From 853a273aa9984cad8b693b4edd637940c351a2b5 Mon Sep 17 00:00:00 2001
From: arnaucube <git@arnaucube.com>
Date: Fri, 15 Aug 2025 16:06:08 +0000
Subject: [PATCH] add observer for FriConfig & FriParams in verifier circuit,
 to match the updated version of plonky2 at PR#1678
 (https://github.com/0xPolygonZero/plonky2/pull/1678)

---
 challenger/challenger.go | 23 +++++++++++++++++++++++
 types/common_data.go     | 10 ++++++++++
 types/types.go           | 12 +++++-------
 verifier/verifier.go     |  1 +
 4 files changed, 39 insertions(+), 7 deletions(-)

diff --git a/challenger/challenger.go b/challenger/challenger.go
index 20da42a..48cbf46 100644
--- a/challenger/challenger.go
+++ b/challenger/challenger.go
@@ -86,6 +86,29 @@ func (c *Chip) ObserveOpenings(openings fri.Openings) {
 	}
 }
 
+func (c *Chip) ObserveFriConfig(
+	config types.FriConfig,
+) {
+	c.ObserveElement(gl.NewVariable(config.RateBits))
+	c.ObserveElement(gl.NewVariable(config.CapHeight))
+	c.ObserveElement(gl.NewVariable(config.ProofOfWorkBits))
+	for i := 0; i < len(config.ReductionStrategy); i++ {
+		c.ObserveElement(gl.NewVariable(config.ReductionStrategy[i]))
+	}
+	c.ObserveElement(gl.NewVariable(config.NumQueryRounds))
+}
+
+func (c *Chip) ObserveFriParams(
+	params types.FriParams,
+) {
+	c.ObserveFriConfig(params.Config)
+	c.ObserveElement(gl.NewVariable(0)) // Hiding, should be always false as hiding is not supported in gnark-plonky2-verifier
+	c.ObserveElement(gl.NewVariable(params.DegreeBits))
+	for i := 0; i < len(params.ReductionArityBits); i++ {
+		c.ObserveElement(gl.NewVariable(params.ReductionArityBits[i]))
+	}
+}
+
 func (c *Chip) GetChallenge() gl.Variable {
 	if len(c.inputBuffer) != 0 || len(c.outputBuffer) == 0 {
 		c.duplexing()
diff --git a/types/common_data.go b/types/common_data.go
index c3a6177..aca0852 100644
--- a/types/common_data.go
+++ b/types/common_data.go
@@ -86,6 +86,13 @@ func ReadCommonCircuitData(path string) CommonCircuitData {
 	commonCircuitData.Config.FriConfig.RateBits = raw.Config.FriConfig.RateBits
 	commonCircuitData.Config.FriConfig.CapHeight = raw.Config.FriConfig.CapHeight
 	commonCircuitData.Config.FriConfig.ProofOfWorkBits = raw.Config.FriConfig.ProofOfWorkBits
+	// since the only reduction_strategy supported in gnark-plonky2-verifier is
+	// ConstantArityBits, set the first element of the array to 1 (following
+	// https://github.com/0xPolygonZero/plonky2/blob/main/plonky2/src/fri/reduction_strategies.rs#L70).
+	// Ideally this first value set to 1 would already be set by plonky2 serialization of the
+	// CommonCircuitData, but the serializer available in plonky2's implementation does not
+	// include this first value
+	commonCircuitData.Config.FriConfig.ReductionStrategy = append([]uint64{1}, raw.Config.FriConfig.ReductionStrategy.ConstantArityBits...)
 	commonCircuitData.Config.FriConfig.NumQueryRounds = raw.Config.FriConfig.NumQueryRounds
 
 	commonCircuitData.FriParams.DegreeBits = raw.FriParams.DegreeBits
@@ -93,6 +100,9 @@ func ReadCommonCircuitData(path string) CommonCircuitData {
 	commonCircuitData.FriParams.Config.RateBits = raw.FriParams.Config.RateBits
 	commonCircuitData.FriParams.Config.CapHeight = raw.FriParams.Config.CapHeight
 	commonCircuitData.FriParams.Config.ProofOfWorkBits = raw.FriParams.Config.ProofOfWorkBits
+	// set FriParams.ReductionStrategy[0]=1, for the same reason as in
+	// FriConfig.ReductionStrategy few lines above
+	commonCircuitData.FriParams.Config.ReductionStrategy = append([]uint64{1}, raw.FriParams.Config.ReductionStrategy.ConstantArityBits...)
 	commonCircuitData.FriParams.Config.NumQueryRounds = raw.FriParams.Config.NumQueryRounds
 	commonCircuitData.FriParams.ReductionArityBits = raw.FriParams.ReductionArityBits
 
diff --git a/types/types.go b/types/types.go
index 03bd38f..217925f 100644
--- a/types/types.go
+++ b/types/types.go
@@ -5,13 +5,11 @@ import (
 )
 
 type FriConfig struct {
-	RateBits        uint64
-	CapHeight       uint64
-	ProofOfWorkBits uint64
-	NumQueryRounds  uint64
-	// Note that we do not need `reduction_strategy` of type FriReductionStrategy as the plonky2 FriConfig has.
-	// reduction_strategy is only used for computing `reduction_arity_bits`, which is serialized in the
-	// CommonCircuitData.
+	RateBits          uint64
+	CapHeight         uint64
+	ProofOfWorkBits   uint64
+	ReductionStrategy []uint64 // only ConstantArityBits is supported
+	NumQueryRounds    uint64
 }
 
 func (fc *FriConfig) Rate() float64 {
diff --git a/verifier/verifier.go b/verifier/verifier.go
index 0d47f23..99f0b58 100644
--- a/verifier/verifier.go
+++ b/verifier/verifier.go
@@ -53,6 +53,7 @@ func (c *VerifierChip) GetChallenges(
 
 	var circuitDigest = verifierData.CircuitDigest
 
+	challenger.ObserveFriParams(c.commonData.FriParams)
 	challenger.ObserveBN254Hash(circuitDigest)
 	challenger.ObserveHash(publicInputsHash)
 	challenger.ObserveCap(proof.WiresCap)