Pow

Sha256 based proof of work over a typed piece of data.

Any type that implementes serde::Deserialize can be tagged with a proof of work.

Examples

Prove we did work targeting a phrase.

use pow::Pow;

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

let phrase = b"Phrase to tag.".to_vec();
let pw = Pow::prove_work(&phrase, difficulty).unwrap();
assert!(pw.score(&phrase).unwrap() >= difficulty);

Prove more difficult work. This time targeting a time.

// more diffcult, takes around 100_000 hashes to generate proof
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.score(&now).unwrap() >= difficulty);

Score scheme

To score a proof of work for a given (target, Pow) pair: Sha256 is calculated over the concatenation SALT + target + Pow. The first 16 bytes of the hash are interpreted as a 128 bit unsigned integer. That integer is the score. A constant, SALT, is used as prefix to prevent pow reuse from other systems such as proof of work blockchains.

In other words:

fn score<T: Serialize>(target: &T, pow_tag: &Pow<T>) -> u128 {
    let bytes = serialize(&SALT) + serialize(target) + serialize(pow_tag);
    let hash = sha256(&bytes);
    deserialize(&hash[..16])
}

Serialization encoding.

It shouldn't matter to users of this library, but the bincode crate is used for cheap deterministic serialization. All values are serialized using network byte order.

Threshold scheme

Given a minimum score m. A Pow p satisfies the minimum score for target t iff score(t, p) >= m.

Choosing a difficulty setting.

Difficulty settings are usually best adjusted dynamically a la bitcoin.

To manually select a difficulty, choose the average number of hashes required.

fn 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, to calculate probable number of hashes required to satisfy a given minimum difficulty.

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