The Block is the main unit of the Filecoin blockchain.

The Block structure in the Filecoin blockchain is composed of: i) the Block Header, ii) the list of messages inside the block, and iii) the signed messages. This is represented inside the FullBlock abstraction. The messages indicate the required set of changes to apply in order to arrive at a deterministic state of the chain.

The Lotus implementation of the block has the following struct:

type FullBlock struct {
	Header        *BlockHeader
	BlsMessages   []*Message
	SecpkMessages []*SignedMessage

A block is functionally the same as a block header in the Filecoin protocol. While a block header contains Merkle links to the full system state, messages, and message receipts, a block can be thought of as the full set of this information (not just the Merkle roots, but rather the full data of the state tree, message tree, receipts tree, etc.). Because a full block is large in size, the Filecoin blockchain consists of block headers rather than full blocks. We often use the terms block and block header interchangeably.

A BlockHeader is a canonical representation of a block. BlockHeaders are propagated between miner nodes. From the BlockHeader message, a miner has all the required information to apply the associated FullBlock’s state and update the chain. In order to be able to do this, the minimum set of information items that need to be included in the BlockHeader are shown below and include among others: the miner’s address, the Ticket, the Proof of SpaceTime, the CID of the parents where this block evolved from in the IPLD DAG, as well as the messages’ own CIDs.

The Lotus implementation of the block header has the following structs:

type BlockHeader struct {
	Miner address.Address // 0 unique per block/miner

	Ticket                *Ticket           // 1 unique per block/miner: should be a valid VRF
	ElectionProof         *ElectionProof    // 2 unique per block/miner: should be a valid VRF
	BeaconEntries         []BeaconEntry     // 3 identical for all blocks in same tipset
	WinPoStProof          []proof.PoStProof // 4 unique per block/miner
	Parents               []cid.Cid         // 5 identical for all blocks in same tipset
	ParentWeight          BigInt            // 6 identical for all blocks in same tipset
	Height                abi.ChainEpoch    // 7 identical for all blocks in same tipset
	ParentStateRoot       cid.Cid           // 8 identical for all blocks in same tipset
	ParentMessageReceipts cid.Cid           // 9 identical for all blocks in same tipset
	Messages              cid.Cid           // 10 unique per block
	BLSAggregate          *crypto.Signature // 11 unique per block: aggrregate of BLS messages from above
	Timestamp             uint64            // 12 identical for all blocks in same tipset / hard-tied to the value of Height above
	BlockSig              *crypto.Signature // 13 unique per block/miner: miner signature
	ForkSignaling         uint64            // 14 currently unused/undefined
	ParentBaseFee         abi.TokenAmount   // 15 identical for all blocks in same tipset: the base fee after executing parent tipset

	validated bool // internal, true if the signature has been validated
type Ticket struct {
	VRFProof []byte
type ElectionProof struct {
	WinCount int64
	VRFProof []byte
type BeaconEntry struct {
	Round uint64
	Data  []byte

The BlockHeader structure has to refer to the TicketWinner of the current round which ensures the correct winner is passed to ChainSync.

func IsTicketWinner(vrfTicket []byte, mypow BigInt, totpow BigInt) bool

The Message structure has to include the source (From) and destination (To) addresses, a Nonce and the GasPrice.

The Lotus implementation of the message has the following structure:

type Message struct {
	Version uint64

	To   address.Address
	From address.Address

	Nonce uint64

	Value abi.TokenAmount

	GasLimit   int64
	GasFeeCap  abi.TokenAmount
	GasPremium abi.TokenAmount

	Method abi.MethodNum
	Params []byte

The message is also validated before it is passed to the chain synchronization logic:

func (m *Message) ValidForBlockInclusion(minGas int64, version network.Version) error {
	if m.Version != 0 {
		return xerrors.New("'Version' unsupported")

	if m.To == address.Undef {
		return xerrors.New("'To' address cannot be empty")

	if m.To == build.ZeroAddress && version >= network.Version7 {
		return xerrors.New("invalid 'To' address")

	if !abi.AddressValidForNetworkVersion(m.To, version) {
		return xerrors.New("'To' address protocol unsupported for network version")

	if m.From == address.Undef {
		return xerrors.New("'From' address cannot be empty")

	if !abi.AddressValidForNetworkVersion(m.From, version) {
		return xerrors.New("'From' address protocol unsupported for network version")

	if m.Value.Int == nil {
		return xerrors.New("'Value' cannot be nil")

	if m.Value.LessThan(big.Zero()) {
		return xerrors.New("'Value' field cannot be negative")

	if m.Value.GreaterThan(TotalFilecoinInt) {
		return xerrors.New("'Value' field cannot be greater than total filecoin supply")

	if m.GasFeeCap.Int == nil {
		return xerrors.New("'GasFeeCap' cannot be nil")

	if m.GasFeeCap.LessThan(big.Zero()) {
		return xerrors.New("'GasFeeCap' field cannot be negative")

	if m.GasPremium.Int == nil {
		return xerrors.New("'GasPremium' cannot be nil")

	if m.GasPremium.LessThan(big.Zero()) {
		return xerrors.New("'GasPremium' field cannot be negative")

	if m.GasPremium.GreaterThan(m.GasFeeCap) {
		return xerrors.New("'GasFeeCap' less than 'GasPremium'")

	if m.GasLimit > build.BlockGasLimit {
		return xerrors.Errorf("'GasLimit' field cannot be greater than a block's gas limit (%d > %d)", m.GasLimit, build.BlockGasLimit)

	if m.GasLimit <= 0 {
		return xerrors.Errorf("'GasLimit' field %d must be positive", m.GasLimit)

	// since prices might vary with time, this is technically semantic validation
	if m.GasLimit < minGas {
		return xerrors.Errorf("'GasLimit' field cannot be less than the cost of storing a message on chain %d < %d", m.GasLimit, minGas)

	return nil
Block syntax validation

Syntax validation refers to validation that should be performed on a block and its messages without reference to outside information such as the parent state tree. This type of validation is sometimes called static validation.

An invalid block must not be transmitted or referenced as a parent.

A syntactically valid block header must decode into fields matching the definitions below, must be a valid CBOR PubSub BlockMsg message and must have:

  • between 1 and 5*ec.ExpectedLeaders Parents CIDs if Epoch is greater than zero (else empty Parents),
  • a non-negative ParentWeight,
  • less than or equal to BlockMessageLimit number of messages,
  • aggregate message CIDs, encapsulated in the MsgMeta structure, serialized to the Messages CID in the block header,
  • a Miner address which is an ID-address. The Miner Address in the block header should be present and correspond to a public-key address in the current chain state.
  • Block signature (BlockSig) that belongs to the public-key address retrieved for the Miner
  • a non-negative Epoch,
  • a positive Timestamp,
  • a Ticket with non-empty VRFResult,
  • ElectionPoStOutput containing:
    • a Candidates array with between 1 and EC.ExpectedLeaders values (inclusive),
    • a non-empty PoStRandomness field,
    • a non-empty Proof field,
  • a non-empty ForkSignal field.

A syntactically valid full block must have:

  • all referenced messages syntactically valid,
  • all referenced parent receipts syntactically valid,
  • the sum of the serialized sizes of the block header and included messages is no greater than block.BlockMaxSize,
  • the sum of the gas limit of all explicit messages is no greater than block.BlockGasLimit.

Note that validation of the block signature requires access to the miner worker address and public key from the parent tipset state, so signature validation forms part of semantic validation. Similarly, message signature validation requires lookup of the public key associated with each message’s From account actor in the block’s parent state.

Block semantic validation

Semantic validation refers to validation that requires reference to information outside the block header and messages themselves. Semantic validation relates to the parent tipset and state on which the block is built.

In order to proceed to semantic validation the FullBlock must be assembled from the received block header retrieving its Filecoin messages. Block message CIDs can be retrieved from the network and be decoded into valid CBOR Message/SignedMessage.

In the Lotus implementation the semantic validation of a block is carried out by the Syncer module:

ValidateBlock should match up with ‘Semantical Validation’ in in the spec
func (syncer *Syncer) ValidateBlock(ctx context.Context, b *types.FullBlock, useCache bool) (err error) {
	defer func() {
		// b.Cid() could panic for empty blocks that are used in tests.
		if rerr := recover(); rerr != nil {
			err = xerrors.Errorf("validate block panic: %w", rerr)

	if useCache {
		isValidated, err :=, b.Cid())
		if err != nil {
			return xerrors.Errorf("check block validation cache %s: %w", b.Cid(), err)

		if isValidated {
			return nil

	validationStart := build.Clock.Now()
	defer func() {
		stats.Record(ctx, metrics.BlockValidationDurationMilliseconds.M(metrics.SinceInMilliseconds(validationStart)))
		log.Infow("block validation", "took", time.Since(validationStart), "height", b.Header.Height, "age", time.Since(time.Unix(int64(b.Header.Timestamp), 0)))

	ctx, span := trace.StartSpan(ctx, "validateBlock")
	defer span.End()

	if err := syncer.consensus.ValidateBlock(ctx, b); err != nil {
		return err

	if useCache {
		if err :=, b.Cid()); err != nil {
			return xerrors.Errorf("caching block validation %s: %w", b.Cid(), err)

	return nil

Messages are retrieved through the Syncer. There are the following two steps followed by the Syncer: 1- Assemble a FullTipSet populated with the single block received earlier. The Block’s ParentWeight is greater than the one from the (first block of the) heaviest tipset. 2- Retrieve all tipsets from the received Block down to our chain. Validation is expanded to every block inside these tipsets. The validation should ensure that: - Beacon entires are ordered by their round number. - The Tipset Parents CIDs match the fetched parent tipset through BlockSync.

A semantically valid block must meet all of the following requirements.


  • Parents listed in lexicographic order of their header’s Ticket.
  • ParentStateRoot CID of the block matches the state CID computed from the parent Tipset.
  • ParentState matches the state tree produced by executing the parent tipset’s messages (as defined by the VM interpreter) against that tipset’s parent state.
  • ParentMessageReceipts identifying the receipt list produced by parent tipset execution, with one receipt for each unique message from the parent tipset. In other words, the Block’s ParentMessageReceipts CID matches the receipts CID computed from the parent tipset.
  • ParentWeight matches the weight of the chain up to and including the parent tipset.


  • Epoch is greater than that of its Parents, and
    • not in the future according to the node’s local clock reading of the current epoch,
      • blocks with future epochs should not be rejected, but should not be evaluated (validated or included in a tipset) until the appropriate epoch
    • not farther in the past than the soft finality as defined by SPC Finality,
      • this rule only applies when receiving new gossip blocks (i.e. from the current chain head), not when syncing to the chain for the first time.
  • The Timestamp included is in seconds that:
    • must not be bigger than current time plus ΑllowableClockDriftSecs
    • must not be smaller than previous block’s Timestamp plus BlockDelay (including null blocks)
    • is of the precise value implied by the genesis block’s timestamp, the network’s Βlock time and the Βlock’s Epoch.


  • The Miner is active in the storage power table in the parent tipset state. The Miner’s address is registered in the Claims HAMT of the Power Actor
  • The TipSetState should be included for each tipset being validated.
    • Every Block in the tipset should belong to different a miner.
  • The Actor associated with the message’s From address exists, is an account actor and its Nonce matches the message Nonce.
  • Valid proofs that the Miner proved access to sealed versions of the sectors it was challenged for are included. In order to achieve that:
    • draw randomness for current epoch with WinningPoSt domain separation tag.
    • get list of sectors challanged in this epoch for this miner, based on the randomness drawn.
  • Miner is not slashed in StoragePowerActor.

Beacon- & Ticket-Related

  • Valid BeaconEntries should be included:
    • Check that every one of the BeaconEntries is a signature of a message: previousSignature || round signed using DRAND’s public key.
    • All entries between MaxBeaconRoundForEpoch down to prevEntry (from previous tipset) should be included.
  • A Ticket derived from the minimum ticket from the parent tipset’s block headers,
    • Ticket.VRFResult validly signed by the Miner actor’s worker account public key,
  • ElectionProof Ticket is computed correctly by checking BLS signature using miner’s key. The ElectionProof ticket should be a winning ticket.

Message- & Signature-Related

  • secp256k1 messages are correctly signed by their sending actor’s (From) worker account key,
  • A BLSAggregate signature is included that signs the array of CIDs of all the BLS messages referenced by the block with their sending actor’s key.
  • A valid Signature over the block header’s fields from the block’s Miner actor’s worker account public key is included.
  • For each message in ValidForBlockInclusion() the following hold:
    • Message fields Version, To, From, Value, GasPrice, and GasLimit are correctly defined.
    • Message GasLimit is under the message minimum gas cost (derived from chain height and message length).
  • For each message in ApplyMessage (that is before a message is executed), the following hold:
    • Basic gas and value checks in checkMessage():
      • The Message GasLimit is bigger than zero.
      • The Message GasPrice and Value are set.
    • The Message’s storage gas cost is under the message’s GasLimit.
    • The Message’s Nonce matches the nonce in the Actor retrieved from the message’s From address.
    • The Message’s maximum gas cost (derived from its GasLimit, GasPrice, and Value) is under the balance of the Actor retrieved from message’s From address.
    • The Message’s transfer Value is under the balance of the Actor retrieved from message’s From address.

There is no semantic validation of the messages included in a block beyond validation of their signatures. If all messages included in a block are syntactically valid then they may be executed and produce a receipt.

A chain sync system may perform syntactic and semantic validation in stages in order to minimize unnecessary resource expenditure.

If all of the above tests are successful, the block is marked as validated. Ultimately, an invalid block must not be propagated further or validated as a parent node.