span-batches: encode bitlists as big-endian integers, use standard Go big.Int...

span-batches: encode bitlists as big-endian integers, use standard Go big.Int encoding/decodig, update spec

op-node/rollup/derive/span_batch.go

@@ -41,7 +41,7 @@ type spanBatchPrefix struct {
 
 type spanBatchPayload struct {
 	blockCount    uint64        // Number of L2 block in the span
-	originBits    *big.Int      // Bitlist of blockCount bits. Each bit indicates if the L1 origin is changed at the L2 block.
+	originBits    *big.Int      // Standard span-batch bitlist of blockCount bits. Each bit indicates if the L1 origin is changed at the L2 block.
 	blockTxCounts []uint64      // List of transaction counts for each L2 block
 	txs           *spanBatchTxs // Transactions encoded in SpanBatch specs
 }
@@ -58,34 +58,12 @@ func (b *RawSpanBatch) GetBatchType() int {
 }
 
 // decodeOriginBits parses data into bp.originBits
-// originBits is bitlist right-padded to a multiple of 8 bits
 func (bp *spanBatchPayload) decodeOriginBits(r *bytes.Reader) error {
-	originBitBufferLen := bp.blockCount / 8
-	if bp.blockCount%8 != 0 {
-		originBitBufferLen++
-	}
-	// avoid out of memory before allocation
-	if originBitBufferLen > MaxSpanBatchSize {
-		return ErrTooBigSpanBatchSize
-	}
-	originBitBuffer := make([]byte, originBitBufferLen)
-	_, err := io.ReadFull(r, originBitBuffer)
+	bits, err := decodeSpanBatchBits(r, bp.blockCount)
 	if err != nil {
-		return fmt.Errorf("failed to read origin bits: %w", err)
-	}
-	originBits := new(big.Int)
-	for i := 0; i < int(bp.blockCount); i += 8 {
-		end := i + 8
-		if end > int(bp.blockCount) {
-			end = int(bp.blockCount)
-		}
-		bits := originBitBuffer[i/8]
-		for j := i; j < end; j++ {
-			bit := uint((bits >> (j - i)) & 1)
-			originBits.SetBit(originBits, j, bit)
-		}
+		return fmt.Errorf("failed to decode origin bits: %w", err)
 	}
-	bp.originBits = originBits
+	bp.originBits = bits
 	return nil
 }
 
@@ -293,26 +271,9 @@ func (bp *spanBatchPrefix) encodePrefix(w io.Writer) error {
 }
 
 // encodeOriginBits encodes bp.originBits
-// originBits is bitlist right-padded to a multiple of 8 bits
 func (bp *spanBatchPayload) encodeOriginBits(w io.Writer) error {
-	originBitBufferLen := bp.blockCount / 8
-	if bp.blockCount%8 != 0 {
-		originBitBufferLen++
-	}
-	originBitBuffer := make([]byte, originBitBufferLen)
-	for i := 0; i < int(bp.blockCount); i += 8 {
-		end := i + 8
-		if end > int(bp.blockCount) {
-			end = int(bp.blockCount)
-		}
-		var bits uint = 0
-		for j := i; j < end; j++ {
-			bits |= bp.originBits.Bit(j) << (j - i)
-		}
-		originBitBuffer[i/8] = byte(bits)
-	}
-	if _, err := w.Write(originBitBuffer); err != nil {
-		return fmt.Errorf("cannot write origin bits: %w", err)
+	if err := encodeSpanBatchBits(w, bp.blockCount, bp.originBits); err != nil {
+		return fmt.Errorf("failed to encode origin bits: %w", err)
 	}
 	return nil
 }

op-node/rollup/derive/span_batch_txs.go

@@ -19,14 +19,14 @@ type spanBatchTxs struct {
 	totalBlockTxCount uint64
 
 	// 8 fields
-	contractCreationBits *big.Int
-	yParityBits          *big.Int
+	contractCreationBits *big.Int // standard span-batch bitlist
+	yParityBits          *big.Int // standard span-batch bitlist
 	txSigs               []spanBatchSignature
 	txNonces             []uint64
 	txGases              []uint64
 	txTos                []common.Address
 	txDatas              []hexutil.Bytes
-	protectedBits        *big.Int
+	protectedBits        *big.Int // standard span-batch bitlist
 
 	// intermediate variables which can be recovered
 	txTypes            []int
@@ -39,114 +39,35 @@ type spanBatchSignature struct {
 	s *uint256.Int
 }
 
