doc: add a simple membership circuit example

examples/simple_membership_circuit.rs

@@ -0,0 +1,138 @@
+use std::ops::BitOr;
+
+use ark_crypto_primitives::snark::{CircuitSpecificSetupSNARK, SNARK};
+use ark_ec::pairing::Pairing;
+use ark_ff::PrimeField;
+use ark_groth16::{prepare_verifying_key, Groth16};
+use ark_r1cs_std::{alloc::AllocVar, boolean::Boolean, eq::EqGadget, uint8::UInt8};
+use ark_relations::r1cs::{ConstraintSynthesizer, ConstraintSystemRef, SynthesisError};
+use ark_std::{
+    rand::{RngCore, SeedableRng},
+    test_rng,
+};
+
+struct SecretIsPartOfPublicInputsCircuit<'a, const N_PUBLIC_INPUTS: usize> {
+    secret_value: Option<u8>,
+    public_inputs: Option<&'a [u8; N_PUBLIC_INPUTS]>,
+}
+
+// Those constraints make sure the that secret is present in the public input
+// values, wihout revealing it's actual value to the verifier
+impl<'a, const N_PUBLIC_INPUTS: usize, ConstraintF: PrimeField> ConstraintSynthesizer<ConstraintF>
+    for SecretIsPartOfPublicInputsCircuit<'a, N_PUBLIC_INPUTS>
+{
+    fn generate_constraints(
+        self,
+        cs: ConstraintSystemRef<ConstraintF>,
+    ) -> Result<(), SynthesisError> {
+        let secret_value = UInt8::new_witness(cs.clone(), || {
+            self.secret_value
+                .as_ref()
+                .ok_or(SynthesisError::AssignmentMissing)
+        })?;
+
+        let mut is_found = Boolean::new_constant(cs.clone(), false)?;
+
+        for i in 0..N_PUBLIC_INPUTS {
+            let public_inputs = self
+                .public_inputs
+                .as_ref()
+                .ok_or(SynthesisError::AssignmentMissing)?;
+            let public_input = UInt8::new_input(cs.clone(), || {
+                public_inputs
+                    .get(i)
+                    .ok_or(SynthesisError::AssignmentMissing)
+            })?;
+            let is_eq = public_input.is_eq(&secret_value)?;
+
+            is_found = is_found.bitor(&is_eq);
+        }
+
+        is_found.enforce_equal(&Boolean::constant(true))?;
+
+        Ok(())
+    }
+}
+
+/// Setup, proove and verify the `SecretIsPartOfPublicInputsCircuit`.
+///
+/// * Args
+/// 1) `secret`: the value you want to prove is part of the public input without
+///    revealing it
+/// ..) `public_inputs`: some numbers that will be used as the public inputs to
+/// the circuit
+///
+/// * Usase
+/// ```shell
+/// cargo run --example simple_membership_circuit 42 1 21 42
+/// ```
+///
+/// In order to be able to generate a proof, the secret should be part of the
+/// public inputs. The number of public input is constrained by the
+/// `N_PUB_INPUTS` constant and should be respected.
+fn main() {
+    // Edit this const to change the number of public inputs
+    const N_PUB_INPUTS: usize = 3;
+
+    // Read args
+    let (secret_value, public_inputs) = {
+        let mut args = std::env::args();
+        let secret = str::parse::<u8>(&args.nth(1).expect("arguments should be given"))
+            .expect("all arguments should be valid u8");
+
+        let mut public_inputs = [0u8; N_PUB_INPUTS];
+        let mut n_inputs = 0;
+        for (i, arg) in args.enumerate() {
+            public_inputs[i] = str::parse::<u8>(&arg).expect("all arguments should be valid u8");
+            n_inputs += 1;
+        }
+        assert_eq!(
+            n_inputs, N_PUB_INPUTS,
+            "exactly N_PUB_INPUTS public inputs should be given"
+        );
+
+        (secret, public_inputs)
+    };
+
+    // Setup circuit and generate keys
+    let mut rng = ark_std::rand::rngs::StdRng::seed_from_u64(test_rng().next_u64());
+    let (pk, vk) = Groth16::<ark_bn254::Bn254>::setup(
+        SecretIsPartOfPublicInputsCircuit::<N_PUB_INPUTS> {
+            secret_value: None,
+            public_inputs: Some(&[0u8; N_PUB_INPUTS]),
+        },
+        &mut rng,
+    )
+    .unwrap();
+    let pvk = prepare_verifying_key::<ark_bn254::Bn254>(&vk);
+
+    // Generate proof
+    let proof = Groth16::<ark_bn254::Bn254>::prove(
+        &pk,
+        SecretIsPartOfPublicInputsCircuit::<N_PUB_INPUTS> {
+            secret_value: Some(secret_value),
+            public_inputs: Some(&public_inputs),
+        },
+        &mut rng,
+    )
+    .unwrap();
+
+    // We use `Uint8` as inputs, each one is represented as a sequence of 8
+    // bigendian `Boolean`. Therfore we have to feed the verifier 8 times more
+    // values than our number of public inputs.
+    let mut verifier_inputs = Vec::with_capacity(public_inputs.len() * 8);
+    for input in public_inputs {
+        for i in 0..8 {
+            let mask = 1 << i;
+            verifier_inputs.push(<ark_bn254::Bn254 as Pairing>::ScalarField::from(
+                mask & input != 0,
+            ));
+        }
+    }
+
+    // Verify proof
+    assert!(
+        Groth16::<ark_bn254::Bn254>::verify_with_processed_vk(&pvk, &verifier_inputs, &proof)
+            .unwrap()
+    );
+}