add hash_to_scalar

ecgfp5/src/curve/scalar_field.rs

@@ -9,14 +9,19 @@ use core::{
     iter::{Product, Sum},
     ops::{Add, AddAssign, Div, DivAssign, Mul, MulAssign, Neg, Sub, SubAssign},
 };
+use plonky2::hash::hash_types::HashOut;
 use plonky2_field::extension::quintic::QuinticExtension;
 use rand::RngCore;
 
 use itertools::Itertools;
 use num::{bigint::BigUint, One};
 use serde::{Deserialize, Serialize};
 
-use plonky2_field::types::{Field, PrimeField, PrimeField64, Sample};
+use plonky2_field::{
+    extension::FieldExtension,
+    goldilocks_field::GoldilocksField,
+    types::{Field, PrimeField, PrimeField64, Sample},
+};
 
 use super::GFp5;
 
@@ -462,6 +467,13 @@ impl Scalar {
         Self::from_noncanonical_biguint(biguint_from_array(limbs.map(|l| l.to_canonical_u64())))
     }
 
+    pub fn from_hashout(x: HashOut<GoldilocksField>) -> Self {
+        let mut arr: [GoldilocksField; 5] = [GoldilocksField::ZERO; 5];
+        arr[1..].copy_from_slice(&x.elements);
+        let gfp5 = GFp5::from_basefield_array(arr);
+        Self::from_gfp5(gfp5)
+    }
+
     /// Decode the provided byte slice into a scalar. The bytes are
     /// interpreted into an integer in little-endian unsigned convention.
     /// All slice bytes are read, and the value is REDUCED modulo n. This

ecgfp5/src/gadgets/mod.rs

@@ -1,4 +1,5 @@
 pub mod base_field;
 pub mod curve;
+pub mod poseidon;
 pub mod scalar_field;
 pub mod schnorr;

ecgfp5/src/gadgets/poseidon.rs

@@ -0,0 +1,95 @@
+use crate::curve::scalar_field::Scalar;
+use plonky2::{
+    hash::poseidon::PoseidonHash,
+    iop::target::Target,
+    plonk::{circuit_builder::CircuitBuilder, config::Hasher},
+};
+use plonky2_ecdsa::gadgets::nonnative::NonNativeTarget;
+use plonky2_field::{goldilocks_field::GoldilocksField, types::Field};
+
+use super::base_field::{CircuitBuilderGFp5, QuinticExtensionTarget};
+
+pub fn hash_to_scalar(domain: &[u8], msg: &[u8]) -> Scalar {
+    let f_domain = u8_to_goldilocks(domain);
+    let f_msg = u8_to_goldilocks(msg);
+    let hashout = PoseidonHash::hash_no_pad(&[f_domain.as_slice(), f_msg.as_slice()].concat());
+    Scalar::from_hashout(hashout)
+}
+
+pub fn hash_to_scalar_target(
+    builder: &mut CircuitBuilder<GoldilocksField, 2>,
+    domain: &[u8],
+    msg: Vec<Target>,
+) -> NonNativeTarget<Scalar> {
+    let mut preimage = vec![];
+    let f_domain: Vec<Target> =
+        u8_to_goldilocks(domain).iter().map(|x| builder.constant(*x)).collect();
+    preimage.extend(f_domain);
+    preimage.extend(msg);
+    let hashout = builder.hash_n_to_hash_no_pad::<PoseidonHash>(preimage);
+    let mut limbs = [builder.zero(); 5];
+    limbs[1..].copy_from_slice(&hashout.elements);
+    let result = QuinticExtensionTarget::new(limbs);
+    builder.encode_quintic_ext_as_scalar(result)
+}
+
+/// Convert [u8; 8] to one GoldilocksField
+///
+/// non-canoncial [u8; 8] will panic
+pub fn u8_to_goldilocks(data: &[u8]) -> Vec<GoldilocksField> {
+    const CHUNK_SIZE: usize = 8;
+    data.chunks(CHUNK_SIZE)
+        .map(|chunk| {
+            let mut padded = [0u8; CHUNK_SIZE];
+            let len = chunk.len().min(CHUNK_SIZE);
+            padded[..len].copy_from_slice(&chunk[..len]);
+            GoldilocksField::from_canonical_u64(u64::from_le_bytes(padded))
+        })
+        .collect::<Vec<GoldilocksField>>()
+}
+
+#[cfg(test)]
+mod tests {
+    use plonky2::{
+        iop::witness::PartialWitness,
+        plonk::{
+            circuit_data::CircuitConfig,
+            config::{GenericConfig, PoseidonGoldilocksConfig},
+        },
+    };
+    use plonky2_ecdsa::gadgets::nonnative::PartialWitnessNonNative;
+    use plonky2_field::types::Field64;
+
+    use super::*;
+
+    #[test]
+    #[should_panic]
+    fn test_u8_to_goldilocks_noncanonical() {
+        let order_plus_one = (GoldilocksField::ORDER + 1).to_le_bytes();
+        // noncanonical u64 will panic
+        u8_to_goldilocks(&order_plus_one);
+    }
+
+    #[test]
+    fn test_hash_to_scalar() {
+        const D: usize = 2;
+        type C = PoseidonGoldilocksConfig;
+        type F = <C as GenericConfig<D>>::F;
+
+        let domain = b"domain-plonky2-ecgfp5-poseidon";
+        let msg = b"msg-to-hash-to-scalar";
+        let scalar = hash_to_scalar(domain, msg);
+
+        let config = CircuitConfig::standard_recursion_config();
+        let mut builder = CircuitBuilder::<F, D>::new(config);
+        let msg_target: Vec<Target> =
+            u8_to_goldilocks(msg).iter().map(|x| builder.constant(*x)).collect();
+        let scalar_target = hash_to_scalar_target(&mut builder, domain, msg_target);
+
+        let circuit = builder.build::<C>();
+        let mut pw = PartialWitness::new();
+        pw.set_nonnative_target(scalar_target, scalar);
+        let proof = circuit.prove(pw).unwrap();
+        circuit.verify(proof).unwrap();
+    }
+}