From 8cb89307b2a94239b1c7b98efc9ea1be8cc6a102 Mon Sep 17 00:00:00 2001
From: ycen <yycen@live.com>
Date: Thu, 10 Aug 2023 17:35:17 +0800
Subject: [PATCH] Allow empty proofFac, roll back rejectSample for
 compatibility

---
 common/hash_utils.go     | 15 +--------------
 ecdsa/keygen/messages.go |  5 +++--
 2 files changed, 4 insertions(+), 16 deletions(-)

diff --git a/common/hash_utils.go b/common/hash_utils.go
index 9fa24e5b..4fd26196 100644
--- a/common/hash_utils.go
+++ b/common/hash_utils.go
@@ -7,25 +7,12 @@
 package common
 
 import (
-	"crypto/sha256"
 	"math/big"
 )
 
 // RejectionSample implements the rejection sampling logic for converting a
 // SHA512/256 hash to a value between 0-q
 func RejectionSample(q *big.Int, eHash *big.Int) *big.Int { // e' = eHash
-	auxiliary := new(big.Int).Set(eHash)
-	e := new(big.Int).Set(q)
-	qBytesLen := len(q.Bytes())
-	if qBytesLen > 32 {
-		panic("invalid q size")
-	}
-	one := new(big.Int).SetInt64(1)
-	for e.Cmp(q) != -1 {
-		eHashAdded := auxiliary.Add(auxiliary, one)
-		eHashReSample := sha256.Sum256(eHashAdded.Bytes())
-		// sample qBytesLen bytes
-		e = new(big.Int).SetBytes(eHashReSample[:qBytesLen])
-	}
+	e := eHash.Mod(eHash, q)
 	return e
 }
diff --git a/ecdsa/keygen/messages.go b/ecdsa/keygen/messages.go
index 18da1dc9..e3f43d78 100644
--- a/ecdsa/keygen/messages.go
+++ b/ecdsa/keygen/messages.go
@@ -128,8 +128,9 @@ func NewKGRound2Message1(
 
 func (m *KGRound2Message1) ValidateBasic() bool {
 	return m != nil &&
-		common.NonEmptyBytes(m.GetShare()) &&
-		common.NonEmptyMultiBytes(m.GetFacProof(), facproof.ProofFacBytesParts)
+		common.NonEmptyBytes(m.GetShare())
+	// This is commented for backward compatibility, which msg has no proof
+	// && common.NonEmptyMultiBytes(m.GetFacProof(), facproof.ProofFacBytesParts)
 }
 
 func (m *KGRound2Message1) UnmarshalShare() *big.Int {