Crate dusk_poseidon[−][src]

Expand description

Dusk-Poseidon

Reference implementation for the Poseidon Hashing algorithm.

Reference

Starkad and Poseidon: New Hash Functions for Zero Knowledge Proof Systems

This repository has been created so there’s a unique library that holds the tools & functions required to perform Poseidon Hashes.

This hashes heavily rely on the Hades permutation, which is one of the key parts that Poseidon needs in order to work. This library uses the reference implementation of Dusk-Hades which has been designed & build by the Dusk-Network team.

The library provides the two hashing techniques of Poseidon:

Sponge Hash

The Sponge techniqe in Poseidon allows to hash an unlimited ammount of data into a single Scalar. The sponge hash techniqe requires a padding to be applied before the data can be hashed.

This is done to avoid hash collitions as stated in the paper of the Poseidon Hash algorithm. See: https://eprint.iacr.org/2019/458.pdf. The inputs of the sponge_hash are always Scalar or need to be capable of being represented as it.

The module provides two sponge hash implementations:

Sponge hash using Scalar as backend. Which hashes the inputed Scalars and returns a single Scalar.
Sponge hash gadget using dusk_plonk::Variable as a backend. This techniqe is used/required when you want to proof pre-images of unconstrained data inside of Zero-Knowledge PLONK circuits.

Merkle Hash

The Merkle Level Hashing is a technique that Poseidon is optimized-by-design to perform. This technique allows us to perform hashes of an entire Merkle Tree using Dusk-Hades as backend.

The technique requires the computation of a bitflags element which is always positioned as the first item of the level when we hash it, and it basically generated in respect of the presence or absence of a leaf in the tree level. This allows to prevent hashing collitions.

At the moment, this library is designed and optimized to work only with trees of ARITY up to 4. That means that trees with a bigger ARITY SHOULD NEVER be used with this lib. The module contains the implementation of 4 variants of the same algorithm to support the majority of the configurations that the user may need:

Scalar backend for hashing Merkle Tree levels outside of ZK-Circuits whith two variants: One of them computes the bitflags item while the other assumes that it has already been computed and placed in the first Level position.
dusk_plonk::Variable backend for hashing Merkle Tree levels inside of ZK-Circuits, specifically, PLONK circuits. This implementation comes also whith two variants; One of them computes the bitflags item while the other assumes that it has already been computed and placed in the first Level position.

Zero Knowledge Merkle Opening Proof example:

#[cfg(feature = "canon")]
{
use canonical_derive::Canon;
use dusk_plonk::prelude::*;
use dusk_poseidon::tree::{PoseidonAnnotation, PoseidonLeaf, PoseidonTree, merkle_opening};
use rand_core::OsRng;

// Constant depth of the merkle tree
const DEPTH: usize = 17;

// Leaf representation
#[derive(Debug, Default, Clone, Copy, PartialOrd, Ord, PartialEq, Eq, Canon)]
struct DataLeaf {
    data: BlsScalar,
    pos: u64,
}

// Example helper
impl From<u64> for DataLeaf {
    fn from(n: u64) -> DataLeaf {
        DataLeaf {
            data: BlsScalar::from(n),
            pos: n,
        }
    }
}

// Any leaf of the poseidon tree must implement `PoseidonLeaf`
impl PoseidonLeaf for DataLeaf {
    // Cryptographic hash of the data leaf
    fn poseidon_hash(&self) -> BlsScalar {
        self.data
    }

    // Position on the tree
    fn pos(&self) -> &u64 {
        &self.pos
    }

    // Method used to set the position on the tree after the `PoseidonTree::push` call
    fn set_pos(&mut self, pos: u64) {
        self.pos = pos;
    }
}

fn main() -> Result<(), Error> {
    // Create the ZK keys
    let pub_params = PublicParameters::setup(1 << 15, &mut OsRng)?;
    let (ck, ok) = pub_params.trim(1 << 15)?;

    // Instantiate a new tree
    let mut tree: PoseidonTree<DataLeaf, PoseidonAnnotation, DEPTH> =
        PoseidonTree::new();

    // Append 1024 elements to the tree
    for i in 0..1024 {
        let l = DataLeaf::from(i as u64);
        tree.push(l).unwrap();
    }

    // Create a merkle opening tester gadget
    let gadget_tester =
        |composer: &mut StandardComposer,
         tree: &PoseidonTree<DataLeaf, PoseidonAnnotation, DEPTH>,
         n: usize| {
            let branch = tree.branch(n as u64).unwrap().unwrap();
            let root = tree.root().unwrap();

            let root_p = merkle_opening::<DEPTH>(composer, &branch);
            composer.constrain_to_constant(root_p, BlsScalar::zero(), Some(-root));
        };

    // Define the transcript initializer for the ZK backend
    let label = b"opening_gadget";
    let pos = 0;

    // Create a merkle opening ZK proof
    let mut prover = Prover::new(label);
    gadget_tester(prover.mut_cs(), &tree, pos);
    prover.preprocess(&ck)?;
    let proof = prover.prove(&ck)?;

    // Verify the merkle opening proof
    let mut verifier = Verifier::new(label);
    gadget_tester(verifier.mut_cs(), &tree, pos);
    verifier.preprocess(&ck)?;
    let pi = verifier.mut_cs().construct_dense_pi_vec();
    verifier.verify(&proof, &ok, &pi).unwrap();

    Ok(())
}

}

Canonical

The canonical implementations aim to make available a single representation of the Merkle tree to constrained (referred to as “hosted”) and unconstrained (referred to as “host”) environments.

For that, we rely on the feature canon.

canon feature will require all the crates needed for the Merkle tree to function.

If you want to contribute to this repository/project please, check CONTRIBUTING.md
If you want to report a bug or request a new feature addition, please open an issue on this repository.