make step_circuit mandatory; add support for longer hashes (#30)

src/circuit.rs

@@ -104,7 +104,7 @@ where
 {
   params: NIFSVerifierCircuitParams,
   inputs: Option<NIFSVerifierCircuitInputs<G>>,
-  step_circuit: Option<SC>, // The function that is applied for each step. may be None.
+  step_circuit: SC, // The function that is applied for each step
   poseidon_constants: NovaPoseidonConstants<G::Base>,
 }
 
@@ -119,7 +119,7 @@ where
   pub fn new(
     params: NIFSVerifierCircuitParams,
     inputs: Option<NIFSVerifierCircuitInputs<G>>,
-    step_circuit: Option<SC>,
+    step_circuit: SC,
     poseidon_constants: NovaPoseidonConstants<G::Base>,
   ) -> Self
   where
@@ -289,7 +289,7 @@ where
     // U2' = default if i == 0, otherwise NIFS.V(pp, u_new, U, T)
     /***********************************************************************/
 
-    //Allocate 0 and 1
+    // Allocate 0 and 1
     let zero = alloc_zero(cs.namespace(|| "zero"))?;
     // Hack: We just do this because the number of inputs must be even!!
     zero.inputize(cs.namespace(|| "allocate zero as input"))?;
@@ -477,15 +477,14 @@ where
       |lc| lc + next_i.get_variable() - CS::one() - i.get_variable(),
     );
 
-    if self.step_circuit.is_some() {
     /***********************************************************************/
-      //Allocate z0
+    // Allocate z0
     /***********************************************************************/
 
     let z_0 = AllocatedNum::alloc(cs.namespace(|| "z0"), || Ok(self.inputs.get()?.z0))?;
 
     /***********************************************************************/
-      //Allocate zi
+    // Allocate zi
     /***********************************************************************/
 
     let z_i = AllocatedNum::alloc(cs.namespace(|| "zi"), || Ok(self.inputs.get()?.zi))?;
@@ -537,7 +536,7 @@ where
     h1_hash.absorb(z_0.clone());
     h1_hash.absorb(z_i.clone());
 
-      let hash_bits = h1_hash.get_challenge(cs.namespace(|| "Input hash"))?;
+    let hash_bits = h1_hash.get_challenge(cs.namespace(|| "Input hash"))?; // TODO: use get_hash method
     let hash = le_bits_to_num(cs.namespace(|| "bits to hash"), hash_bits)?;
 
     cs.enforce(
@@ -553,7 +552,6 @@ where
 
     let z_next = self
       .step_circuit
-        .unwrap()
       .synthesize(&mut cs.namespace(|| "F"), z_i)?;
 
     /***********************************************************************/
@@ -583,75 +581,9 @@ where
     h1_hash.absorb(next_i.clone());
     h1_hash.absorb(z_0);
     h1_hash.absorb(z_next);
-      let h1_new_bits = h1_hash.get_challenge(cs.namespace(|| "h1_new bits"))?;
+    let h1_new_bits = h1_hash.get_challenge(cs.namespace(|| "h1_new bits"))?; // TODO: use get_hash method
     let h1_new = le_bits_to_num(cs.namespace(|| "h1_new"), h1_new_bits)?;
     let _ = h1_new.inputize(cs.namespace(|| "output h1_new"))?;
-    } else {
-      /***********************************************************************/
-      // Check that h1 = Hash(params, u2, i)
-      /***********************************************************************/
-
-      let mut h1_hash: PoseidonROGadget<G::Base> = PoseidonROGadget::new(self.poseidon_constants);
-
-      h1_hash.absorb(params.clone());
-      h1_hash.absorb(W_r.x);
-      h1_hash.absorb(W_r.y);
-      h1_hash.absorb(W_r.is_infinity);
-      h1_hash.absorb(E_r.x);
-      h1_hash.absorb(E_r.y);
-      h1_hash.absorb(E_r.is_infinity);
-      h1_hash.absorb(u_r);
-      h1_hash.absorb(i.clone());
-
-      // absorb each of the limbs of X_r[0]
-      for limb in Xr0_bn.into_iter() {
-        h1_hash.absorb(limb);
-      }
-
-      // absorb each of the limbs of X_r[1]
-      for limb in Xr1_bn.into_iter() {
-        h1_hash.absorb(limb);
-      }
-
-      let hash_bits = h1_hash.get_challenge(cs.namespace(|| "Input hash"))?;
-      let hash = le_bits_to_num(cs.namespace(|| "bits to hash"), hash_bits)?;
-
-      cs.enforce(
-        || "check h1",
-        |lc| lc,
-        |lc| lc,
-        |lc| lc + h1.get_variable() - hash.get_variable(),
-      );
-
-      /***********************************************************************/
-      // Compute the new hash H(params, u2')
-      /***********************************************************************/
-
-      h1_hash.flush_state();
-      h1_hash.absorb(params);
-      h1_hash.absorb(W_new.x.clone());
-      h1_hash.absorb(W_new.y.clone());
-      h1_hash.absorb(W_new.is_infinity);
-      h1_hash.absorb(E_new.x.clone());
-      h1_hash.absorb(E_new.y.clone());
-      h1_hash.absorb(E_new.is_infinity);
-      h1_hash.absorb(u_new);
-      h1_hash.absorb(next_i.clone());
-
-      // absorb each of the limbs of X_r_new[0]
-      for limb in Xr0_new_bn.into_iter() {
-        h1_hash.absorb(limb);
-      }
-
-      // absorb each of the limbs of X_r_new[1]
-      for limb in Xr1_new_bn.into_iter() {
-        h1_hash.absorb(limb);
-      }
-
-      let h1_new_bits = h1_hash.get_challenge(cs.namespace(|| "h1_new bits"))?;
-      let h1_new = le_bits_to_num(cs.namespace(|| "h1_new"), h1_new_bits)?;
-      let _ = h1_new.inputize(cs.namespace(|| "output h1_new"))?;
-    }
 
