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zkinterface_backend.rs
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zkinterface_backend.rs
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//! A zkInterface backend using Bulletproofs.
extern crate curve25519_dalek;
extern crate merlin;
extern crate rand;
extern crate zkinterface;
use self::zkinterface::{
Result, Reader, consumers::reader::Term,
};
use curve25519_dalek::scalar::Scalar;
use errors::R1CSError;
use failure::Fail;
use merlin::Transcript;
use r1cs::ConstraintSystem;
use r1cs::LinearCombination;
use r1cs::Prover;
use r1cs::R1CSProof;
use r1cs::Variable;
use r1cs::Verifier;
use std::cmp::min;
use std::collections::HashMap;
use BulletproofGens;
use PedersenGens;
/// Generate a proof using zkInterface messages:
/// - `Circuit` contains the public inputs.
/// - `R1CSConstraints` contains an R1CS which we convert to an arithmetic circuit on the fly.
/// - `Witness` contains the values to assign to all variables.
pub fn prove(messages: &Reader) -> Result<R1CSProof> {
// Common
let pc_gens = PedersenGens::default();
let bp_gens = BulletproofGens::new(128, 1);
let mut transcript = Transcript::new(b"zkInterfaceGadget");
// /Common
// 1. Create a prover
let prover = Prover::new(&bp_gens, &pc_gens, &mut transcript);
// 2. There are no high-level variables.
// 3. Build a CS
let mut cs = prover.finalize_inputs();
gadget_from_messages(&mut cs, messages, true)?;
// 4. Make a proof
let proof = cs.prove().map_err(|e| e.compat())?;
Ok(proof)
}
/// Verify a proof using zkInterface messages:
/// - `Circuit` contains the public inputs.
/// - `R1CSConstraints` contains an R1CS which we convert to an arithmetic circuit on the fly.
pub fn verify(messages: &Reader, proof: &R1CSProof) -> Result<()> {
// Common
let pc_gens = PedersenGens::default();
let bp_gens = BulletproofGens::new(128, 1);
let mut transcript = Transcript::new(b"zkInterfaceGadget");
// /Common
// 1. Create a verifier
let verifier = Verifier::new(&bp_gens, &pc_gens, &mut transcript);
// 2. There are no high-level variables.
// 3. Build a CS
let mut cs = verifier.finalize_inputs();
gadget_from_messages(&mut cs, messages, false)?;
// 4. Verify the proof
cs.verify(&proof)
.map_err(|_| R1CSError::VerificationError.compat().into())
}
/// A gadget using a circuit in zkInterface messages.
pub fn gadget_from_messages<CS: ConstraintSystem>(
cs: &mut CS,
messages: &Reader,
prover: bool,
) -> Result<()> {
let public_vars = messages
.instance_variables()
.ok_or("Missing Circuit.connections")?;
let private_vars = messages
.private_variables()
.ok_or("Missing Circuit.connections")?;
// Map zkif variables to Bulletproofs's equivalent, LinearCombination.
let mut id_to_lc = HashMap::<u64, LinearCombination>::new();
// Prover tracks the values assigned to zkif variables in order to evaluate the gates.
let mut id_to_value = HashMap::<u64, Scalar>::new();
// Map constant one.
id_to_lc.insert(0, Variable::One().into());
if prover {
id_to_value.insert(0, Scalar::one());
}
// Map public inputs.
for var in public_vars {
let val = scalar_from_zkif(var.value)?;
id_to_lc.insert(var.id, val.into());
if prover {
id_to_value.insert(var.id, val);
}
// eprintln!("public{} = {:?}", var.id, val);
}
// eprintln!();
// Map witness (if prover).
if prover {
for var in private_vars.iter() {
let val = scalar_from_zkif(var.value)?;
id_to_value.insert(var.id, val);
// eprintln!("private{} = {:?}", var.id, val);
}
// eprintln!();
}
// Step 1: Allocate one mult gate per R1CS constraint.
let mut gates_a = vec![];
let mut gates_b = vec![];
let mut gates_c = vec![];
for constraint in messages.iter_constraints() {
let (gate_a, gate_b, gate_c) = cs
.allocate(|| {
Ok((
// Prover evaluates the incoming linear combinations using the witness.
eval_zkif_lc(&id_to_value, &constraint.a),
eval_zkif_lc(&id_to_value, &constraint.b),
eval_zkif_lc(&id_to_value, &constraint.c),
))
})
.map_err(|e| e.compat())?;
gates_a.push(gate_a);
gates_b.push(gate_b);
gates_c.push(gate_c);
// XXX: If constraint.a/b/c is just x, insert id_to_lc[x.id] = gate_var
}
// Step 2: Allocate extra gates for variables that are not yet defined.
for circuit_var in private_vars.iter() {
if !id_to_lc.contains_key(&circuit_var.id) {
let (gate_var, _, _) = cs
.allocate(|| {
// Prover takes the value from witness.
let val = id_to_value.get(&circuit_var.id);
Ok((
val.unwrap().clone(),
Scalar::zero(), // Dummy.