-// contractCreationBits is bitlist right-padded to a multiple of 8 bits
 func (btx *spanBatchTxs) encodeContractCreationBits(w io.Writer) error {
-	contractCreationBitBufferLen := btx.totalBlockTxCount / 8
-	if btx.totalBlockTxCount%8 != 0 {
-		contractCreationBitBufferLen++
-	}
-	contractCreationBitBuffer := make([]byte, contractCreationBitBufferLen)
-	for i := 0; i < int(btx.totalBlockTxCount); i += 8 {
-		end := i + 8
-		if end > int(btx.totalBlockTxCount) {
-			end = int(btx.totalBlockTxCount)
-		}
-		var bits uint = 0
-		for j := i; j < end; j++ {
-			bits |= btx.contractCreationBits.Bit(j) << (j - i)
-		}
-		contractCreationBitBuffer[i/8] = byte(bits)
-	}
-	if _, err := w.Write(contractCreationBitBuffer); err != nil {
-		return fmt.Errorf("cannot write contract creation bits: %w", err)
+	if err := encodeSpanBatchBits(w, btx.totalBlockTxCount, btx.contractCreationBits); err != nil {
+		return fmt.Errorf("failed to encode contract creation bits: %w", err)
 	}
 	return nil
 }
 
-// contractCreationBits is bitlist right-padded to a multiple of 8 bits
 func (btx *spanBatchTxs) decodeContractCreationBits(r *bytes.Reader) error {
-	contractCreationBitBufferLen := btx.totalBlockTxCount / 8
-	if btx.totalBlockTxCount%8 != 0 {
-		contractCreationBitBufferLen++
-	}
-	// avoid out of memory before allocation
-	if contractCreationBitBufferLen > MaxSpanBatchSize {
-		return ErrTooBigSpanBatchSize
-	}
-	contractCreationBitBuffer := make([]byte, contractCreationBitBufferLen)
-	_, err := io.ReadFull(r, contractCreationBitBuffer)
+	bits, err := decodeSpanBatchBits(r, btx.totalBlockTxCount)
 	if err != nil {
-		return fmt.Errorf("failed to read contract creation bits: %w", err)
+		return fmt.Errorf("failed to decode contract creation bits: %w", err)
 	}
-	contractCreationBits := new(big.Int)
-	for i := 0; i < int(btx.totalBlockTxCount); i += 8 {
-		end := i + 8
-		if end > int(btx.totalBlockTxCount) {
-			end = int(btx.totalBlockTxCount)
-		}
-		bits := contractCreationBitBuffer[i/8]
-		for j := i; j < end; j++ {
-			bit := uint((bits >> (j - i)) & 1)
-			contractCreationBits.SetBit(contractCreationBits, j, bit)
-		}
-	}
-	btx.contractCreationBits = contractCreationBits
+	btx.contractCreationBits = bits
 	return nil
 }
 
-// protectedBits is bitlist right-padded to a multiple of 8 bits
 func (btx *spanBatchTxs) encodeProtectedBits(w io.Writer) error {
-	protectedBitBufferLen := btx.totalLegacyTxCount / 8
-	if btx.totalLegacyTxCount%8 != 0 {
-		protectedBitBufferLen++
-	}
-	protectedBitBuffer := make([]byte, protectedBitBufferLen)
-	for i := 0; i < int(btx.totalLegacyTxCount); i += 8 {
-		end := i + 8
-		if end > int(btx.totalLegacyTxCount) {
-			end = int(btx.totalLegacyTxCount)
-		}
-		var bits uint = 0
-		for j := i; j < end; j++ {
-			bits |= btx.protectedBits.Bit(j) << (j - i)
-		}
-		protectedBitBuffer[i/8] = byte(bits)
-	}
-	if _, err := w.Write(protectedBitBuffer); err != nil {
-		return fmt.Errorf("cannot write protected bits: %w", err)
+	if err := encodeSpanBatchBits(w, btx.totalLegacyTxCount, btx.protectedBits); err != nil {
+		return fmt.Errorf("failed to encode protected bits: %w", err)
 	}
 	return nil
-
 }
 
-// protectedBits is bitlist right-padded to a multiple of 8 bits
 func (btx *spanBatchTxs) decodeProtectedBits(r *bytes.Reader) error {
-	protectedBitBufferLen := btx.totalLegacyTxCount / 8
-	if btx.totalLegacyTxCount%8 != 0 {
-		protectedBitBufferLen++
-	}
-	// avoid out of memory before allocation
-	if protectedBitBufferLen > MaxSpanBatchSize {
-		return ErrTooBigSpanBatchSize
-	}
-	protectedBitBuffer := make([]byte, protectedBitBufferLen)
-	_, err := io.ReadFull(r, protectedBitBuffer)
+	bits, err := decodeSpanBatchBits(r, btx.totalLegacyTxCount)
 	if err != nil {
-		return fmt.Errorf("failed to read protected bits: %w", err)
-	}
-	protectedBits := new(big.Int)
-	for i := 0; i < int(btx.totalLegacyTxCount); i += 8 {
-		end := i + 8
-		if end > int(btx.totalLegacyTxCount) {
-			end = int(btx.totalLegacyTxCount)
-		}
-		bits := protectedBitBuffer[i/8]
-		for j := i; j < end; j++ {
-			bit := uint((bits >> (j - i)) & 1)
-			protectedBits.SetBit(protectedBits, j, bit)
-		}
+		return fmt.Errorf("failed to decode protected bits: %w", err)
 	}
-	btx.protectedBits = protectedBits
+	btx.protectedBits = bits
 	return nil
 }
 
