Fix modexp when partial base/exp/mod is set

core/vm/contracts_zkevm.go

@@ -400,18 +400,24 @@ func (c *bigModExp_zkevm) Run(input []byte) ([]byte, error) {
 		mod = big.NewInt(0)
 	)
 
-	if len(input) >= 96 + int(baseLen) {
-		base = new(big.Int).SetBytes(getData(input, 96, uint64(baseLen)))
+	// Extract `base`, `exp`, and `mod` with padding as needed
+	baseData := getData(input, 96, uint64(baseLen))
+	if uint64(len(baseData)) < baseLen {
+		baseData = common.RightPadBytes(baseData, int(baseLen))
 	}
-	if len(input) >= 96 + int(baseLen) + int(expLen) {
-		exp = new(big.Int).SetBytes(getData(input, 96 + uint64(baseLen), uint64(expLen)))
-	}
-	if len(input) >= 96 + int(baseLen) + int(expLen) + int(modLen) {
-		mod = new(big.Int).SetBytes(getData(input, 96 + uint64(baseLen) + uint64(expLen), uint64(modLen)))
+	base.SetBytes(baseData)
+
+	expData := getData(input, 96+uint64(baseLen), uint64(expLen))
+	if uint64(len(expData)) < expLen {
+		expData = common.RightPadBytes(expData, int(expLen))
 	}
-	if len(input) < 96 + int(baseLen) + int(expLen) + int(modLen) {
-		input = common.LeftPadBytes(input, 96 + int(baseLen) + int(expLen) + int(modLen))
+	exp.SetBytes(expData)
+
+	modData := getData(input, 96+uint64(baseLen)+uint64(expLen), uint64(modLen))
+	if uint64(len(modData)) < modLen {
+		modData = common.RightPadBytes(modData, int(modLen))
 	}
+	mod.SetBytes(modData)
 
 	// Retrieve the operands and execute the exponentiation
 	var (
@@ -422,7 +428,7 @@ func (c *bigModExp_zkevm) Run(input []byte) ([]byte, error) {
 	)
 
 	if modBitLen == 0 {
-		return []byte{}, nil
+		return common.LeftPadBytes([]byte{}, int(modLen)), nil
 	}
 
 	if baseBitLen == 0 {