fix: add padding to challenge preimage for missing chunks (#1203)

Co-authored-by: Thegaram <Thegaram@users.noreply.github.com>

common/types/encoding/codecv1/codecv1.go

@@ -210,6 +210,10 @@ func NewDABatch(batch *encoding.Batch) (*DABatch, error) {
 		return nil, fmt.Errorf("too many chunks in batch")
 	}
 
+	if len(batch.Chunks) == 0 {
+		return nil, fmt.Errorf("too few chunks in batch")
+	}
+
 	// batch data hash
 	dataHash, err := computeBatchDataHash(batch.Chunks, batch.TotalL1MessagePoppedBefore)
 	if err != nil {
@@ -284,9 +288,6 @@ func constructBlobPayload(chunks []*encoding.Chunk) (*kzg4844.Blob, *kzg4844.Poi
 	// the raw (un-padded) blob payload
 	blobBytes := make([]byte, metadataLength)
 
-	// the number of chunks that contain at least one L2 transaction
-	numNonEmptyChunks := 0
-
 	// challenge digest preimage
 	// 1 hash for metadata and 1 for each chunk
 	challengePreimage := make([]byte, (1+MaxNumChunks)*32)
@@ -294,42 +295,47 @@ func constructBlobPayload(chunks []*encoding.Chunk) (*kzg4844.Blob, *kzg4844.Poi
 	// the challenge point z
 	var z kzg4844.Point
 
+	// the chunk data hash used for calculating the challenge preimage
+	var chunkDataHash common.Hash
+
+	// blob metadata: num_chunks
+	binary.BigEndian.PutUint16(blobBytes[0:], uint16(len(chunks)))
+
 	// encode blob metadata and L2 transactions,
 	// and simultaneously also build challenge preimage
 	for chunkID, chunk := range chunks {
 		currentChunkStartIndex := len(blobBytes)
-		hasL2Tx := false
 
 		for _, block := range chunk.Blocks {
 			for _, tx := range block.Transactions {
 				if tx.Type != types.L1MessageTxType {
-					hasL2Tx = true
 					// encode L2 txs into blob payload
 					rlpTxData, err := encoding.ConvertTxDataToRLPEncoding(tx)
 					if err != nil {
 						return nil, nil, err
 					}
 					blobBytes = append(blobBytes, rlpTxData...)
-					continue
 				}
 			}
 		}
 
 		// blob metadata: chunki_size
-		chunkSize := len(blobBytes) - currentChunkStartIndex
-		binary.BigEndian.PutUint32(blobBytes[2+4*chunkID:], uint32(chunkSize))
-
-		if hasL2Tx {
-			numNonEmptyChunks++
+		if chunkSize := len(blobBytes) - currentChunkStartIndex; chunkSize != 0 {
+			binary.BigEndian.PutUint32(blobBytes[2+4*chunkID:], uint32(chunkSize))
 		}
 
 		// challenge: compute chunk data hash
-		hash := crypto.Keccak256Hash(blobBytes[currentChunkStartIndex:])
-		copy(challengePreimage[32+chunkID*32:], hash[:])
+		chunkDataHash = crypto.Keccak256Hash(blobBytes[currentChunkStartIndex:])
+		copy(challengePreimage[32+chunkID*32:], chunkDataHash[:])
 	}
 
-	// blob metadata: num_chunks
-	binary.BigEndian.PutUint16(blobBytes[0:], uint16(numNonEmptyChunks))
+	// if we have fewer than MaxNumChunks chunks, the rest
+	// of the blob metadata is correctly initialized to 0,
+	// but we need to add padding to the challenge preimage
+	for chunkID := len(chunks); chunkID < MaxNumChunks; chunkID++ {
+		// use the last chunk's data hash as padding
+		copy(challengePreimage[32+chunkID*32:], chunkDataHash[:])
+	}
 
 	// challenge: compute metadata hash
 	hash := crypto.Keccak256Hash(blobBytes[0:metadataLength])

common/version/version.go

@@ -5,7 +5,7 @@ import (
 	"runtime/debug"
 )
 
-var tag = "v4.3.74"
+var tag = "v4.3.75"
 
 var commit = func() string {
 	if info, ok := debug.ReadBuildInfo(); ok {