@@ -164,27 +85,19 @@ func (btx *spanBatchTxs) contractCreationCount() (uint64, error) {
 	return result, nil
 }
 
-// yParityBits is bitlist right-padded to a multiple of 8 bits
 func (btx *spanBatchTxs) encodeYParityBits(w io.Writer) error {
-	yParityBitBufferLen := btx.totalBlockTxCount / 8
-	if btx.totalBlockTxCount%8 != 0 {
-		yParityBitBufferLen++
-	}
-	yParityBitBuffer := make([]byte, yParityBitBufferLen)
-	for i := 0; i < int(btx.totalBlockTxCount); i += 8 {
-		end := i + 8
-		if end > int(btx.totalBlockTxCount) {
-			end = int(btx.totalBlockTxCount)
-		}
-		var bits uint = 0
-		for j := i; j < end; j++ {
-			bits |= btx.yParityBits.Bit(j) << (j - i)
-		}
-		yParityBitBuffer[i/8] = byte(bits)
+	if err := encodeSpanBatchBits(w, btx.totalBlockTxCount, btx.yParityBits); err != nil {
+		return fmt.Errorf("failed to encode y-parity bits: %w", err)
 	}
-	if _, err := w.Write(yParityBitBuffer); err != nil {
-		return fmt.Errorf("cannot write y parity bits: %w", err)
+	return nil
+}
+
+func (btx *spanBatchTxs) decodeYParityBits(r *bytes.Reader) error {
+	bits, err := decodeSpanBatchBits(r, btx.totalBlockTxCount)
+	if err != nil {
+		return fmt.Errorf("failed to decode y-parity bits: %w", err)
 	}
+	btx.yParityBits = bits
 	return nil
 }
 
@@ -242,37 +155,6 @@ func (btx *spanBatchTxs) encodeTxDatas(w io.Writer) error {
 	return nil
 }
 
-// yParityBits is bitlist right-padded to a multiple of 8 bits
-func (btx *spanBatchTxs) decodeYParityBits(r *bytes.Reader) error {
-	yParityBitBufferLen := btx.totalBlockTxCount / 8
-	if btx.totalBlockTxCount%8 != 0 {
-		yParityBitBufferLen++
-	}
-	// avoid out of memory before allocation
-	if yParityBitBufferLen > MaxSpanBatchSize {
-		return ErrTooBigSpanBatchSize
-	}
-	yParityBitBuffer := make([]byte, yParityBitBufferLen)
-	_, err := io.ReadFull(r, yParityBitBuffer)
-	if err != nil {
-		return fmt.Errorf("failed to read y parity bits: %w", err)
-	}
-	yParityBits := new(big.Int)
-	for i := 0; i < int(btx.totalBlockTxCount); i += 8 {
-		end := i + 8
-		if end > int(btx.totalBlockTxCount) {
-			end = int(btx.totalBlockTxCount)
-		}
-		bits := yParityBitBuffer[i/8]
-		for j := i; j < end; j++ {
-			bit := uint((bits >> (j - i)) & 1)
-			yParityBits.SetBit(yParityBits, j, bit)
-		}
-	}
-	btx.yParityBits = yParityBits
-	return nil
-}
-
 func (btx *spanBatchTxs) decodeTxSigsRS(r *bytes.Reader) error {
 	var txSigs []spanBatchSignature
 	var sigBuffer [32]byte

