Nova/examples/minroot.rs

//! Demonstrates how to use Nova to produce a recursive proof of the correct execution of
//! iterations of the MinRoot function, thereby realizing a Nova-based verifiable delay function (VDF).
//! We execute a configurable number of iterations of the MinRoot function per step of Nova's recursion.
type G1 = pasta_curves::pallas::Point;
type G2 = pasta_curves::vesta::Point;
use ::bellperson::{gadgets::num::AllocatedNum, ConstraintSystem, SynthesisError};
use ff::PrimeField;
use generic_array::typenum::U2;
use neptune::{
  circuit::poseidon_hash,
  poseidon::{Poseidon, PoseidonConstants},
  Strength,
};
use nova_snark::{
  traits::{Group, StepCircuit},
  CompressedSNARK, PublicParams, RecursiveSNARK,
};
use num_bigint::BigUint;
use std::marker::PhantomData;
use std::time::Instant;

#[derive(Clone, Debug)]
struct MinRootIteration<F: PrimeField> {
  x_i: F,
  y_i: F,
  x_i_plus_1: F,
  y_i_plus_1: F,
}

impl<F: PrimeField> MinRootIteration<F> {
  // produces a sample non-deterministic advice, executing one invocation of MinRoot per step
  fn new(num_iters: usize, x_0: &F, y_0: &F, pc: &PoseidonConstants<F, U2>) -> (F, Vec<Self>) {
    // although this code is written generically, it is tailored to Pallas' scalar field
    // (p - 3 / 5)
    let exp = BigUint::parse_bytes(
      b"23158417847463239084714197001737581570690445185553317903743794198714690358477",
      10,
    )
    .unwrap();

    let mut res = Vec::new();
    let mut x_i = *x_0;
    let mut y_i = *y_0;
    for _i in 0..num_iters {
      let x_i_plus_1 = (x_i + y_i).pow_vartime(exp.to_u64_digits()); // computes the fifth root of x_i + y_i

      // sanity check
      let sq = x_i_plus_1 * x_i_plus_1;
      let quad = sq * sq;
      let fifth = quad * x_i_plus_1;
      debug_assert_eq!(fifth, x_i + y_i);

      let y_i_plus_1 = x_i;

      res.push(Self {
        x_i,
        y_i,
        x_i_plus_1,
        y_i_plus_1,
      });

      x_i = x_i_plus_1;
      y_i = y_i_plus_1;
    }

    let z0 = Poseidon::<F, U2>::new_with_preimage(&[*x_0, *y_0], pc).hash();

    (z0, res)
  }
}

#[derive(Clone, Debug)]
struct MinRootCircuit<F: PrimeField> {
  seq: Vec<MinRootIteration<F>>,
  pc: PoseidonConstants<F, U2>,
}

impl<F> StepCircuit<F> for MinRootCircuit<F>
where
  F: PrimeField,
{
  fn synthesize<CS: ConstraintSystem<F>>(
    &self,
    cs: &mut CS,
    z: AllocatedNum<F>,
  ) -> Result<AllocatedNum<F>, SynthesisError> {
    let mut z_out: Result<AllocatedNum<F>, SynthesisError> = Err(SynthesisError::AssignmentMissing);
    for i in 0..self.seq.len() {
      // Allocate four variables for holding non-deterministic advice: x_i, y_i, x_i_plus_1, y_i_plus_1
      let x_i = AllocatedNum::alloc(cs.namespace(|| format!("x_i_iter_{}", i)), || {
        Ok(self.seq[i].x_i)
      })?;
      let y_i = AllocatedNum::alloc(cs.namespace(|| format!("y_i_iter_{}", i)), || {
        Ok(self.seq[i].y_i)
      })?;
      let x_i_plus_1 =
        AllocatedNum::alloc(cs.namespace(|| format!("x_i_plus_1_iter_{}", i)), || {
          Ok(self.seq[i].x_i_plus_1)
        })?;

