fullcodec_plonk/commitment_scheme/kzg10/

srs.rs

// This Source Code Form is subject to the terms of the Mozilla Public
// License, v. 2.0. If a copy of the MPL was not distributed with this
// file, You can obtain one at http://mozilla.org/MPL/2.0/.
//
// Copyright (c) DUSK NETWORK. All rights reserved.

//! The Public Parameters can also be referred to as the Structured Reference
//! String (SRS).
use super::key::{CommitKey, OpeningKey};
use crate::{error::Error, util};
use dusk_bls12_381::{G1Affine, G1Projective, G2Affine};
use dusk_bytes::{DeserializableSlice, Serializable};
use parity_scale_codec::{Decode, Encode};
use rand_core::RngCore;
use serde::{Deserialize, Serialize};
use sp_std::vec;
use sp_std::vec::Vec;

/// The Public Parameters can also be referred to as the Structured Reference
/// String (SRS). It is available to both the prover and verifier and allows the
/// verifier to efficiently verify and make claims about polynomials up to and
/// including a configured degree.
#[derive(Debug, Clone, Encode, Decode, Serialize, Deserialize, PartialEq)]
pub struct PublicParameters {
    /// Key used to generate proofs for composed circuits.
    pub(crate) commit_key: CommitKey,
    /// Key used to verify proofs for composed circuits.
    pub(crate) opening_key: OpeningKey,
}

impl PublicParameters {
    /// Returns an untrimmed [`CommitKey`] reference contained in the
    /// `PublicParameters` instance.
    pub fn commit_key(&self) -> &CommitKey {
        &self.commit_key
    }

    /// Returns an [`OpeningKey`] reference contained in the
    /// `PublicParameters` instance.
    pub fn opening_key(&self) -> &OpeningKey {
        &self.opening_key
    }

    /// Setup generates the public parameters using a random number generator.
    /// This method will in most cases be used for testing and exploration.
    /// In reality, a `Trusted party` or a `Multiparty Computation` will be used
    /// to generate the SRS. Returns an error if the configured degree is less
    /// than one.
    pub fn setup(
        max_degree: usize,
        mut rng: impl RngCore,
    ) -> Result<PublicParameters, Error> {
        // Cannot commit to constants
        if max_degree < 1 {
            return Err(Error::DegreeIsZero);
        }

        // Generate the secret scalar beta
        let beta = util::random_scalar(&mut rng);

        // Compute powers of beta up to and including beta^max_degree
        let powers_of_beta = util::powers_of(&beta, max_degree);

        // Powers of G1 that will be used to commit to a specified polynomial
        let g = util::random_g1_point(&mut rng);
        let powers_of_g: Vec<G1Projective> =
            util::slow_multiscalar_mul_single_base(&powers_of_beta, g);
        assert_eq!(powers_of_g.len(), max_degree + 1);

        // Normalise all projective points
        let mut normalised_g = vec![G1Affine::identity(); max_degree + 1];
        G1Projective::batch_normalize(&powers_of_g, &mut normalised_g);

        // Compute beta*G2 element and stored cached elements for verifying
        // multiple proofs.
        let h: G2Affine = util::random_g2_point(&mut rng).into();
        let beta_h: G2Affine = (h * beta).into();

        Ok(PublicParameters {
            commit_key: CommitKey {
                powers_of_g: normalised_g,
            },
            opening_key: OpeningKey::new(g.into(), h, beta_h),
        })
    }

    /// Serialize the [`PublicParameters`] into bytes.
    ///
    /// This operation is designed to store the raw representation of the
    /// contents of the PublicParameters. Therefore, the size of the bytes
    /// outputed by this function is expected to be the double than the one
    /// that [`PublicParameters::to_var_bytes`].
    ///
    /// # Note
    /// This function should be used when we want to serialize the
    /// PublicParameters allowing a really fast deserialization later.
    /// This functions output should not be used by the regular
    /// [`PublicParameters::from_slice`] fn.
    pub fn to_raw_var_bytes(&self) -> Vec<u8> {
        let mut bytes = self.opening_key.to_bytes().to_vec();
        bytes.extend(&self.commit_key.to_raw_var_bytes());

        bytes
    }

    /// Deserialize [`PublicParameters`] from a set of bytes created by
    /// [`PublicParameters::to_raw_var_bytes`].
    ///
    /// The bytes source is expected to be trusted and no checks will be
    /// performed reggarding the content of the points that the bytes
    /// contain serialized.
    ///
    /// # Safety
    /// This function will not produce any memory errors but can deal to the
    /// generation of invalid or unsafe points/keys. To make sure this does not
    /// happen, the inputed bytes must match the ones that were generated by
    /// the encoding functions of this lib.
    pub unsafe fn from_slice_unchecked(bytes: &[u8]) -> Self {
        let opening_key = &bytes[..OpeningKey::SIZE];
        let opening_key = OpeningKey::from_slice(opening_key)
            .expect("Error at OpeningKey deserialization");

        let commit_key = &bytes[OpeningKey::SIZE..];
        let commit_key = CommitKey::from_slice_unchecked(commit_key);