op-node/rollup/derive/span_batch_util.go

+package derive
+
+import (
+	"bytes"
+	"fmt"
+	"io"
+	"math/big"
+)
+
+// decodeSpanBatchBits decodes a standard span-batch bitlist.
+// The bitlist is encoded as big-endian integer, left-padded with zeroes to a multiple of 8 bits.
+// The encoded bitlist cannot be longer than MaxSpanBatchSize.
+func decodeSpanBatchBits(r *bytes.Reader, bitLength uint64) (*big.Int, error) {
+	// Round up, ensure enough bytes when number of bits is not a multiple of 8.
+	// Alternative of (L+7)/8 is not overflow-safe.
+	bufLen := bitLength / 8
+	if bitLength%8 != 0 {
+		bufLen++
+	}
+	// avoid out of memory before allocation
+	if bufLen > MaxSpanBatchSize {
+		return nil, ErrTooBigSpanBatchSize
+	}
+	buf := make([]byte, bufLen)
+	_, err := io.ReadFull(r, buf)
+	if err != nil {
+		return nil, fmt.Errorf("failed to read bits: %w", err)
+	}
+	out := new(big.Int)
+	out.SetBytes(buf)
+	// We read the correct number of bytes, but there may still be trailing bits
+	if l := uint64(out.BitLen()); l > bitLength {
+		return nil, fmt.Errorf("bitfield has %d bits, but expected no more than %d", l, bitLength)
+	}
+	return out, nil
+}
+
+// encodeSpanBatchBits encodes a standard span-batch bitlist.
+// The bitlist is encoded as big-endian integer, left-padded with zeroes to a multiple of 8 bits.
+// The encoded bitlist cannot be longer than MaxSpanBatchSize.
+func encodeSpanBatchBits(w io.Writer, bitLength uint64, bits *big.Int) error {
+	if l := uint64(bits.BitLen()); l > bitLength {
+		return fmt.Errorf("bitfield is larger than bitLength: %d > %d", l, bitLength)
+	}
+	// Round up, ensure enough bytes when number of bits is not a multiple of 8.
+	// Alternative of (L+7)/8 is not overflow-safe.
+	bufLen := bitLength / 8
+	if bitLength%8 != 0 { // rounding up this way is safe against overflows
+		bufLen++
+	}
+	if bufLen > MaxSpanBatchSize {
+		return ErrTooBigSpanBatchSize
+	}
+	buf := make([]byte, bufLen)
+	bits.FillBytes(buf) // zero-extended, big-endian
+	if _, err := w.Write(buf); err != nil {
+		return fmt.Errorf("cannot write bits: %w", err)
+	}
+	return nil
+}

specs/span-batches.md

@@ -86,6 +86,9 @@ Notation:
 
 [protobuf spec]: https://protobuf.dev/programming-guides/encoding/#varints
 
+Standard bitlists, in the context of span-batches, are encoded as big-endian integers,
+left-padded with zeroes to the next multiple of 8 bits.
+
 Where:
 
 - `prefix = rel_timestamp ++ l1_origin_num ++ parent_check ++ l1_origin_check`
@@ -98,15 +101,15 @@ Where:
     The hash is truncated to 20 bytes for efficiency, i.e. `span_end.l1_origin.hash[:20]`.
 - `payload = block_count ++ origin_bits ++ block_tx_counts ++ txs`:
   - `block_count`: `uvarint` number of L2 blocks. This is at least 1, empty span batches are invalid.
-  - `origin_bits`: bitlist of `block_count` bits, right-padded to a multiple of 8 bits:
+  - `origin_bits`: standard bitlist of `block_count` bits:
     1 bit per L2 block, indicating if the L1 origin changed this L2 block.
   - `block_tx_counts`: for each block, a `uvarint` of `len(block.transactions)`.
   - `txs`: L2 transactions which is reorganized and encoded as below.
 - `txs = contract_creation_bits ++ y_parity_bits ++
         tx_sigs ++ tx_tos ++ tx_datas ++ tx_nonces ++ tx_gases ++ protected_bits`
-  - `contract_creation_bits`: bit list of `sum(block_tx_counts)` bits, right-padded to a multiple of 8 bits,
+  - `contract_creation_bits`: standard bitlist of `sum(block_tx_counts)` bits:
     1 bit per L2 transactions, indicating if transaction is a contract creation transaction.
-  - `y_parity_bits`: bit list of `sum(block_tx_counts)` bits, right-padded to a multiple of 8 bits,
+  - `y_parity_bits`: standard bitlist of `sum(block_tx_counts)` bits:
     1 bit per L2 transactions, indicating the y parity value when recovering transaction sender address.
   - `tx_sigs`: concatenated list of transaction signatures
     - `r` is encoded as big-endian `uint256`
@@ -122,7 +125,7 @@ Where:
     - `legacy`: `gasLimit`
     - `1`: ([EIP-2930]): `gasLimit`
     - `2`: ([EIP-1559]): `gas_limit`
-  - `protected_bits`: bit list of length of number of legacy transactions, right-padded to a multiple of 8 bits,
+  - `protected_bits`: standard bitlist of length of number of legacy transactions:
     1 bit per L2 legacy transactions, indicating if transacion is protected([EIP-155]) or not.
 
 Introduce version `2` to the [batch-format](./derivation.md#batch-format) table: