config option to turn off randomizing unused wires (makes debugging easier as it becomes deterministic)

if you turn it off apparently you have something like a 10^-13 chance of the proof failing, and since it's deterministic (unless zero knowledge), it will always fail.

remark: now it's off by default. Don't forget to turn it back in production

plonky2/src/plonk/circuit_builder.rs

@@ -1095,7 +1095,12 @@ impl<F: RichField + Extendable<D>, const D: usize> CircuitBuilder<F, D> {
         {
             self.connect(hash_part, Target::wire(pi_gate, wire))
         }
-        self.randomize_unused_pi_wires(pi_gate);
+
+        // See <https://github.com/0xPolygonZero/plonky2/issues/456>
+        // however randomization makes debugging harder as runs are not deterministic
+        if self.config.randomize_unused_wires {
+            self.randomize_unused_pi_wires(pi_gate);
+        }
 
         // Place LUT-related gates.
         self.add_all_lookups();

plonky2/src/plonk/circuit_data.rs

@@ -80,6 +80,9 @@ pub struct CircuitConfig {
     /// A boolean to activate the zero-knowledge property. When this is set to `false`, proofs *may*
     /// leak additional information.
     pub zero_knowledge: bool,
+    /// See <https://github.com/0xPolygonZero/plonky2/issues/456>
+    /// Can be turned off for deterministic results
+    pub randomize_unused_wires: bool,
     /// A cap on the quotient polynomial's degree factor. The actual degree factor is derived
     /// systematically, but will never exceed this value.
     pub max_quotient_degree_factor: usize,
@@ -107,6 +110,7 @@ impl CircuitConfig {
             security_bits: 100,
             num_challenges: 2,
             zero_knowledge: false,
+            randomize_unused_wires: false,
             max_quotient_degree_factor: 8,
             fri_config: FriConfig {
                 rate_bits: 3,

plonky2/src/util/serialization/mod.rs

@@ -661,6 +661,7 @@ pub trait Read {
         let max_quotient_degree_factor = self.read_usize()?;
         let use_base_arithmetic_gate = self.read_bool()?;
         let zero_knowledge = self.read_bool()?;
+        let randomize_unused_wires = self.read_bool()?;
         let fri_config = self.read_fri_config()?;
 
         Ok(CircuitConfig {
@@ -672,6 +673,7 @@ pub trait Read {
             max_quotient_degree_factor,
             use_base_arithmetic_gate,
             zero_knowledge,
+            randomize_unused_wires,
             fri_config,
         })
     }
@@ -1696,6 +1698,7 @@ pub trait Write {
             max_quotient_degree_factor,
             use_base_arithmetic_gate,
             zero_knowledge,
+            randomize_unused_wires,
             fri_config,
         } = config;
 
@@ -1707,6 +1710,7 @@ pub trait Write {
         self.write_usize(*max_quotient_degree_factor)?;
         self.write_bool(*use_base_arithmetic_gate)?;
         self.write_bool(*zero_knowledge)?;
+        self.write_bool(*randomize_unused_wires)?;
         self.write_fri_config(fri_config)?;
 
         Ok(())