        Self {
            commit_key,
            opening_key,
        }
    }

    /// Serialises a [`PublicParameters`] struct into a slice of bytes.
    pub fn to_var_bytes(&self) -> Vec<u8> {
        let mut bytes = self.opening_key.to_bytes().to_vec();
        bytes.extend(self.commit_key.to_var_bytes().iter());
        bytes
    }

    /// Deserialise a slice of bytes into a Public Parameter struct performing
    /// security and consistency checks for each point that the bytes
    /// contain.
    ///
    /// # Note
    /// This function can be really slow if the [`PublicParameters`] have a
    /// certain degree. If the bytes come from a trusted source such as a
    /// local file, we recommend to use
    /// [`PublicParameters::from_slice_unchecked`] and
    /// [`PublicParameters::to_raw_var_bytes`].
    pub fn from_slice(bytes: &[u8]) -> Result<PublicParameters, Error> {
        if bytes.len() <= OpeningKey::SIZE {
            return Err(Error::NotEnoughBytes);
        }
        let mut buf = bytes;
        let opening_key = OpeningKey::from_reader(&mut buf)?;
        let commit_key = CommitKey::from_slice(buf)?;

        let pp = PublicParameters {
            commit_key,
            opening_key,
        };

        Ok(pp)
    }

    /// Trim truncates the [`PublicParameters`] to allow the prover to commit to
    /// polynomials up to the and including the truncated degree.
    /// Returns the [`CommitKey`] and [`OpeningKey`] used to generate and verify
    /// proofs.
    ///
    /// Returns an error if the truncated degree is larger than the public
    /// parameters configured degree.
    pub fn trim(
        &self,
        truncated_degree: usize,
    ) -> Result<(CommitKey, OpeningKey), Error> {
        let truncated_prover_key =
            self.commit_key.truncate(truncated_degree)?;
        let opening_key = self.opening_key.clone();
        Ok((truncated_prover_key, opening_key))
    }

    /// Max degree specifies the largest Polynomial
    /// that this prover key can commit to.
    pub fn max_degree(&self) -> usize {
        self.commit_key.max_degree()
    }
}

#[cfg(feature = "std")]
#[cfg(test)]
mod test {
    use super::*;
    use dusk_bls12_381::BlsScalar;
    use rand_core::OsRng;

    #[test]
    fn test_powers_of() {
        let x = BlsScalar::from(10u64);
        let degree = 100u64;

        let powers_of_x = util::powers_of(&x, degree as usize);

        for (i, x_i) in powers_of_x.iter().enumerate() {
            assert_eq!(*x_i, x.pow(&[i as u64, 0, 0, 0]))
        }

        let last_element = powers_of_x.last().unwrap();
        assert_eq!(*last_element, x.pow(&[degree, 0, 0, 0]))
    }

    #[test]
    fn test_serialise_deserialise_public_parameter() {
        let pp = PublicParameters::setup(1 << 7, &mut OsRng).unwrap();

        let got_pp = PublicParameters::from_slice(&pp.to_var_bytes()).unwrap();

        assert_eq!(got_pp.commit_key.powers_of_g, pp.commit_key.powers_of_g);
        assert_eq!(got_pp.opening_key.g, pp.opening_key.g);
        assert_eq!(got_pp.opening_key.h, pp.opening_key.h);
        assert_eq!(got_pp.opening_key.beta_h, pp.opening_key.beta_h);
    }

    #[test]
    fn public_parameters_bytes_unchecked() {
        let pp = PublicParameters::setup(1 << 7, &mut OsRng).unwrap();

        let pp_p = unsafe {
            let bytes = pp.to_raw_var_bytes();
            PublicParameters::from_slice_unchecked(&bytes)
        };

        assert_eq!(pp.commit_key, pp_p.commit_key);
        assert_eq!(pp.opening_key.g, pp_p.opening_key.g);
        assert_eq!(pp.opening_key.h, pp_p.opening_key.h);
        assert_eq!(pp.opening_key.beta_h, pp_p.opening_key.beta_h);
    }
}