can be used to prove work was done on a given serializable input. The input merely needs to implement `serde::Deserialize` to be used.

This is a fork of the [`pow` library](https://github.com/bddap/pow) by bddap with some new additions. Primary of these being:

- PoW datatype now saves the calculation result to be used for checking proof validity given input
- `is_valid_proof` method to do the above mentioned

Other small changes have also been included of various importance but mostly just stylistic/ease of use improvements.

# Examples

Prove work was done, specifically targeting a phrase.

```rust
use PoW_SHA256::PoW;

// Very easy difficulty
let difficulty = u128::max_value() - u128::max_value() / 2;

let phrase = b"Phrase to be used.".to_vec();
let pw = PoW::prove_work(&phrase, difficulty).unwrap();

// Asserting that the result is of sufficient difficulty
assert!(pw.result >= difficulty);

// Asserting that the PoW was generated from the provided phrase
assert!(pw.is_valid_proof(&phrase))
```

Prove more difficult work. This time targeting a time.

```rust
// Greater diffculty this time around. Takes around 100,000 hashes to find a nonce of the correct difficulty.
let difficulty = u128::max_value() - u128::max_value() / 100_000;

let now: u64 = get_unix_time_seconds();
let pw = PoW::prove_work(&now, difficulty).unwrap();

assert!(pw.result >= difficulty);
assert!(pw.is_valid_proof(&phrase))
```


# Hashing Scheme

SHA256 is calculated over the concatenation of the:
- SALT
- Serialized Input `T` 
- Nonce

The first 16 bytes of the resulting hash are interpreted as a 128 bit unsigned integer.

A randomly generated constant, `SALT`, is used as prefix to prevent PoW reuse from other systems such as proof of work blockchains.

# Choosing a difficulty setting.

Depending on your use case, difficulty settings often are best set dynamically a la bitcoin.

However if your use case requires manual setting then it is trivial to set one yourself. One way to manually select a difficulty is to choose the average Hnumber of hashes desired with a function like this:

```rust
fn get_difficulty(average: u128) -> u128 {
    debug_assert_ne!(average, 0, "It is impossible to prove work in zero attempts.");
    let m = u128::max_value();
    m - m / average
}
```

Conversely we can use the same equation to calculate the probable number of hashes required to satisfy a given difficulty:

```rust
fn est_average(difficulty: u128) -> u128 {
    let m = u128::max_value();
    if difficulty == m {
        return m;
    } 
    m / (m - difficulty)
}
```

# License

This project is dual-licensed under `Apache License Version 2.0` & `MIT license`.