      // check that z = hash(x_i, y_i), where z is an output from the prior step
      if i == 0 {
        let z_hash = poseidon_hash(
          cs.namespace(|| "input hash"),
          vec![x_i.clone(), y_i.clone()],
          &self.pc,
        )?;
        cs.enforce(
          || "z =? z_hash",
          |lc| lc + z_hash.get_variable(),
          |lc| lc + CS::one(),
          |lc| lc + z.get_variable(),
        );
      }

      // check the following conditions hold:
      // (i) x_i_plus_1 = (x_i + y_i)^{1/5}, which can be more easily checked with x_i_plus_1^5 = x_i + y_i
      // (ii) y_i_plus_1 = x_i
      // (1) constraints for condition (i) are below
      // (2) constraints for condition (ii) is avoided because we just used x_i wherever y_i_plus_1 is used
      let x_i_plus_1_sq =
        x_i_plus_1.square(cs.namespace(|| format!("x_i_plus_1_sq_iter_{}", i)))?;
      let x_i_plus_1_quad =
        x_i_plus_1_sq.square(cs.namespace(|| format!("x_i_plus_1_quad_{}", i)))?;
      let x_i_plus_1_pow_5 = x_i_plus_1_quad.mul(
        cs.namespace(|| format!("x_i_plus_1_pow_5_{}", i)),
        &x_i_plus_1,
      )?;
      cs.enforce(
        || format!("x_i_plus_1_pow_5 = x_i + y_i_iter_{}", i),
        |lc| lc + x_i_plus_1_pow_5.get_variable(),
        |lc| lc + CS::one(),
        |lc| lc + x_i.get_variable() + y_i.get_variable(),
      );

      // return hash(x_i_plus_1, y_i_plus_1) since Nova circuits expect a single output
      if i == self.seq.len() - 1 {
        z_out = poseidon_hash(
          cs.namespace(|| "output hash"),
          vec![x_i_plus_1, x_i.clone()],
          &self.pc,
        );
      }
    }

    z_out
  }

  fn compute(&self, z: &F) -> F {
    // sanity check
    let z_hash =
      Poseidon::<F, U2>::new_with_preimage(&[self.seq[0].x_i, self.seq[0].y_i], &self.pc).hash();
    debug_assert_eq!(z, &z_hash);

    // compute output hash using advice
    let iters = self.seq.len();
    Poseidon::<F, U2>::new_with_preimage(
      &[
        self.seq[iters - 1].x_i_plus_1,
        self.seq[iters - 1].y_i_plus_1,
      ],
      &self.pc,
    )
    .hash()
  }
}

fn main() {
  let num_steps = 10;
  let num_iters_per_step = 10; // number of iterations of MinRoot per Nova's recursive step

  let pc = PoseidonConstants::<<G1 as Group>::Scalar, U2>::new_with_strength(Strength::Standard);
  let circuit_primary = MinRootCircuit {
    seq: vec![
      MinRootIteration {
        x_i: <G1 as Group>::Scalar::zero(),
        y_i: <G1 as Group>::Scalar::zero(),
        x_i_plus_1: <G1 as Group>::Scalar::zero(),
        y_i_plus_1: <G1 as Group>::Scalar::zero(),
      };
      num_iters_per_step
    ],
    pc: pc.clone(),
  };

  let circuit_secondary = TrivialTestCircuit {
    _p: Default::default(),
  };

  println!("Nova-based VDF with MinRoot delay function");
  println!("==========================================");
  println!(
    "Proving {} iterations of MinRoot per step",
    num_iters_per_step
  );

  // produce public parameters
  println!("Producing public parameters...");
  let pp = PublicParams::<
    G1,
    G2,
    MinRootCircuit<<G1 as Group>::Scalar>,
    TrivialTestCircuit<<G2 as Group>::Scalar>,
  >::setup(circuit_primary, circuit_secondary.clone());
  println!(
    "Number of constraints per step (primary circuit): {}",
    pp.num_constraints().0
  );
  println!(
    "Number of constraints per step (secondary circuit): {}",
    pp.num_constraints().1
  );