     Ok(())
   }
@@ -700,9 +632,9 @@ mod tests {
       NIFSVerifierCircuit::new(
         params.clone(),
         None,
-        Some(TestCircuit {
+        TestCircuit {
           _p: Default::default(),
-        }),
+        },
         poseidon_constants1.clone(),
       );
     // First create the shape
@@ -718,7 +650,14 @@ mod tests {
     let poseidon_constants2: NovaPoseidonConstants<<G1 as Group>::Base> =
       NovaPoseidonConstants::new();
     let circuit2: NIFSVerifierCircuit<G1, TestCircuit<<G1 as Group>::Base>> =
-      NIFSVerifierCircuit::new(params.clone(), None, None, poseidon_constants2);
+      NIFSVerifierCircuit::new(
+        params.clone(),
+        None,
+        TestCircuit {
+          _p: Default::default(),
+        },
+        poseidon_constants2,
+      );
     // First create the shape
     let mut cs: ShapeCS<G2> = ShapeCS::new();
     let _ = circuit2.synthesize(&mut cs);
@@ -755,9 +694,9 @@ mod tests {
       NIFSVerifierCircuit::new(
         params,
         Some(inputs),
-        Some(TestCircuit {
+        TestCircuit {
           _p: Default::default(),
-        }),
+        },
         poseidon_constants1,
       );
     let _ = circuit.synthesize(&mut cs);

src/poseidon.rs

@@ -80,10 +80,8 @@ where
     self.state.push(e);
   }
 
-  /// Compute a challenge by hashing the current state
-  #[allow(dead_code)]
-  pub fn get_challenge(&mut self) -> Scalar {
-    let hash = match self.state.len() {
+  fn hash_inner(&mut self) -> Scalar {
+    match self.state.len() {
       25 => {
         Poseidon::<Scalar, U25>::new_with_preimage(&self.state, &self.constants.constants25).hash()
       }
@@ -96,7 +94,13 @@ where
       _ => {
         panic!("Number of elements in the RO state does not match any of the arities used in Nova")
       }
-    };
+    }
+  }
+
+  /// Compute a challenge by hashing the current state
+  #[allow(dead_code)]
+  pub fn get_challenge(&mut self) -> Scalar {
+    let hash = self.hash_inner();
     // Only keep 128 bits
     let bits = hash.to_le_bits();
     let mut res = Scalar::zero();
@@ -109,6 +113,22 @@ where
     }
     res
   }
+
+  #[allow(dead_code)]
+  pub fn get_hash(&mut self) -> Scalar {
+    let hash = self.hash_inner();
+    // Only keep 250 bits
+    let bits = hash.to_le_bits();
+    let mut res = Scalar::zero();
+    let mut coeff = Scalar::one();
+    for bit in bits[0..250].into_iter() {
+      if *bit {
+        res += coeff;
+      }
+      coeff += coeff;
+    }
+    res
+  }
 }
 
 /// A Poseidon-based RO gadget to use inside the verifier circuit.
@@ -145,9 +165,7 @@ where
     self.state.push(e);
   }
 
-  /// Compute a challenge by hashing the current state
-  #[allow(dead_code)]
-  pub fn get_challenge<CS>(&mut self, mut cs: CS) -> Result<Vec<AllocatedBit>, SynthesisError>
+  fn hash_inner<CS>(&mut self, mut cs: CS) -> Result<Vec<AllocatedBit>, SynthesisError>
   where
     CS: ConstraintSystem<Scalar>,
   {
@@ -171,17 +189,40 @@ where
         panic!("Number of elements in the RO state does not match any of the arities used in Nova")
       }
     };
-    // Only keep 128 bits
-    let bits: Vec<AllocatedBit> = out
+
+    // return the hash as a vector of bits
+    Ok(
+      out
         .to_bits_le_strict(cs.namespace(|| "poseidon hash to boolean"))?
         .iter()
         .map(|boolean| match boolean {
           Boolean::Is(ref x) => x.clone(),
           _ => panic!("Wrong type of input. We should have never reached there"),
         })
-      .collect();
+        .collect(),
+    )
+  }
+
+  /// Compute a challenge by hashing the current state
+  #[allow(dead_code)]
+  pub fn get_challenge<CS>(&mut self, mut cs: CS) -> Result<Vec<AllocatedBit>, SynthesisError>
+  where
+    CS: ConstraintSystem<Scalar>,
+  {
+    let bits = self.hash_inner(cs.namespace(|| "hash"))?;
+    // Only keep 128 bits
     Ok(bits[..128].into())
   }
+
+  #[allow(dead_code)]
+  pub fn get_hash<CS>(&mut self, mut cs: CS) -> Result<Vec<AllocatedBit>, SynthesisError>
+  where
+    CS: ConstraintSystem<Scalar>,
+  {
+    let bits = self.hash_inner(cs.namespace(|| "hash"))?;
+    // Only keep 250 bits
+    Ok(bits[..250].into())
+  }
 }
 
 #[cfg(test)]