beacon/light: add CommitteeChain (#27766)

This change implements CommitteeChain which is a key component of the beacon light client. It is a passive data structure that can validate, hold and update a chain of beacon light sync committees and updates, starting from a checkpoint that proves the starting committee through a beacon block hash, header and corresponding state. Once synced to the current sync period, CommitteeChain can also validate signed beacon headers.

beacon/light/canonical.go

@@ -0,0 +1,125 @@
+// Copyright 2023 The go-ethereum Authors
+// This file is part of the go-ethereum library.
+//
+// The go-ethereum library is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Lesser General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// The go-ethereum library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU Lesser General Public License for more details.
+//
+// You should have received a copy of the GNU Lesser General Public License
+// along with the go-ethereum library. If not, see <http://www.gnu.org/licenses/>.
+
+package light
+
+import (
+	"encoding/binary"
+	"fmt"
+
+	"github.com/ethereum/go-ethereum/common/lru"
+	"github.com/ethereum/go-ethereum/ethdb"
+	"github.com/ethereum/go-ethereum/log"
+	"github.com/ethereum/go-ethereum/rlp"
+)
+
+// canonicalStore stores instances of the given type in a database and caches
+// them in memory, associated with a continuous range of period numbers.
+// Note: canonicalStore is not thread safe and it is the caller's responsibility
+// to avoid concurrent access.
+type canonicalStore[T any] struct {
+	keyPrefix []byte
+	periods   periodRange
+	cache     *lru.Cache[uint64, T]
+}
+
+// newCanonicalStore creates a new canonicalStore and loads all keys associated
+// with the keyPrefix in order to determine the ranges available in the database.
+func newCanonicalStore[T any](db ethdb.Iteratee, keyPrefix []byte) (*canonicalStore[T], error) {
+	cs := &canonicalStore[T]{
+		keyPrefix: keyPrefix,
+		cache:     lru.NewCache[uint64, T](100),
+	}
+	var (
+		iter  = db.NewIterator(keyPrefix, nil)
+		kl    = len(keyPrefix)
+		first = true
+	)
+	defer iter.Release()
+
+	for iter.Next() {
+		if len(iter.Key()) != kl+8 {
+			log.Warn("Invalid key length in the canonical chain database", "key", fmt.Sprintf("%#x", iter.Key()))
+			continue
+		}
+		period := binary.BigEndian.Uint64(iter.Key()[kl : kl+8])
+		if first {
+			cs.periods.Start = period
+		} else if cs.periods.End != period {
+			return nil, fmt.Errorf("gap in the canonical chain database between periods %d and %d", cs.periods.End, period-1)
+		}
+		first = false
+		cs.periods.End = period + 1
+	}
+	return cs, nil
+}
+
+// databaseKey returns the database key belonging to the given period.
+func (cs *canonicalStore[T]) databaseKey(period uint64) []byte {
+	return binary.BigEndian.AppendUint64(append([]byte{}, cs.keyPrefix...), period)
+}
+
+// add adds the given item to the database. It also ensures that the range remains
+// continuous. Can be used either with a batch or database backend.
+func (cs *canonicalStore[T]) add(backend ethdb.KeyValueWriter, period uint64, value T) error {
+	if !cs.periods.canExpand(period) {
+		return fmt.Errorf("period expansion is not allowed, first: %d, next: %d, period: %d", cs.periods.Start, cs.periods.End, period)
+	}
+	enc, err := rlp.EncodeToBytes(value)
+	if err != nil {
+		return err
+	}
+	if err := backend.Put(cs.databaseKey(period), enc); err != nil {
+		return err
+	}
+	cs.cache.Add(period, value)
+	cs.periods.expand(period)
+	return nil
+}
+
+// deleteFrom removes items starting from the given period.
+func (cs *canonicalStore[T]) deleteFrom(db ethdb.KeyValueWriter, fromPeriod uint64) (deleted periodRange) {
+	keepRange, deleteRange := cs.periods.split(fromPeriod)
+	deleteRange.each(func(period uint64) {
+		db.Delete(cs.databaseKey(period))
+		cs.cache.Remove(period)
+	})
+	cs.periods = keepRange
+	return deleteRange
+}
+
+// get returns the item at the given period or the null value of the given type
+// if no item is present.
+func (cs *canonicalStore[T]) get(backend ethdb.KeyValueReader, period uint64) (T, bool) {
+	var null, value T
+	if !cs.periods.contains(period) {
+		return null, false
+	}
+	if value, ok := cs.cache.Get(period); ok {
+		return value, true
+	}
+	enc, err := backend.Get(cs.databaseKey(period))
+	if err != nil {
+		log.Error("Canonical store value not found", "period", period, "start", cs.periods.Start, "end", cs.periods.End)
+		return null, false
+	}
+	if err := rlp.DecodeBytes(enc, &value); err != nil {
+		log.Error("Error decoding canonical store value", "error", err)
+		return null, false
+	}
+	cs.cache.Add(period, value)
+	return value, true
+}

