implement proof-based signatures using plonky2 proofs (#112)

* implement proof-based signatures using plonky2 proofs

* proof-based sigs: polish & document

src/backends/plonky2/primitives/signature.rs

@@ -0,0 +1,194 @@
+//! Proof-based signatures using Plonky2 proofs, following
+//! https://eprint.iacr.org/2024/1553 .
+use anyhow::Result;
+use plonky2::{
+    field::types::Sample,
+    hash::{
+        hash_types::{HashOut, HashOutTarget},
+        poseidon::PoseidonHash,
+    },
+    iop::{
+        target::Target,
+        witness::{PartialWitness, WitnessWrite},
+    },
+    plonk::{
+        circuit_builder::CircuitBuilder,
+        circuit_data::{CircuitConfig, ProverCircuitData, VerifierCircuitData},
+        config::Hasher,
+        proof::ProofWithPublicInputs,
+    },
+};
+
+use crate::backends::plonky2::basetypes::{Proof, Value, C, D, F, VALUE_SIZE};
+
+pub struct ProverParams {
+    prover: ProverCircuitData<F, C, D>,
+    circuit: SignatureCircuit,
+}
+
+#[derive(Debug)]
+pub struct VerifierParams(VerifierCircuitData<F, C, D>);
+
+#[derive(Clone, Debug)]
+pub struct SecretKey(Value);
+
+#[derive(Clone, Debug)]
+pub struct PublicKey(Value);
+
+#[derive(Clone, Debug)]
+pub struct Signature(Proof);
+
+/// Implements the key generation and the computation of proof-based signatures.
+impl SecretKey {
+    pub fn new() -> Self {
+        // note: the `F::rand()` internally uses `rand::rngs::OsRng`
+        Self(Value(std::array::from_fn(|_| F::rand())))
+    }
+
+    pub fn public_key(&self) -> PublicKey {
+        PublicKey(Value(PoseidonHash::hash_no_pad(&self.0 .0).elements))
+    }
+
+    pub fn sign(&self, pp: &ProverParams, msg: Value) -> Result<Signature> {
+        let pk = self.public_key();
+        let s = Value(PoseidonHash::hash_no_pad(&[pk.0 .0, msg.0].concat()).elements);
+
+        let mut pw = PartialWitness::<F>::new();
+        pp.circuit.set_targets(&mut pw, self.clone(), pk, msg, s)?;
+
+        let proof = pp.prover.prove(pw)?;
+
+        Ok(Signature(proof.proof))
+    }
+}
+
+/// Implements the parameters generation and the verification of proof-based
+/// signatures.
+impl Signature {
+    pub fn params() -> Result<(ProverParams, VerifierParams)> {
+        let (builder, circuit) = Self::builder()?;
+        let prover = builder.build_prover::<C>();
+
+        let (builder, _) = Self::builder()?;
+        let circuit_data = builder.build::<C>();
+        let vp = circuit_data.verifier_data();
+
+        Ok((ProverParams { prover, circuit }, VerifierParams(vp)))
+    }
+
+    fn builder() -> Result<(CircuitBuilder<F, D>, SignatureCircuit)> {
+        // notice that we use the 'zk' config
+        let config = CircuitConfig::standard_recursion_zk_config();
+
+        let mut builder = CircuitBuilder::<F, D>::new(config);
+        let circuit = SignatureCircuit::add_targets(&mut builder)?;
+
+        Ok((builder, circuit))
+    }
+
+    pub fn verify(&self, vp: &VerifierParams, pk: &PublicKey, msg: Value) -> Result<()> {
+        // prepare public inputs as [pk, msg, s]
+        let s = Value(PoseidonHash::hash_no_pad(&[pk.0 .0, msg.0].concat()).elements);
+        let public_inputs: Vec<F> = [pk.0 .0, msg.0, s.0].concat();
+
+        // verify plonky2 proof
+        vp.0.verify(ProofWithPublicInputs {
+            proof: self.0.clone(),
+            public_inputs,
+        })
+    }
+}
+
+/// The SignatureCircuit implements the circuit used for the proof of the
+/// argument described at https://eprint.iacr.org/2024/1553.
+///
+/// The circuit proves that for the given public inputs (pk, msg, s), the Prover
+/// knows the secret (sk) such that:
+/// i) pk == H(sk)
+/// ii) s == H(pk, msg)
+struct SignatureCircuit {
+    sk_targ: Vec<Target>,
+    pk_targ: HashOutTarget,
+    msg_targ: Vec<Target>,
+    s_targ: HashOutTarget,
+}
+
+impl SignatureCircuit {
+    /// creates the targets and defines the logic of the circuit
+    fn add_targets(builder: &mut CircuitBuilder<F, D>) -> Result<Self> {
+        // create the targets
+        let sk_targ = builder.add_virtual_targets(VALUE_SIZE);
+        let pk_targ = builder.add_virtual_hash();
+        let msg_targ = builder.add_virtual_targets(VALUE_SIZE);
+        let s_targ = builder.add_virtual_hash();
+
+        // define the public inputs
+        builder.register_public_inputs(&pk_targ.elements);
+        builder.register_public_inputs(&msg_targ);
+        builder.register_public_inputs(&s_targ.elements);
+
+        // define the logic
+        let computed_pk_targ = builder.hash_n_to_hash_no_pad::<PoseidonHash>(sk_targ.clone());
+        builder.connect_array::<VALUE_SIZE>(computed_pk_targ.elements, pk_targ.elements);
+
+        let inp: Vec<Target> = [pk_targ.elements.to_vec(), msg_targ.clone()].concat();
+        let computed_s_targ = builder.hash_n_to_hash_no_pad::<PoseidonHash>(inp);
+        builder.connect_array::<VALUE_SIZE>(computed_s_targ.elements, s_targ.elements);
+
+        // return the targets
+        Ok(Self {
+            sk_targ,
+            pk_targ,
+            msg_targ,
+            s_targ,
+        })
+    }
+
+    /// assigns the given values to the targets
+    fn set_targets(
+        &self,
+        pw: &mut PartialWitness<F>,
+        sk: SecretKey,
+        pk: PublicKey,
+        msg: Value,
+        s: Value,
+    ) -> Result<()> {
+        pw.set_target_arr(&self.sk_targ, &sk.0 .0.to_vec())?;
+        pw.set_hash_target(self.pk_targ, HashOut::<F>::from_vec(pk.0 .0.to_vec()))?;
+        pw.set_target_arr(&self.msg_targ, &msg.0.to_vec())?;
+        pw.set_hash_target(self.s_targ, HashOut::<F>::from_vec(s.0.to_vec()))?;
+
+        Ok(())
+    }
+}
+
+#[cfg(test)]
+pub mod tests {
+    use crate::backends::plonky2::basetypes::Hash;
+
+    use super::*;
+
+    // Note: this test must be run with the `--release` flag.
+    #[test]
+    fn test_signature() -> Result<()> {
+        let (pp, vp) = Signature::params()?;
+
+        let sk = SecretKey::new();
+        let pk = sk.public_key();
+
+        let msg = Value::from(42);
+        let sig = sk.sign(&pp, msg)?;
+        sig.verify(&vp, &pk, msg)?;
+
+        // expect the signature verification to fail when using a different msg
+        let v = sig.verify(&vp, &pk, Value::from(24));
+        assert!(v.is_err(), "should fail to verify");
+
+        // perform a 2nd signature over another msg and verify it
+        let msg_2 = Value::from(Hash::from("message"));
+        let sig2 = sk.sign(&pp, msg_2)?;
+        sig2.verify(&vp, &pk, msg_2)?;
+
+        Ok(())
+    }
+}