From 39bd2dc346b1103057c788f785bf5479fa7cb3df Mon Sep 17 00:00:00 2001
From: Scott Fairclough <scott@hexosoft.co.uk>
Date: Thu, 31 Oct 2024 12:07:39 +0000
Subject: [PATCH] logic change to mod exp revert rules

---
 core/vm/contracts_zkevm.go | 44 +++++++++++++++++++++++++-------------
 1 file changed, 29 insertions(+), 15 deletions(-)

diff --git a/core/vm/contracts_zkevm.go b/core/vm/contracts_zkevm.go
index 73afa136536..a01e3025b7a 100644
--- a/core/vm/contracts_zkevm.go
+++ b/core/vm/contracts_zkevm.go
@@ -302,15 +302,20 @@ func (c *bigModExp_zkevm) RequiredGas(input []byte) uint64 {
 
 	// Retrieve the operands and execute the exponentiation
 	var (
-		base = new(big.Int).SetBytes(getData(input, 0, baseLen.Uint64()))
-		exp  = new(big.Int).SetBytes(getData(input, baseLen.Uint64(), expLen.Uint64()))
-		mod  = new(big.Int).SetBytes(getData(input, baseLen.Uint64()+expLen.Uint64(), modLen.Uint64()))
+		base       = new(big.Int).SetBytes(getData(input, 0, baseLen.Uint64()))
+		exp        = new(big.Int).SetBytes(getData(input, baseLen.Uint64(), expLen.Uint64()))
+		mod        = new(big.Int).SetBytes(getData(input, baseLen.Uint64()+expLen.Uint64(), modLen.Uint64()))
+		baseBitLen = base.BitLen()
+		expBitLen  = exp.BitLen()
+		modBitLen  = mod.BitLen()
 	)
 
+	// special revert case for ZK
+	if baseBitLen == 0 && modBitLen > 8192 {
+		return 0
+	}
+
 	// limit to 8192 bits for base, exp, and mod in ZK - revert if we go over
-	baseBitLen := base.BitLen()
-	expBitLen := exp.BitLen()
-	modBitLen := mod.BitLen()
 	if baseBitLen > 8192 || expBitLen > 8192 || modBitLen > 8192 {
 		return 0
 	}
@@ -389,22 +394,31 @@ func (c *bigModExp_zkevm) Run(input []byte) ([]byte, error) {
 	} else {
 		input = input[:0]
 	}
-	// Handle a special case when both the base and mod length is zero
-	if baseLen == 0 && modLen == 0 {
+
+	if modLen == 0 {
 		return []byte{}, nil
 	}
+
 	// Retrieve the operands and execute the exponentiation
 	var (
-		base = new(big.Int).SetBytes(getData(input, 0, baseLen))
-		exp  = new(big.Int).SetBytes(getData(input, baseLen, expLen))
-		mod  = new(big.Int).SetBytes(getData(input, baseLen+expLen, modLen))
-		v    []byte
+		base       = new(big.Int).SetBytes(getData(input, 0, baseLen))
+		exp        = new(big.Int).SetBytes(getData(input, baseLen, expLen))
+		mod        = new(big.Int).SetBytes(getData(input, baseLen+expLen, modLen))
+		v          []byte
+		baseBitLen = base.BitLen()
+		expBitLen  = exp.BitLen()
+		modBitLen  = mod.BitLen()
 	)
 
+	if baseBitLen == 0 {
+		if modBitLen > 8192 {
+			return nil, ErrExecutionReverted
+		} else {
+			return []byte{}, nil
+		}
+	}
+
 	// limit to 8192 bits for base, exp, and mod in ZK
-	baseBitLen := base.BitLen()
-	expBitLen := exp.BitLen()
-	modBitLen := mod.BitLen()
 	if baseBitLen > 8192 || expBitLen > 8192 || modBitLen > 8192 {
 		return nil, ErrExecutionReverted
 	}