beacon/light/committee_chain.go

@@ -0,0 +1,514 @@
+// Copyright 2023 The go-ethereum Authors
+// This file is part of the go-ethereum library.
+//
+// The go-ethereum library is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Lesser General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// The go-ethereum library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU Lesser General Public License for more details.
+//
+// You should have received a copy of the GNU Lesser General Public License
+// along with the go-ethereum library. If not, see <http://www.gnu.org/licenses/>.
+
+package light
+
+import (
+	"errors"
+	"fmt"
+	"math"
+	"sync"
+	"time"
+
+	"github.com/ethereum/go-ethereum/beacon/params"
+	"github.com/ethereum/go-ethereum/beacon/types"
+	"github.com/ethereum/go-ethereum/common"
+	"github.com/ethereum/go-ethereum/common/lru"
+	"github.com/ethereum/go-ethereum/common/mclock"
+	"github.com/ethereum/go-ethereum/core/rawdb"
+	"github.com/ethereum/go-ethereum/ethdb"
+	"github.com/ethereum/go-ethereum/log"
+)
+
+var (
+	ErrNeedCommittee      = errors.New("sync committee required")
+	ErrInvalidUpdate      = errors.New("invalid committee update")
+	ErrInvalidPeriod      = errors.New("invalid update period")
+	ErrWrongCommitteeRoot = errors.New("wrong committee root")
+	ErrCannotReorg        = errors.New("can not reorg committee chain")
+)
+
+// CommitteeChain is a passive data structure that can validate, hold and update
+// a chain of beacon light sync committees and updates. It requires at least one
+// externally set fixed committee root at the beginning of the chain which can
+// be set either based on a BootstrapData or a trusted source (a local beacon
+// full node). This makes the structure useful for both light client and light
+// server setups.
+//
+// It always maintains the following consistency constraints:
+//   - a committee can only be present if its root hash matches an existing fixed
+//     root or if it is proven by an update at the previous period
+//   - an update can only be present if a committee is present at the same period
+//     and the update signature is valid and has enough participants.
+//     The committee at the next period (proven by the update) should also be
+//     present (note that this means they can only be added together if neither
+//     is present yet). If a fixed root is present at the next period then the
+//     update can only be present if it proves the same committee root.
+//
+// Once synced to the current sync period, CommitteeChain can also validate
+// signed beacon headers.
+type CommitteeChain struct {
+	// chainmu guards against concurrent access to the canonicalStore structures
+	// (updates, committees, fixedCommitteeRoots) and ensures that they stay consistent
+	// with each other and with committeeCache.
+	chainmu             sync.RWMutex
+	db                  ethdb.KeyValueStore
+	updates             *canonicalStore[*types.LightClientUpdate]
+	committees          *canonicalStore[*types.SerializedSyncCommittee]
+	fixedCommitteeRoots *canonicalStore[common.Hash]
+	committeeCache      *lru.Cache[uint64, syncCommittee] // cache deserialized committees
+
+	clock       mclock.Clock         // monotonic clock (simulated clock in tests)
+	unixNano    func() int64         // system clock (simulated clock in tests)
+	sigVerifier committeeSigVerifier // BLS sig verifier (dummy verifier in tests)
+
+	config             *types.ChainConfig
+	signerThreshold    int
+	minimumUpdateScore types.UpdateScore
+	enforceTime        bool // enforceTime specifies whether the age of a signed header should be checked
+}
+
+// NewCommitteeChain creates a new CommitteeChain.
+func NewCommitteeChain(db ethdb.KeyValueStore, config *types.ChainConfig, signerThreshold int, enforceTime bool) *CommitteeChain {
+	return newCommitteeChain(db, config, signerThreshold, enforceTime, blsVerifier{}, &mclock.System{}, func() int64 { return time.Now().UnixNano() })
+}
+
+// newCommitteeChain creates a new CommitteeChain with the option of replacing the
+// clock source and signature verification for testing purposes.
+func newCommitteeChain(db ethdb.KeyValueStore, config *types.ChainConfig, signerThreshold int, enforceTime bool, sigVerifier committeeSigVerifier, clock mclock.Clock, unixNano func() int64) *CommitteeChain {
+	s := &CommitteeChain{
+		committeeCache:  lru.NewCache[uint64, syncCommittee](10),
+		db:              db,
+		sigVerifier:     sigVerifier,
+		clock:           clock,
+		unixNano:        unixNano,
+		config:          config,
+		signerThreshold: signerThreshold,
+		enforceTime:     enforceTime,
+		minimumUpdateScore: types.UpdateScore{
+			SignerCount:    uint32(signerThreshold),
+			SubPeriodIndex: params.SyncPeriodLength / 16,
+		},
+	}
+
+	var err1, err2, err3 error
+	if s.fixedCommitteeRoots, err1 = newCanonicalStore[common.Hash](db, rawdb.FixedCommitteeRootKey); err1 != nil {
+		log.Error("Error creating fixed committee root store", "error", err1)
+	}
+	if s.committees, err2 = newCanonicalStore[*types.SerializedSyncCommittee](db, rawdb.SyncCommitteeKey); err2 != nil {
+		log.Error("Error creating committee store", "error", err2)
+	}
+	if s.updates, err3 = newCanonicalStore[*types.LightClientUpdate](db, rawdb.BestUpdateKey); err3 != nil {
+		log.Error("Error creating update store", "error", err3)
+	}
+	if err1 != nil || err2 != nil || err3 != nil || !s.checkConstraints() {
+		log.Info("Resetting invalid committee chain")
+		s.Reset()
+	}
+	// roll back invalid updates (might be necessary if forks have been changed since last time)
+	for !s.updates.periods.isEmpty() {
+		update, ok := s.updates.get(s.db, s.updates.periods.End-1)
+		if !ok {
+			log.Error("Sync committee update missing", "period", s.updates.periods.End-1)
+			s.Reset()
+			break
+		}
+		if valid, err := s.verifyUpdate(update); err != nil {
+			log.Error("Error validating update", "period", s.updates.periods.End-1, "error", err)
+		} else if valid {
+			break
+		}
+		if err := s.rollback(s.updates.periods.End); err != nil {
+			log.Error("Error writing batch into chain database", "error", err)
+		}
+	}
+	if !s.committees.periods.isEmpty() {
+		log.Trace("Sync committee chain loaded", "first period", s.committees.periods.Start, "last period", s.committees.periods.End-1)
+	}
+	return s
+}
+
+// checkConstraints checks committee chain validity constraints
+func (s *CommitteeChain) checkConstraints() bool {
+	isNotInFixedCommitteeRootRange := func(r periodRange) bool {
+		return s.fixedCommitteeRoots.periods.isEmpty() ||
+			r.Start < s.fixedCommitteeRoots.periods.Start ||