  // produce non-deterministic advice
  let (z0_primary, minroot_iterations) = MinRootIteration::new(
    num_iters_per_step * num_steps,
    &<G1 as Group>::Scalar::zero(),
    &<G1 as Group>::Scalar::one(),
    &pc,
  );
  let minroot_circuits = (0..num_steps)
    .map(|i| MinRootCircuit {
      seq: (0..num_iters_per_step)
        .map(|j| MinRootIteration {
          x_i: minroot_iterations[i * num_iters_per_step + j].x_i,
          y_i: minroot_iterations[i * num_iters_per_step + j].y_i,
          x_i_plus_1: minroot_iterations[i * num_iters_per_step + j].x_i_plus_1,
          y_i_plus_1: minroot_iterations[i * num_iters_per_step + j].y_i_plus_1,
        })
        .collect::<Vec<_>>(),
      pc: pc.clone(),
    })
    .collect::<Vec<_>>();

  let z0_secondary = <G2 as Group>::Scalar::zero();

  type C1 = MinRootCircuit<<G1 as Group>::Scalar>;
  type C2 = TrivialTestCircuit<<G2 as Group>::Scalar>;
  // produce a recursive SNARK
  println!("Generating a RecursiveSNARK...");
  let mut recursive_snark: Option<RecursiveSNARK<G1, G2, C1, C2>> = None;

  for (i, circuit_primary) in minroot_circuits.iter().take(num_steps).enumerate() {
    let start = Instant::now();
    let res = RecursiveSNARK::prove_step(
      &pp,
      recursive_snark,
      circuit_primary.clone(),
      circuit_secondary.clone(),
      z0_primary,
      z0_secondary,
    );
    assert!(res.is_ok());
    println!(
      "RecursiveSNARK::prove_step {}: {:?}, took {:?} ",
      i,
      res.is_ok(),
      start.elapsed()
    );
    recursive_snark = Some(res.unwrap());
  }

  assert!(recursive_snark.is_some());
  let recursive_snark = recursive_snark.unwrap();

  // verify the recursive SNARK
  println!("Verifying a RecursiveSNARK...");
  let start = Instant::now();
  let res = recursive_snark.verify(&pp, num_steps, z0_primary, z0_secondary);
  println!(
    "RecursiveSNARK::verify: {:?}, took {:?}",
    res.is_ok(),
    start.elapsed()
  );
  assert!(res.is_ok());

  // produce a compressed SNARK
  println!("Generating a CompressedSNARK using Spartan with IPA-PC...");
  let start = Instant::now();
  type S1 = nova_snark::spartan_with_ipa_pc::RelaxedR1CSSNARK<G1>;
  type S2 = nova_snark::spartan_with_ipa_pc::RelaxedR1CSSNARK<G2>;
  let res = CompressedSNARK::<_, _, _, _, S1, S2>::prove(&pp, &recursive_snark);
  println!(
    "CompressedSNARK::prove: {:?}, took {:?}",
    res.is_ok(),
    start.elapsed()
  );
  assert!(res.is_ok());
  let compressed_snark = res.unwrap();

  // verify the compressed SNARK
  println!("Verifying a CompressedSNARK...");
  let start = Instant::now();
  let res = compressed_snark.verify(&pp, num_steps, z0_primary, z0_secondary);
  println!(
    "CompressedSNARK::verify: {:?}, took {:?}",
    res.is_ok(),
    start.elapsed()
  );
  assert!(res.is_ok());
}

// A trivial test circuit that we use on the secondary curve
#[derive(Clone, Debug)]
struct TrivialTestCircuit<F: PrimeField> {
  _p: PhantomData<F>,
}

impl<F> StepCircuit<F> for TrivialTestCircuit<F>
where
  F: PrimeField,
{
  fn synthesize<CS: ConstraintSystem<F>>(
    &self,
    _cs: &mut CS,
    z: AllocatedNum<F>,
  ) -> Result<AllocatedNum<F>, SynthesisError> {
    Ok(z)
  }

  fn compute(&self, z: &F) -> F {
    *z
  }
}