Scalar::zero(), // Dummy.
))
})
.map_err(|e| e.compat())?;
id_to_lc.insert(circuit_var.id, gate_var.into());
// eprintln!("private{} allocated to {:?}", circuit_var.id, gate_var);
}
}
// eprintln!();
// Step 3: Add linear constraints into each wire of each gate.
for (i, constraint) in messages.iter_constraints().enumerate() {
// eprintln!("constraint {}:", i);
let lc_a = convert_zkif_lc(&id_to_lc, &constraint.a)?;
// eprintln!(" A = {:?}", lc_a);
cs.constrain(lc_a - gates_a[i]);
let lc_b = convert_zkif_lc(&id_to_lc, &constraint.b)?;
// eprintln!(" B = {:?}", lc_b);
cs.constrain(lc_b - gates_b[i]);
let lc_c = convert_zkif_lc(&id_to_lc, &constraint.c)?;
// eprintln!(" C = {:?}", lc_c);
cs.constrain(lc_c - gates_c[i]);
// eprintln!();
// XXX: Skip trivial constraints where the lc was defined as just the gate var.
}
// XXX: optimize gate allocation.
// - Detect trivial LC wires = 1 * x. Then use the gate wire as variable x.
// Skip dummy allocation in step 2, and skip constraint in step 3.
// - Detect when the LC going into a gate contains a single new variable 1.x,
// set x = wire - (other terms in existing variables).
// - Allocate two variables at once (left, right, ignore output)?
// - Try to reorder the constraints to minimize dummy gates allocations.
Ok(())
}
/// This is a gadget equivalent to the zkinterface example circuit: x^2 + y^2 = zz
fn _example_gadget<CS: ConstraintSystem>(cs: &mut CS) -> Result<()> {
let x = LinearCombination::from(3 as u64);
let y = LinearCombination::from(4 as u64);
let zz = LinearCombination::from(25 as u64);
let (_, _, xx) = cs.multiply(x.clone(), x);
let (_, _, yy) = cs.multiply(y.clone(), y);
cs.constrain(xx + yy - zz);
Ok(())
}
/// Convert zkInterface little-endian bytes to Dalek Scalar.
fn scalar_from_zkif(le_bytes: &[u8]) -> Result<Scalar> {
let mut bytes32 = [0; 32];
let l = min(le_bytes.len(), 32);
bytes32[..l].copy_from_slice(&le_bytes[..l]);
Scalar::from_canonical_bytes(bytes32).ok_or("Invalid scalar encoding".into())
}
fn convert_zkif_lc(
id_to_lc: &HashMap<u64, LinearCombination>,
zkif_terms: &[Term],
) -> Result<LinearCombination> {
let mut lc = LinearCombination::default();
for term in zkif_terms {
let var = id_to_lc
.get(&term.id)
.ok_or(format!("Unknown var {}", term.id))?;
let coeff = scalar_from_zkif(term.value)?;
lc = lc + (var.clone() * coeff);
}
Ok(lc)
}
fn eval_zkif_lc(id_to_value: &HashMap<u64, Scalar>, terms: &[Term]) -> Scalar {
terms
.iter()
.map(|term| {
let val = match id_to_value.get(&term.id) {
Some(s) => s.clone(),
None => Scalar::zero(),
};
let coeff = scalar_from_zkif(term.value).unwrap();
coeff * val
})
.sum()
}
#[test]
fn test_zkinterface_backend() {
use self::zkinterface::producers::examples;
// Load test messages common to the prover and verifier: Circuit and Constraints.
let verifier_messages = {
let mut buf = Vec::<u8>::new();
examples::example_circuit_header().write_into(&mut buf).unwrap();
examples::example_constraints().write_into(&mut buf).unwrap();
let mut msg = Reader::new();
msg.push_message(buf).unwrap();
msg
};
// Prover uses an additional message: Witness.
let prover_messages = {
let mut msg = verifier_messages.clone();
let mut buf = Vec::<u8>::new();
examples::example_witness().write_into(&mut buf).unwrap();
msg.push_message(buf).unwrap();
msg
};
// Prove using the witness.
let proof = prove(&prover_messages).unwrap();
// Verify using the circuit and the proof.
verify(&verifier_messages, &proof).unwrap();
}