darkfi/src/old/sha256.rs

// Say we want to write a circuit that proves we know the preimage to some hash computed
// using SHA-256d (calling SHA-256 twice). The preimage must have a fixed length known in
// advance (because the circuit parameters will depend on it), but can otherwise have any value.
// We take the following strategy:
//
// * Witness each bit of the preimage.
// * Compute hash = SHA-256d(preimage) inside the circuit.
// * Expose hash as a public input using multiscalar packing.
//
use bellman::{
    gadgets::{
        boolean::{AllocatedBit, Boolean},
        multipack,
        sha256::sha256,
    },
    groth16, Circuit, ConstraintSystem, SynthesisError,
};
use bls12_381::Bls12;
use ff::PrimeField;
use rand::rngs::OsRng;
use sha2::{Digest, Sha256};

/// Our own SHA-256d gadget. Input and output are in little-endian bit order.
fn sha256d<Scalar: PrimeField, CS: ConstraintSystem<Scalar>>(
    mut cs: CS,
    data: &[Boolean],
) -> Result<Vec<Boolean>, SynthesisError> {
    // Flip endianness of each input byte
    // NOTE: data is a vec of Bool so it is iterating over 8 'bits' at a time
    // This is needed because Rust sha256 and ZC sha256 have different endianness.
    let input: Vec<_> = data
        .chunks(8)
        .map(|c| c.iter().rev())
        .flatten()
        .cloned()
        .collect();

    let mid = sha256(cs.namespace(|| "SHA-256(input)"), &input)?;
    let res = sha256(cs.namespace(|| "SHA-256(mid)"), &mid)?;

    // Flip endianness of each output byte
    Ok(res
        .chunks(8)
        .map(|c| c.iter().rev())
        .flatten()
        .cloned()
        .collect())
}

struct MyCircuit {
    /// The input to SHA-256d we are proving that we know. Set to `None` when we
    /// are verifying a proof (and do not have the witness data).
    preimage: Option<[u8; 80]>,
}

impl<Scalar: PrimeField> Circuit<Scalar> for MyCircuit {
    fn synthesize<CS: ConstraintSystem<Scalar>>(self, cs: &mut CS) -> Result<(), SynthesisError> {
        // Compute the values for the bits of the preimage. If we are verifying a proof,
        // we still need to create the same constraints, so we return an equivalent-size
        // Vec of None (indicating that the value of each bit is unknown).
        let bit_values = if let Some(preimage) = self.preimage {
            preimage
                .iter()
                .map(|byte| (0..8).map(move |i| (byte >> i) & 1u8 == 1u8))
                .flatten()
                .map(|b| Some(b))
                .collect()
        } else {
            vec![None; 80 * 8]
        };
        assert_eq!(bit_values.len(), 80 * 8);

        // Witness the bits of the preimage.
        let preimage_bits = bit_values
            .into_iter()
            .enumerate()
            // Allocate each bit.
            .map(|(i, b)| AllocatedBit::alloc(cs.namespace(|| format!("preimage bit {}", i)), b))
            // Convert the AllocatedBits into Booleans (required for the sha256 gadget).
            .map(|b| b.map(Boolean::from))
            .collect::<Result<Vec<_>, _>>()?;

        // Compute hash = SHA-256d(preimage).
        let hash = sha256d(cs.namespace(|| "SHA-256d(preimage)"), &preimage_bits)?;

        // Expose the vector of 32 boolean variables as compact public inputs.
        multipack::pack_into_inputs(cs.namespace(|| "pack hash"), &hash)
    }
}

fn main() {
    use std::time::Instant;

    let start = Instant::now();
    println!("Starting...");
    // Create parameters for our circuit. In a production deployment these would
    // be generated securely using a multiparty computation.
    let params = {
        let c = MyCircuit { preimage: None };
        groth16::generate_random_parameters::<Bls12, _, _>(c, &mut OsRng).unwrap()
    };
    println!("Generated random params. [{:?}]", start.elapsed());

    let start = Instant::now();
    // Prepare the verification key (for proof verification).
    let pvk = groth16::prepare_verifying_key(&params.vk);
    println!("Prepared verify key [{:?}]", start.elapsed());

    let start = Instant::now();
    // Pick a preimage and compute its hash.
    let preimage = [42; 80];
    let hash = Sha256::digest(&Sha256::digest(&preimage));
    println!(
        "Computed sha256(sha256(preimage)) witness data [{:?}]",
        start.elapsed()
    );

    // Create an instance of our circuit (with the preimage as a witness).
    let c = MyCircuit {
        preimage: Some(preimage),
    };

    let start = Instant::now();
    // Create a Groth16 proof with our parameters.
    let proof = groth16::create_random_proof(c, &params, &mut OsRng).unwrap();
    println!("Generated random proof [{:?}]", start.elapsed());

    let start = Instant::now();
    // Pack the hash as inputs for proof verification.
    let hash_bits = multipack::bytes_to_bits_le(&hash);
    let inputs = multipack::compute_multipacking(&hash_bits);
    println!("Packed data and verifying proof... [{:?}]", start.elapsed());

    let start = Instant::now();
    // Check the proof!
    assert!(groth16::verify_proof(&pvk, &proof, &inputs).is_ok());
    println!("Done! [{:?}]", start.elapsed());
}