Change pedersen hash outside the circuit to use window table lookups.

benches/pedersen_hash.rs

@@ -0,0 +1,23 @@
+#![feature(test)]
+
+extern crate rand;
+extern crate test;
+extern crate pairing;
+extern crate sapling_crypto;
+
+use rand::{Rand, thread_rng};
+use pairing::bls12_381::Bls12;
+use sapling_crypto::jubjub::JubjubBls12;
+use sapling_crypto::pedersen_hash::{pedersen_hash, Personalization};
+
+#[bench]
+fn bench_pedersen_hash(b: &mut test::Bencher) {
+    let params = JubjubBls12::new();
+    let rng = &mut thread_rng();
+    let bits = (0..510).map(|_| bool::rand(rng)).collect::<Vec<_>>();
+    let personalization = Personalization::MerkleTree(31);
+
+    b.iter(|| {
+        pedersen_hash::<Bls12, _>(personalization, bits.clone(), &params)
+    });
+}

src/jubjub/mod.rs

@@ -112,6 +112,8 @@ pub trait JubjubParams<E: JubjubEngine>: Sized {
     fn scale(&self) -> &E::Fr;
     /// Returns the generators (for each segment) used in all Pedersen commitments.
     fn pedersen_hash_generators(&self) -> &[edwards::Point<E, PrimeOrder>];
+    /// Returns the exp table for Pedersen hashes.
+    fn pedersen_hash_exp_table(&self) -> &[Vec<Vec<edwards::Point<E, PrimeOrder>>>];
     /// Returns the maximum number of chunks per segment of the Pedersen hash.
     fn pedersen_hash_chunks_per_generator(&self) -> usize;
     /// Returns the pre-computed window tables [-4, 3, 2, 1, 1, 2, 3, 4] of different
@@ -126,6 +128,9 @@ pub trait JubjubParams<E: JubjubEngine>: Sized {
     /// Returns a window table [0, 1, ..., 8] for different magnitudes of some
     /// fixed generator.
     fn circuit_generators(&self, FixedGenerators) -> &[Vec<(E::Fr, E::Fr)>];
+    /// Returns the window size for exponentiation of Pedersen hash generators
+    /// outside the circuit
+    fn pedersen_hash_exp_window_size() -> u32;
 }
 
 impl JubjubEngine for Bls12 {
@@ -140,6 +145,7 @@ pub struct JubjubBls12 {
     scale: Fr,
 
     pedersen_hash_generators: Vec<edwards::Point<Bls12, PrimeOrder>>,
+    pedersen_hash_exp: Vec<Vec<Vec<edwards::Point<Bls12, PrimeOrder>>>>,
     pedersen_circuit_generators: Vec<Vec<Vec<(Fr, Fr)>>>,
 
     fixed_base_generators: Vec<edwards::Point<Bls12, PrimeOrder>>,
@@ -154,6 +160,9 @@ impl JubjubParams<Bls12> for JubjubBls12 {
     fn pedersen_hash_generators(&self) -> &[edwards::Point<Bls12, PrimeOrder>] {
         &self.pedersen_hash_generators
     }
+    fn pedersen_hash_exp_table(&self) -> &[Vec<Vec<edwards::Point<Bls12, PrimeOrder>>>] {
+        &self.pedersen_hash_exp
+    }
     fn pedersen_hash_chunks_per_generator(&self) -> usize {
         63
     }
@@ -171,6 +180,9 @@ impl JubjubParams<Bls12> for JubjubBls12 {
     {
         &self.fixed_base_circuit_generators[base as usize][..]
     }
+    fn pedersen_hash_exp_window_size() -> u32 {
+        8
+    }
 }
 
 impl JubjubBls12 {
@@ -191,6 +203,7 @@ impl JubjubBls12 {
 
             // We'll initialize these below
             pedersen_hash_generators: vec![],
+            pedersen_hash_exp: vec![],
             pedersen_circuit_generators: vec![],
             fixed_base_generators: vec![],
             fixed_base_circuit_generators: vec![],
@@ -258,6 +271,42 @@ impl JubjubBls12 {
             tmp_params.pedersen_hash_generators = pedersen_hash_generators;
         }
 
+        // Create the exp table for the Pedersen hash generators
+        {
+            let mut pedersen_hash_exp = vec![];
+
+            for g in &tmp_params.pedersen_hash_generators {
+                let mut g = g.clone();
+
+                let window = JubjubBls12::pedersen_hash_exp_window_size();
+
+                let mut tables = vec![];
+
+                let mut num_bits = 0;
+                while num_bits <= fs::Fs::NUM_BITS {
+                    let mut table = Vec::with_capacity(1 << window);
+
+                    let mut base = edwards::Point::zero();
+
+                    for _ in 0..(1 << window) {
+                        table.push(base.clone());
+                        base = base.add(&g, &tmp_params);
+                    }
+
+                    tables.push(table);
+                    num_bits += window;
+
+                    for _ in 0..window {
+                        g = g.double(&tmp_params);
+                    }
+                }
+
+                pedersen_hash_exp.push(tables);
+            }
+
+            tmp_params.pedersen_hash_exp = pedersen_hash_exp;
+        }
+
         // Create the bases for other parts of the protocol
         {
             let mut fixed_base_generators = vec![edwards::Point::zero(); FixedGenerators::Max as usize];

src/pedersen_hash.rs

@@ -1,6 +1,7 @@
 use jubjub::*;
 use pairing::*;
 
+#[derive(Copy, Clone)]
 pub enum Personalization {
     NoteCommitment,
     MerkleTree(usize)
@@ -31,7 +32,7 @@ pub fn pedersen_hash<E, I>(
     let mut bits = personalization.get_bits().into_iter().chain(bits.into_iter());
 
     let mut result = edwards::Point::zero();
-    let mut generators = params.pedersen_hash_generators().iter();
+    let mut generators = params.pedersen_hash_exp_table().iter();
 
     loop {
         let mut acc = E::Fs::zero();
@@ -78,8 +79,23 @@ pub fn pedersen_hash<E, I>(
             break;
         }
 
-        let mut tmp = generators.next().expect("we don't have enough generators").clone();
+        let mut table: &[Vec<edwards::Point<E, _>>] = &generators.next().expect("we don't have enough generators");
-        tmp = tmp.mul(acc, params);
+        let window = JubjubBls12::pedersen_hash_exp_window_size();
+        let window_mask = (1 << window) - 1;
+
+        let mut acc = acc.into_repr();
+        
+        let mut tmp = edwards::Point::zero();
+
+        while !acc.is_zero() {
+            let i = (acc.as_ref()[0] & window_mask) as usize;
+
+            tmp = tmp.add(&table[0][i], params);
+
+            acc.shr(window);
+            table = &table[1..];
+        }
+
         result = result.add(&tmp, params);
     }