+			r.Start >= s.fixedCommitteeRoots.periods.End
+	}
+
+	valid := true
+	if !s.updates.periods.isEmpty() {
+		if isNotInFixedCommitteeRootRange(s.updates.periods) {
+			log.Error("Start update is not in the fixed roots range")
+			valid = false
+		}
+		if s.committees.periods.Start > s.updates.periods.Start || s.committees.periods.End <= s.updates.periods.End {
+			log.Error("Missing committees in update range")
+			valid = false
+		}
+	}
+	if !s.committees.periods.isEmpty() {
+		if isNotInFixedCommitteeRootRange(s.committees.periods) {
+			log.Error("Start committee is not in the fixed roots range")
+			valid = false
+		}
+		if s.committees.periods.End > s.fixedCommitteeRoots.periods.End && s.committees.periods.End > s.updates.periods.End+1 {
+			log.Error("Last committee is neither in the fixed roots range nor proven by updates")
+			valid = false
+		}
+	}
+	return valid
+}
+
+// Reset resets the committee chain.
+func (s *CommitteeChain) Reset() {
+	s.chainmu.Lock()
+	defer s.chainmu.Unlock()
+
+	if err := s.rollback(0); err != nil {
+		log.Error("Error writing batch into chain database", "error", err)
+	}
+}
+
+// CheckpointInit initializes a CommitteeChain based on the checkpoint.
+// Note: if the chain is already initialized and the committees proven by the
+// checkpoint do match the existing chain then the chain is retained and the
+// new checkpoint becomes fixed.
+func (s *CommitteeChain) CheckpointInit(bootstrap *types.BootstrapData) error {
+	s.chainmu.Lock()
+	defer s.chainmu.Unlock()
+
+	if err := bootstrap.Validate(); err != nil {
+		return err
+	}
+
+	period := bootstrap.Header.SyncPeriod()
+	if err := s.deleteFixedCommitteeRootsFrom(period + 2); err != nil {
+		s.Reset()
+		return err
+	}
+	if s.addFixedCommitteeRoot(period, bootstrap.CommitteeRoot) != nil {
+		s.Reset()
+		if err := s.addFixedCommitteeRoot(period, bootstrap.CommitteeRoot); err != nil {
+			s.Reset()
+			return err
+		}
+	}
+	if err := s.addFixedCommitteeRoot(period+1, common.Hash(bootstrap.CommitteeBranch[0])); err != nil {
+		s.Reset()
+		return err
+	}
+	if err := s.addCommittee(period, bootstrap.Committee); err != nil {
+		s.Reset()
+		return err
+	}
+	return nil
+}
+
+// addFixedCommitteeRoot sets a fixed committee root at the given period.
+// Note that the period where the first committee is added has to have a fixed
+// root which can either come from a BootstrapData or a trusted source.
+func (s *CommitteeChain) addFixedCommitteeRoot(period uint64, root common.Hash) error {
+	if root == (common.Hash{}) {
+		return ErrWrongCommitteeRoot
+	}
+
+	batch := s.db.NewBatch()
+	oldRoot := s.getCommitteeRoot(period)
+	if !s.fixedCommitteeRoots.periods.canExpand(period) {
+		// Note: the fixed committee root range should always be continuous and
+		// therefore the expected syncing method is to forward sync and optionally
+		// backward sync periods one by one, starting from a checkpoint. The only
+		// case when a root that is not adjacent to the already fixed ones can be
+		// fixed is when the same root has already been proven by an update chain.
+		// In this case the all roots in between can and should be fixed.
+		// This scenario makes sense when a new trusted checkpoint is added to an
+		// existing chain, ensuring that it will not be rolled back (might be
+		// important in case of low signer participation rate).
+		if root != oldRoot {
+			return ErrInvalidPeriod
+		}
+		// if the old root exists and matches the new one then it is guaranteed
+		// that the given period is after the existing fixed range and the roots
+		// in between can also be fixed.
+		for p := s.fixedCommitteeRoots.periods.End; p < period; p++ {
+			if err := s.fixedCommitteeRoots.add(batch, p, s.getCommitteeRoot(p)); err != nil {
+				return err
+			}
+		}
+	}
+	if oldRoot != (common.Hash{}) && (oldRoot != root) {
+		// existing old root was different, we have to reorg the chain
+		if err := s.rollback(period); err != nil {
+			return err
+		}
+	}
+	if err := s.fixedCommitteeRoots.add(batch, period, root); err != nil {
+		return err
+	}
+	if err := batch.Write(); err != nil {
+		log.Error("Error writing batch into chain database", "error", err)
+		return err
+	}
+	return nil
+}
+
+// deleteFixedCommitteeRootsFrom deletes fixed roots starting from the given period.
+// It also maintains chain consistency, meaning that it also deletes updates and
+// committees if they are no longer supported by a valid update chain.
+func (s *CommitteeChain) deleteFixedCommitteeRootsFrom(period uint64) error {
+	if period >= s.fixedCommitteeRoots.periods.End {
+		return nil
+	}
+	batch := s.db.NewBatch()
+	s.fixedCommitteeRoots.deleteFrom(batch, period)
+	if s.updates.periods.isEmpty() || period <= s.updates.periods.Start {
+		// Note: the first period of the update chain should always be fixed so if
+		// the fixed root at the first update is removed then the entire update chain
+		// and the proven committees have to be removed. Earlier committees in the
+		// remaining fixed root range can stay.
+		s.updates.deleteFrom(batch, period)
+		s.deleteCommitteesFrom(batch, period)
+	} else {
+		// The update chain stays intact, some previously fixed committee roots might
+		// get unfixed but are still proven by the update chain. If there were
+		// committees present after the range proven by updates, those should be
+		// removed if the belonging fixed roots are also removed.
+		fromPeriod := s.updates.periods.End + 1 // not proven by updates
+		if period > fromPeriod {
+			fromPeriod = period // also not justified by fixed roots
+		}
+		s.deleteCommitteesFrom(batch, fromPeriod)
+	}
+	if err := batch.Write(); err != nil {
+		log.Error("Error writing batch into chain database", "error", err)
+		return err
+	}
+	return nil
+}
+
+// deleteCommitteesFrom deletes committees starting from the given period.
+func (s *CommitteeChain) deleteCommitteesFrom(batch ethdb.Batch, period uint64) {
+	deleted := s.committees.deleteFrom(batch, period)
+	for period := deleted.Start; period < deleted.End; period++ {
+		s.committeeCache.Remove(period)
+	}
+}
+
+// addCommittee adds a committee at the given period if possible.
+func (s *CommitteeChain) addCommittee(period uint64, committee *types.SerializedSyncCommittee) error {
+	if !s.committees.periods.canExpand(period) {
+		return ErrInvalidPeriod
+	}
+	root := s.getCommitteeRoot(period)
+	if root == (common.Hash{}) {
+		return ErrInvalidPeriod
+	}
+	if root != committee.Root() {
+		return ErrWrongCommitteeRoot
+	}
+	if !s.committees.periods.contains(period) {
+		if err := s.committees.add(s.db, period, committee); err != nil {
+			return err
+		}
+		s.committeeCache.Remove(period)
+	}
+	return nil
+}
+
+// InsertUpdate adds a new update if possible.
+func (s *CommitteeChain) InsertUpdate(update *types.LightClientUpdate, nextCommittee *types.SerializedSyncCommittee) error {
+	s.chainmu.Lock()
+	defer s.chainmu.Unlock()
+
+	period := update.AttestedHeader.Header.SyncPeriod()
+	if !s.updates.periods.canExpand(period) || !s.committees.periods.contains(period) {
+		return ErrInvalidPeriod
+	}
+	if s.minimumUpdateScore.BetterThan(update.Score()) {
+		return ErrInvalidUpdate
+	}
+	oldRoot := s.getCommitteeRoot(period + 1)
+	reorg := oldRoot != (common.Hash{}) && oldRoot != update.NextSyncCommitteeRoot
+	if oldUpdate, ok := s.updates.get(s.db, period); ok && !update.Score().BetterThan(oldUpdate.Score()) {
+		// a better or equal update already exists; no changes, only fail if new one tried to reorg
+		if reorg {
+			return ErrCannotReorg
+		}
+		return nil
+	}
+	if s.fixedCommitteeRoots.periods.contains(period+1) && reorg {
+		return ErrCannotReorg
+	}
+	if ok, err := s.verifyUpdate(update); err != nil {
+		return err
+	} else if !ok {
+		return ErrInvalidUpdate
+	}
+	addCommittee := !s.committees.periods.contains(period+1) || reorg
+	if addCommittee {
+		if nextCommittee == nil {
+			return ErrNeedCommittee
+		}
+		if nextCommittee.Root() != update.NextSyncCommitteeRoot {
+			return ErrWrongCommitteeRoot
+		}
+	}
+	if reorg {
+		if err := s.rollback(period + 1); err != nil {
+			return err
+		}
+	}
+	batch := s.db.NewBatch()
+	if addCommittee {
+		if err := s.committees.add(batch, period+1, nextCommittee); err != nil {
+			return err
+		}
+		s.committeeCache.Remove(period + 1)
+	}
+	if err := s.updates.add(batch, period, update); err != nil {
+		return err
+	}
+	if err := batch.Write(); err != nil {
+		log.Error("Error writing batch into chain database", "error", err)
+		return err
+	}
+	log.Info("Inserted new committee update", "period", period, "next committee root", update.NextSyncCommitteeRoot)
+	return nil
+}
+
+// NextSyncPeriod returns the next period where an update can be added and also
+// whether the chain is initialized at all.
+func (s *CommitteeChain) NextSyncPeriod() (uint64, bool) {
+	s.chainmu.RLock()
+	defer s.chainmu.RUnlock()
+
+	if s.committees.periods.isEmpty() {
+		return 0, false
+	}
+	if !s.updates.periods.isEmpty() {
+		return s.updates.periods.End, true
+	}
+	return s.committees.periods.End - 1, true
+}
+
+// rollback removes all committees and fixed roots from the given period and updates
+// starting from the previous period.
+func (s *CommitteeChain) rollback(period uint64) error {
+	max := s.updates.periods.End + 1
+	if s.committees.periods.End > max {
+		max = s.committees.periods.End
+	}
+	if s.fixedCommitteeRoots.periods.End > max {
+		max = s.fixedCommitteeRoots.periods.End
+	}
+	for max > period {
+		max--
+		batch := s.db.NewBatch()
+		s.deleteCommitteesFrom(batch, max)
+		s.fixedCommitteeRoots.deleteFrom(batch, max)
+		if max > 0 {
+			s.updates.deleteFrom(batch, max-1)
+		}
+		if err := batch.Write(); err != nil {
+			log.Error("Error writing batch into chain database", "error", err)
+			return err
+		}
+	}
+	return nil
+}
+
+// getCommitteeRoot returns the committee root at the given period, either fixed,
+// proven by a previous update or both. It returns an empty hash if the committee
+// root is unknown.
+func (s *CommitteeChain) getCommitteeRoot(period uint64) common.Hash {
+	if root, ok := s.fixedCommitteeRoots.get(s.db, period); ok || period == 0 {
+		return root
+	}
+	if update, ok := s.updates.get(s.db, period-1); ok {
+		return update.NextSyncCommitteeRoot
+	}
+	return common.Hash{}
+}
+
+// getSyncCommittee returns the deserialized sync committee at the given period.
+func (s *CommitteeChain) getSyncCommittee(period uint64) (syncCommittee, error) {
+	if c, ok := s.committeeCache.Get(period); ok {
+		return c, nil
+	}
+	if sc, ok := s.committees.get(s.db, period); ok {
+		c, err := s.sigVerifier.deserializeSyncCommittee(sc)
+		if err != nil {
+			return nil, fmt.Errorf("Sync committee #%d deserialization error: %v", period, err)
+		}
+		s.committeeCache.Add(period, c)
+		return c, nil
+	}
+	return nil, fmt.Errorf("Missing serialized sync committee #%d", period)
+}
+
+// VerifySignedHeader returns true if the given signed header has a valid signature
+// according to the local committee chain. The caller should ensure that the
+// committees advertised by the same source where the signed header came from are
+// synced before verifying the signature.
+// The age of the header is also returned (the time elapsed since the beginning
+// of the given slot, according to the local system clock). If enforceTime is
+// true then negative age (future) headers are rejected.
+func (s *CommitteeChain) VerifySignedHeader(head types.SignedHeader) (bool, time.Duration, error) {
+	s.chainmu.RLock()
+	defer s.chainmu.RUnlock()
+
+	return s.verifySignedHeader(head)
+}
+
+func (s *CommitteeChain) verifySignedHeader(head types.SignedHeader) (bool, time.Duration, error) {
+	var age time.Duration
+	now := s.unixNano()
+	if head.Header.Slot < (uint64(now-math.MinInt64)/uint64(time.Second)-s.config.GenesisTime)/12 {
+		age = time.Duration(now - int64(time.Second)*int64(s.config.GenesisTime+head.Header.Slot*12))
+	} else {
+		age = time.Duration(math.MinInt64)
+	}
+	if s.enforceTime && age < 0 {
+		return false, age, nil
+	}
+	committee, err := s.getSyncCommittee(types.SyncPeriod(head.SignatureSlot))
+	if err != nil {
+		return false, 0, err
+	}
+	if committee == nil {
+		return false, age, nil
+	}
+	if signingRoot, err := s.config.Forks.SigningRoot(head.Header); err == nil {
+		return s.sigVerifier.verifySignature(committee, signingRoot, &head.Signature), age, nil
+	}
+	return false, age, nil
+}
+
+// verifyUpdate checks whether the header signature is correct and the update
+// fits into the specified constraints (assumes that the update has been
+// successfully validated previously)
+func (s *CommitteeChain) verifyUpdate(update *types.LightClientUpdate) (bool, error) {
+	// Note: SignatureSlot determines the sync period of the committee used for signature
+	// verification. Though in reality SignatureSlot is always bigger than update.Header.Slot,
+	// setting them as equal here enforces the rule that they have to be in the same sync
+	// period in order for the light client update proof to be meaningful.
+	ok, age, err := s.verifySignedHeader(update.AttestedHeader)
+	if age < 0 {
+		log.Warn("Future committee update received", "age", age)
+	}
+	return ok, err
+}

beacon/light/committee_chain_test.go

@@ -0,0 +1,356 @@
+// Copyright 2022 The go-ethereum Authors
+// This file is part of the go-ethereum library.
+//
+// The go-ethereum library is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Lesser General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// The go-ethereum library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU Lesser General Public License for more details.
+//
+// You should have received a copy of the GNU Lesser General Public License
+// along with the go-ethereum library. If not, see <http://www.gnu.org/licenses/>.
+
+package light
+
+import (
+	"crypto/rand"
+	"testing"
+	"time"
+
+	"github.com/ethereum/go-ethereum/beacon/params"
+	"github.com/ethereum/go-ethereum/beacon/types"
+	"github.com/ethereum/go-ethereum/common/mclock"
+	"github.com/ethereum/go-ethereum/ethdb/memorydb"
+)
+
+var (
+	testGenesis  = newTestGenesis()
+	testGenesis2 = newTestGenesis()
+
+	tfBase = newTestForks(testGenesis, types.Forks{
+		&types.Fork{Epoch: 0, Version: []byte{0}},
+	})
+	tfAlternative = newTestForks(testGenesis, types.Forks{
+		&types.Fork{Epoch: 0, Version: []byte{0}},
+		&types.Fork{Epoch: 0x700, Version: []byte{1}},
+	})
+	tfAnotherGenesis = newTestForks(testGenesis2, types.Forks{
+		&types.Fork{Epoch: 0, Version: []byte{0}},
+	})
+
+	tcBase                      = newTestCommitteeChain(nil, tfBase, true, 0, 10, 400, false)
+	tcBaseWithInvalidUpdates    = newTestCommitteeChain(tcBase, tfBase, false, 5, 10, 200, false) // signer count too low
+	tcBaseWithBetterUpdates     = newTestCommitteeChain(tcBase, tfBase, false, 5, 10, 440, false)
+	tcReorgWithWorseUpdates     = newTestCommitteeChain(tcBase, tfBase, true, 5, 10, 400, false)
+	tcReorgWithWorseUpdates2    = newTestCommitteeChain(tcBase, tfBase, true, 5, 10, 380, false)
+	tcReorgWithBetterUpdates    = newTestCommitteeChain(tcBase, tfBase, true, 5, 10, 420, false)
+	tcReorgWithFinalizedUpdates = newTestCommitteeChain(tcBase, tfBase, true, 5, 10, 400, true)
+	tcFork                      = newTestCommitteeChain(tcBase, tfAlternative, true, 7, 10, 400, false)
+	tcAnotherGenesis            = newTestCommitteeChain(nil, tfAnotherGenesis, true, 0, 10, 400, false)
+)
+
+func TestCommitteeChainFixedCommitteeRoots(t *testing.T) {
+	for _, reload := range []bool{false, true} {
+		c := newCommitteeChainTest(t, tfBase, 300, true)
+		c.setClockPeriod(7)
+		c.addFixedCommitteeRoot(tcBase, 4, nil)
+		c.addFixedCommitteeRoot(tcBase, 5, nil)
+		c.addFixedCommitteeRoot(tcBase, 6, nil)
+		c.addFixedCommitteeRoot(tcBase, 8, ErrInvalidPeriod) // range has to be continuous
+		c.addFixedCommitteeRoot(tcBase, 3, nil)
+		c.addFixedCommitteeRoot(tcBase, 2, nil)
+		if reload {
+			c.reloadChain()
+		}
+		c.addCommittee(tcBase, 4, nil)
+		c.addCommittee(tcBase, 6, ErrInvalidPeriod) // range has to be continuous
+		c.addCommittee(tcBase, 5, nil)
+		c.addCommittee(tcBase, 6, nil)
+		c.addCommittee(tcAnotherGenesis, 3, ErrWrongCommitteeRoot)
+		c.addCommittee(tcBase, 3, nil)
+		if reload {
+			c.reloadChain()
+		}
+		c.verifyRange(tcBase, 3, 6)
+	}
+}
+
+func TestCommitteeChainCheckpointSync(t *testing.T) {
+	for _, enforceTime := range []bool{false, true} {
+		for _, reload := range []bool{false, true} {
+			c := newCommitteeChainTest(t, tfBase, 300, enforceTime)
+			if enforceTime {
+				c.setClockPeriod(6)
+			}
+			c.insertUpdate(tcBase, 3, true, ErrInvalidPeriod)
+			c.addFixedCommitteeRoot(tcBase, 3, nil)
+			c.addFixedCommitteeRoot(tcBase, 4, nil)
+			c.insertUpdate(tcBase, 4, true, ErrInvalidPeriod) // still no committee
+			c.addCommittee(tcBase, 3, nil)
+			c.addCommittee(tcBase, 4, nil)
+			if reload {
+				c.reloadChain()
+			}
+			c.verifyRange(tcBase, 3, 4)
+			c.insertUpdate(tcBase, 3, false, nil)              // update can be added without committee here
+			c.insertUpdate(tcBase, 4, false, ErrNeedCommittee) // but not here as committee 5 is not there yet
+			c.insertUpdate(tcBase, 4, true, nil)
+			c.verifyRange(tcBase, 3, 5)
+			c.insertUpdate(tcBaseWithInvalidUpdates, 5, true, ErrInvalidUpdate) // signer count too low
+			c.insertUpdate(tcBase, 5, true, nil)
+			if reload {
+				c.reloadChain()
+			}
+			if enforceTime {
+				c.insertUpdate(tcBase, 6, true, ErrInvalidUpdate) // future update rejected
+				c.setClockPeriod(7)
+			}
+			c.insertUpdate(tcBase, 6, true, nil) // when the time comes it's accepted
+			if reload {
+				c.reloadChain()
+			}
+			if enforceTime {
+				c.verifyRange(tcBase, 3, 6) // committee 7 is there but still in the future
+				c.setClockPeriod(8)
+			}
+			c.verifyRange(tcBase, 3, 7) // now period 7 can also be verified
+			// try reverse syncing an update
+			c.insertUpdate(tcBase, 2, false, ErrInvalidPeriod) // fixed committee is needed first
+			c.addFixedCommitteeRoot(tcBase, 2, nil)
+			c.addCommittee(tcBase, 2, nil)
+			c.insertUpdate(tcBase, 2, false, nil)
+			c.verifyRange(tcBase, 2, 7)
+		}
+	}
+}
+
+func TestCommitteeChainReorg(t *testing.T) {
+	for _, reload := range []bool{false, true} {
+		for _, addBetterUpdates := range []bool{false, true} {
+			c := newCommitteeChainTest(t, tfBase, 300, true)
+			c.setClockPeriod(11)
+			c.addFixedCommitteeRoot(tcBase, 3, nil)
+			c.addFixedCommitteeRoot(tcBase, 4, nil)
+			c.addCommittee(tcBase, 3, nil)
+			for period := uint64(3); period < 10; period++ {
+				c.insertUpdate(tcBase, period, true, nil)
+			}
+			if reload {
+				c.reloadChain()
+			}
+			c.verifyRange(tcBase, 3, 10)
+			c.insertUpdate(tcReorgWithWorseUpdates, 5, true, ErrCannotReorg)
+			c.insertUpdate(tcReorgWithWorseUpdates2, 5, true, ErrCannotReorg)
+			if addBetterUpdates {
+				// add better updates for the base chain and expect first reorg to fail
+				// (only add updates as committees should be the same)
+				for period := uint64(5); period < 10; period++ {
+					c.insertUpdate(tcBaseWithBetterUpdates, period, false, nil)
+				}
+				if reload {
+					c.reloadChain()
+				}
+				c.verifyRange(tcBase, 3, 10) // still on the same chain
+				c.insertUpdate(tcReorgWithBetterUpdates, 5, true, ErrCannotReorg)
+			} else {
+				// reorg with better updates
+				c.insertUpdate(tcReorgWithBetterUpdates, 5, false, ErrNeedCommittee)
+				c.verifyRange(tcBase, 3, 10) // no success yet, still on the base chain
+				c.verifyRange(tcReorgWithBetterUpdates, 3, 5)
+				c.insertUpdate(tcReorgWithBetterUpdates, 5, true, nil)
+				// successful reorg, base chain should only match before the reorg period
+				if reload {
+					c.reloadChain()
+				}
+				c.verifyRange(tcBase, 3, 5)
+				c.verifyRange(tcReorgWithBetterUpdates, 3, 6)
+				for period := uint64(6); period < 10; period++ {
+					c.insertUpdate(tcReorgWithBetterUpdates, period, true, nil)
+				}
+				c.verifyRange(tcReorgWithBetterUpdates, 3, 10)
+			}
+			// reorg with finalized updates; should succeed even if base chain updates
+			// have been improved because a finalized update beats everything else
+			c.insertUpdate(tcReorgWithFinalizedUpdates, 5, false, ErrNeedCommittee)
+			c.insertUpdate(tcReorgWithFinalizedUpdates, 5, true, nil)
+			if reload {
+				c.reloadChain()
+			}
+			c.verifyRange(tcReorgWithFinalizedUpdates, 3, 6)
+			for period := uint64(6); period < 10; period++ {
+				c.insertUpdate(tcReorgWithFinalizedUpdates, period, true, nil)
+			}
+			c.verifyRange(tcReorgWithFinalizedUpdates, 3, 10)
+		}
+	}
+}
+
+func TestCommitteeChainFork(t *testing.T) {
+	c := newCommitteeChainTest(t, tfAlternative, 300, true)
+	c.setClockPeriod(11)
+	// trying to sync a chain on an alternative fork with the base chain data
+	c.addFixedCommitteeRoot(tcBase, 0, nil)
+	c.addFixedCommitteeRoot(tcBase, 1, nil)
+	c.addCommittee(tcBase, 0, nil)
+	// shared section should sync without errors
+	for period := uint64(0); period < 7; period++ {
+		c.insertUpdate(tcBase, period, true, nil)
+	}
+	c.insertUpdate(tcBase, 7, true, ErrInvalidUpdate) // wrong fork
+	// committee root #7 is still the same but signatures are already signed with
+	// a different fork id so period 7 should only verify on the alternative fork
+	c.verifyRange(tcBase, 0, 6)
+	c.verifyRange(tcFork, 0, 7)
+	for period := uint64(7); period < 10; period++ {
+		c.insertUpdate(tcFork, period, true, nil)
+	}
+	c.verifyRange(tcFork, 0, 10)
+	// reload the chain while switching to the base fork
+	c.config = tfBase
+	c.reloadChain()
+	// updates 7..9 should be rolled back now
+	c.verifyRange(tcFork, 0, 6) // again, period 7 only verifies on the right fork
+	c.verifyRange(tcBase, 0, 7)
+	c.insertUpdate(tcFork, 7, true, ErrInvalidUpdate) // wrong fork
+	for period := uint64(7); period < 10; period++ {
+		c.insertUpdate(tcBase, period, true, nil)
+	}
+	c.verifyRange(tcBase, 0, 10)
+}
+
+type committeeChainTest struct {
+	t               *testing.T
+	db              *memorydb.Database
+	clock           *mclock.Simulated
+	config          types.ChainConfig
+	signerThreshold int
+	enforceTime     bool
+	chain           *CommitteeChain
+}
+
+func newCommitteeChainTest(t *testing.T, config types.ChainConfig, signerThreshold int, enforceTime bool) *committeeChainTest {
+	c := &committeeChainTest{
+		t:               t,
+		db:              memorydb.New(),
+		clock:           &mclock.Simulated{},
+		config:          config,
+		signerThreshold: signerThreshold,
+		enforceTime:     enforceTime,
+	}
+	c.chain = newCommitteeChain(c.db, &config, signerThreshold, enforceTime, dummyVerifier{}, c.clock, func() int64 { return int64(c.clock.Now()) })
+	return c
+}
+
+func (c *committeeChainTest) reloadChain() {
+	c.chain = newCommitteeChain(c.db, &c.config, c.signerThreshold, c.enforceTime, dummyVerifier{}, c.clock, func() int64 { return int64(c.clock.Now()) })
+}
+
+func (c *committeeChainTest) setClockPeriod(period float64) {
+	target := mclock.AbsTime(period * float64(time.Second*12*params.SyncPeriodLength))
+	wait := time.Duration(target - c.clock.Now())
+	if wait < 0 {
+		c.t.Fatalf("Invalid setClockPeriod")
+	}
+	c.clock.Run(wait)
+}
+
+func (c *committeeChainTest) addFixedCommitteeRoot(tc *testCommitteeChain, period uint64, expErr error) {
+	if err := c.chain.addFixedCommitteeRoot(period, tc.periods[period].committee.Root()); err != expErr {
+		c.t.Errorf("Incorrect error output from addFixedCommitteeRoot at period %d (expected %v, got %v)", period, expErr, err)
+	}
+}
+
+func (c *committeeChainTest) addCommittee(tc *testCommitteeChain, period uint64, expErr error) {
+	if err := c.chain.addCommittee(period, tc.periods[period].committee); err != expErr {
+		c.t.Errorf("Incorrect error output from addCommittee at period %d (expected %v, got %v)", period, expErr, err)
+	}
+}
+
+func (c *committeeChainTest) insertUpdate(tc *testCommitteeChain, period uint64, addCommittee bool, expErr error) {
+	var committee *types.SerializedSyncCommittee
+	if addCommittee {
+		committee = tc.periods[period+1].committee
+	}
+	if err := c.chain.InsertUpdate(tc.periods[period].update, committee); err != expErr {
+		c.t.Errorf("Incorrect error output from InsertUpdate at period %d (expected %v, got %v)", period, expErr, err)
+	}
+}
+
+func (c *committeeChainTest) verifySignedHeader(tc *testCommitteeChain, period float64, expOk bool) {
+	slot := uint64(period * float64(params.SyncPeriodLength))
+	signedHead := GenerateTestSignedHeader(types.Header{Slot: slot}, &tc.config, tc.periods[types.SyncPeriod(slot)].committee, slot+1, 400)
+	if ok, _, _ := c.chain.VerifySignedHeader(signedHead); ok != expOk {
+		c.t.Errorf("Incorrect output from VerifySignedHeader at period %f (expected %v, got %v)", period, expOk, ok)
+	}
+}
+
+func (c *committeeChainTest) verifyRange(tc *testCommitteeChain, begin, end uint64) {
+	if begin > 0 {
+		c.verifySignedHeader(tc, float64(begin)-0.5, false)
+	}
+	for period := begin; period <= end; period++ {
+		c.verifySignedHeader(tc, float64(period)+0.5, true)
+	}
+	c.verifySignedHeader(tc, float64(end)+1.5, false)
+}
+
+func newTestGenesis() types.ChainConfig {
+	var config types.ChainConfig
+	rand.Read(config.GenesisValidatorsRoot[:])
+	return config
+}
+
+func newTestForks(config types.ChainConfig, forks types.Forks) types.ChainConfig {
+	for _, fork := range forks {
+		config.AddFork(fork.Name, fork.Epoch, fork.Version)
+	}
+	return config
+}
+
+func newTestCommitteeChain(parent *testCommitteeChain, config types.ChainConfig, newCommittees bool, begin, end int, signerCount int, finalizedHeader bool) *testCommitteeChain {
+	tc := &testCommitteeChain{
+		config: config,
+	}
+	if parent != nil {
+		tc.periods = make([]testPeriod, len(parent.periods))
+		copy(tc.periods, parent.periods)
+	}
+	if newCommittees {
+		if begin == 0 {
+			tc.fillCommittees(begin, end+1)
+		} else {
+			tc.fillCommittees(begin+1, end+1)
+		}
+	}
+	tc.fillUpdates(begin, end, signerCount, finalizedHeader)
+	return tc
+}
+
+type testPeriod struct {
+	committee *types.SerializedSyncCommittee
+	update    *types.LightClientUpdate
+}
+
+type testCommitteeChain struct {
+	periods []testPeriod
+	config  types.ChainConfig
+}
+
+func (tc *testCommitteeChain) fillCommittees(begin, end int) {
+	if len(tc.periods) <= end {
+		tc.periods = append(tc.periods, make([]testPeriod, end+1-len(tc.periods))...)
+	}
+	for i := begin; i <= end; i++ {
+		tc.periods[i].committee = GenerateTestCommittee()
+	}
+}
+
+func (tc *testCommitteeChain) fillUpdates(begin, end int, signerCount int, finalizedHeader bool) {
+	for i := begin; i <= end; i++ {
+		tc.periods[i].update = GenerateTestUpdate(&tc.config, uint64(i), tc.periods[i].committee, tc.periods[i+1].committee, signerCount, finalizedHeader)
+	}
+}

beacon/light/range.go

@@ -0,0 +1,78 @@
+// Copyright 2023 The go-ethereum Authors
+// This file is part of the go-ethereum library.
+//
+// The go-ethereum library is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Lesser General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// The go-ethereum library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU Lesser General Public License for more details.
+//
+// You should have received a copy of the GNU Lesser General Public License
+// along with the go-ethereum library. If not, see <http://www.gnu.org/licenses/>.
+
+package light
+
+// periodRange represents a (possibly zero-length) range of integers (sync periods).
+type periodRange struct {
+	Start, End uint64
+}
+
+// isEmpty returns true if the length of the range is zero.
+func (a periodRange) isEmpty() bool {
+	return a.End == a.Start
+}
+
+// contains returns true if the range includes the given period.
+func (a periodRange) contains(period uint64) bool {
+	return period >= a.Start && period < a.End
+}
+
+// canExpand returns true if the range includes or can be expanded with the given
+// period (either the range is empty or the given period is inside, right before or
+// right after the range).
+func (a periodRange) canExpand(period uint64) bool {
+	return a.isEmpty() || (period+1 >= a.Start && period <= a.End)
+}
+
+// expand expands the range with the given period.
+// This method assumes that canExpand returned true: otherwise this is a no-op.
+func (a *periodRange) expand(period uint64) {
+	if a.isEmpty() {
+		a.Start, a.End = period, period+1
+		return
+	}
+	if a.Start == period+1 {
+		a.Start--
+	}
+	if a.End == period {
+		a.End++
+	}
+}
+
+// split splits the range into two ranges. The 'fromPeriod' will be the first
+// element in the second range (if present).
+// The original range is unchanged by this operation
+func (a *periodRange) split(fromPeriod uint64) (periodRange, periodRange) {
+	if fromPeriod <= a.Start {
+		// First range empty, everything in second range,
+		return periodRange{}, *a
+	}
+	if fromPeriod >= a.End {
+		// Second range empty, everything in first range,
+		return *a, periodRange{}
+	}
+	x := periodRange{a.Start, fromPeriod}
+	y := periodRange{fromPeriod, a.End}
+	return x, y
+}
+
+// each invokes the supplied function fn once per period in range
+func (a *periodRange) each(fn func(uint64)) {
+	for p := a.Start; p < a.End; p++ {
+		fn(p)
+	}
+}

beacon/light/test_helpers.go

@@ -0,0 +1,152 @@
+// Copyright 2023 The go-ethereum Authors
+// This file is part of the go-ethereum library.
+//
+// The go-ethereum library is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Lesser General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// The go-ethereum library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU Lesser General Public License for more details.
+//
+// You should have received a copy of the GNU Lesser General Public License
+// along with the go-ethereum library. If not, see <http://www.gnu.org/licenses/>.
+
+package light
+
+import (
+	"crypto/rand"
+	"crypto/sha256"
+	mrand "math/rand"
+
+	"github.com/ethereum/go-ethereum/beacon/merkle"
+	"github.com/ethereum/go-ethereum/beacon/params"
+	"github.com/ethereum/go-ethereum/beacon/types"
+	"github.com/ethereum/go-ethereum/common"
+)
+
+func GenerateTestCommittee() *types.SerializedSyncCommittee {
+	s := new(types.SerializedSyncCommittee)
+	rand.Read(s[:32])
+	return s
+}
+
+func GenerateTestUpdate(config *types.ChainConfig, period uint64, committee, nextCommittee *types.SerializedSyncCommittee, signerCount int, finalizedHeader bool) *types.LightClientUpdate {
+	update := new(types.LightClientUpdate)
+	update.NextSyncCommitteeRoot = nextCommittee.Root()
+	var attestedHeader types.Header
+	if finalizedHeader {
+		update.FinalizedHeader = new(types.Header)
+		*update.FinalizedHeader, update.NextSyncCommitteeBranch = makeTestHeaderWithMerkleProof(types.SyncPeriodStart(period)+100, params.StateIndexNextSyncCommittee, merkle.Value(update.NextSyncCommitteeRoot))
+		attestedHeader, update.FinalityBranch = makeTestHeaderWithMerkleProof(types.SyncPeriodStart(period)+200, params.StateIndexFinalBlock, merkle.Value(update.FinalizedHeader.Hash()))
+	} else {
+		attestedHeader, update.NextSyncCommitteeBranch = makeTestHeaderWithMerkleProof(types.SyncPeriodStart(period)+2000, params.StateIndexNextSyncCommittee, merkle.Value(update.NextSyncCommitteeRoot))
+	}
+	update.AttestedHeader = GenerateTestSignedHeader(attestedHeader, config, committee, attestedHeader.Slot+1, signerCount)
+	return update
+}
+
+func GenerateTestSignedHeader(header types.Header, config *types.ChainConfig, committee *types.SerializedSyncCommittee, signatureSlot uint64, signerCount int) types.SignedHeader {
+	bitmask := makeBitmask(signerCount)
+	signingRoot, _ := config.Forks.SigningRoot(header)
+	c, _ := dummyVerifier{}.deserializeSyncCommittee(committee)
+	return types.SignedHeader{
+		Header: header,
+		Signature: types.SyncAggregate{
+			Signers:   bitmask,
+			Signature: makeDummySignature(c.(dummySyncCommittee), signingRoot, bitmask),
+		},
+		SignatureSlot: signatureSlot,
+	}
+}
+
+func GenerateTestCheckpoint(period uint64, committee *types.SerializedSyncCommittee) *types.BootstrapData {
+	header, branch := makeTestHeaderWithMerkleProof(types.SyncPeriodStart(period)+200, params.StateIndexSyncCommittee, merkle.Value(committee.Root()))
+	return &types.BootstrapData{
+		Header:          header,
+		Committee:       committee,
+		CommitteeRoot:   committee.Root(),
+		CommitteeBranch: branch,
+	}
+}
+
+func makeBitmask(signerCount int) (bitmask [params.SyncCommitteeBitmaskSize]byte) {
+	for i := 0; i < params.SyncCommitteeSize; i++ {
+		if mrand.Intn(params.SyncCommitteeSize-i) < signerCount {
+			bitmask[i/8] += byte(1) << (i & 7)
+			signerCount--
+		}
+	}
+	return
+}
+
+func makeTestHeaderWithMerkleProof(slot, index uint64, value merkle.Value) (types.Header, merkle.Values) {
+	var branch merkle.Values
+	hasher := sha256.New()
+	for index > 1 {
+		var proofHash merkle.Value
+		rand.Read(proofHash[:])
+		hasher.Reset()
+		if index&1 == 0 {
+			hasher.Write(value[:])
+			hasher.Write(proofHash[:])
+		} else {
+			hasher.Write(proofHash[:])
+			hasher.Write(value[:])
+		}
+		hasher.Sum(value[:0])
+		index >>= 1
+		branch = append(branch, proofHash)
+	}
+	return types.Header{Slot: slot, StateRoot: common.Hash(value)}, branch
+}
+
+// syncCommittee holds either a blsSyncCommittee or a fake dummySyncCommittee used for testing
+type syncCommittee interface{}
+
+// committeeSigVerifier verifies sync committee signatures (either proper BLS
+// signatures or fake signatures used for testing)
+type committeeSigVerifier interface {
+	deserializeSyncCommittee(s *types.SerializedSyncCommittee) (syncCommittee, error)
+	verifySignature(committee syncCommittee, signedRoot common.Hash, aggregate *types.SyncAggregate) bool
+}
+
+// blsVerifier implements committeeSigVerifier
+type blsVerifier struct{}
+
+// deserializeSyncCommittee implements committeeSigVerifier
+func (blsVerifier) deserializeSyncCommittee(s *types.SerializedSyncCommittee) (syncCommittee, error) {
+	return s.Deserialize()
+}
+
+// verifySignature implements committeeSigVerifier
+func (blsVerifier) verifySignature(committee syncCommittee, signingRoot common.Hash, aggregate *types.SyncAggregate) bool {
+	return committee.(*types.SyncCommittee).VerifySignature(signingRoot, aggregate)
+}
+
+type dummySyncCommittee [32]byte
+
+// dummyVerifier implements committeeSigVerifier
+type dummyVerifier struct{}
+
+// deserializeSyncCommittee implements committeeSigVerifier
+func (dummyVerifier) deserializeSyncCommittee(s *types.SerializedSyncCommittee) (syncCommittee, error) {
+	var sc dummySyncCommittee
+	copy(sc[:], s[:32])
+	return sc, nil
+}
+
+// verifySignature implements committeeSigVerifier
+func (dummyVerifier) verifySignature(committee syncCommittee, signingRoot common.Hash, aggregate *types.SyncAggregate) bool {
+	return aggregate.Signature == makeDummySignature(committee.(dummySyncCommittee), signingRoot, aggregate.Signers)
+}
+
+func makeDummySignature(committee dummySyncCommittee, signingRoot common.Hash, bitmask [params.SyncCommitteeBitmaskSize]byte) (sig [params.BLSSignatureSize]byte) {
+	for i, b := range committee[:] {
+		sig[i] = b ^ signingRoot[i]
+	}
+	copy(sig[32:], bitmask[:])
+	return
+}

beacon/types/light_sync.go

@@ -25,6 +25,24 @@ import (
 	"github.com/ethereum/go-ethereum/common"
 )
 
+// BootstrapData contains a sync committee where light sync can be started,
+// together with a proof through a beacon header and corresponding state.
+// Note: BootstrapData is fetched from a server based on a known checkpoint hash.
+type BootstrapData struct {
+	Header          Header
+	CommitteeRoot   common.Hash
+	Committee       *SerializedSyncCommittee `rlp:"-"`
+	CommitteeBranch merkle.Values
+}
+
+// Validate verifies the proof included in BootstrapData.
+func (c *BootstrapData) Validate() error {
+	if c.CommitteeRoot != c.Committee.Root() {
+		return errors.New("wrong committee root")
+	}
+	return merkle.VerifyProof(c.Header.StateRoot, params.StateIndexSyncCommittee, c.CommitteeBranch, merkle.Value(c.CommitteeRoot))
+}
+
 // LightClientUpdate is a proof of the next sync committee root based on a header
 // signed by the sync committee of the given period. Optionally, the update can
 // prove quasi-finality by the signed header referring to a previous, finalized

core/rawdb/schema.go

@@ -132,6 +132,10 @@ var (
 
 	CliqueSnapshotPrefix = []byte("clique-")
 
+	BestUpdateKey         = []byte("update-")    // bigEndian64(syncPeriod) -> RLP(types.LightClientUpdate)  (nextCommittee only referenced by root hash)
+	FixedCommitteeRootKey = []byte("fixedRoot-") // bigEndian64(syncPeriod) -> committee root hash
+	SyncCommitteeKey      = []byte("committee-") // bigEndian64(syncPeriod) -> serialized committee
+
 	preimageCounter    = metrics.NewRegisteredCounter("db/preimage/total", nil)
 	preimageHitCounter = metrics.NewRegisteredCounter("db/preimage/hits", nil)
 )