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411 lines
17 KiB
411 lines
17 KiB
package light
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import (
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"bytes"
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"context"
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"errors"
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"fmt"
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"time"
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"github.com/tendermint/tendermint/light/provider"
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"github.com/tendermint/tendermint/types"
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)
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// The detector component of the light client detects and handles attacks on the light client.
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// More info here:
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// tendermint/docs/architecture/adr-047-handling-evidence-from-light-client.md
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// detectDivergence is a second wall of defense for the light client.
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//
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// It takes the target verified header and compares it with the headers of a set of
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// witness providers that the light client is connected to. If a conflicting header
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// is returned it verifies and examines the conflicting header against the verified
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// trace that was produced from the primary. If successful, it produces two sets of evidence
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// and sends them to the opposite provider before halting.
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//
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// If there are no conflictinge headers, the light client deems the verified target header
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// trusted and saves it to the trusted store.
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func (c *Client) detectDivergence(ctx context.Context, primaryTrace []*types.LightBlock, now time.Time) error {
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if primaryTrace == nil || len(primaryTrace) < 2 {
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return errors.New("nil or single block primary trace")
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}
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var (
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headerMatched bool
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lastVerifiedHeader = primaryTrace[len(primaryTrace)-1].SignedHeader
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witnessesToRemove = make([]int, 0)
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)
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c.logger.Debug("running detector against trace", "endBlockHeight", lastVerifiedHeader.Height,
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"endBlockHash", lastVerifiedHeader.Hash, "length", len(primaryTrace))
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c.providerMutex.Lock()
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defer c.providerMutex.Unlock()
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if len(c.witnesses) == 0 {
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return ErrNoWitnesses
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}
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// launch one goroutine per witness to retrieve the light block of the target height
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// and compare it with the header from the primary
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errc := make(chan error, len(c.witnesses))
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for i, witness := range c.witnesses {
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go c.compareNewHeaderWithWitness(ctx, errc, lastVerifiedHeader, witness, i)
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}
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// handle errors from the header comparisons as they come in
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for i := 0; i < cap(errc); i++ {
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err := <-errc
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switch e := err.(type) {
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case nil: // at least one header matched
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headerMatched = true
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case errConflictingHeaders:
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// We have conflicting headers. This could possibly imply an attack on the light client.
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// First we need to verify the witness's header using the same skipping verification and then we
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// need to find the point that the headers diverge and examine this for any evidence of an attack.
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//
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// We combine these actions together, verifying the witnesses headers and outputting the trace
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// which captures the bifurcation point and if successful provides the information to create valid evidence.
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err := c.handleConflictingHeaders(ctx, primaryTrace, e.Block, e.WitnessIndex, now)
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if err != nil {
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// return information of the attack
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return err
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}
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// if attempt to generate conflicting headers failed then remove witness
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witnessesToRemove = append(witnessesToRemove, e.WitnessIndex)
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case errBadWitness:
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c.logger.Info("witness returned an error during header comparison, removing...",
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"witness", c.witnesses[e.WitnessIndex], "err", err)
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witnessesToRemove = append(witnessesToRemove, e.WitnessIndex)
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default:
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c.logger.Debug("error in light block request to witness", "err", err)
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}
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}
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// remove witnesses that have misbehaved
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if err := c.removeWitnesses(witnessesToRemove); err != nil {
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return err
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}
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// 1. If we had at least one witness that returned the same header then we
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// conclude that we can trust the header
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if headerMatched {
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return nil
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}
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// 2. Else all witnesses have either not responded, don't have the block or sent invalid blocks.
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return ErrFailedHeaderCrossReferencing
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}
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// compareNewHeaderWithWitness takes the verified header from the primary and compares it with a
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// header from a specified witness. The function can return one of three errors:
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//
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// 1: errConflictingHeaders -> there may have been an attack on this light client
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// 2: errBadWitness -> the witness has either not responded, doesn't have the header or has given us an invalid one
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// Note: In the case of an invalid header we remove the witness
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// 3: nil -> the hashes of the two headers match
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func (c *Client) compareNewHeaderWithWitness(ctx context.Context, errc chan error, h *types.SignedHeader,
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witness provider.Provider, witnessIndex int) {
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lightBlock, err := witness.LightBlock(ctx, h.Height)
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switch err {
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// no error means we move on to checking the hash of the two headers
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case nil:
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break
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// the witness hasn't been helpful in comparing headers, we mark the response and continue
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// comparing with the rest of the witnesses
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case provider.ErrNoResponse, provider.ErrLightBlockNotFound:
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errc <- err
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return
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// the witness' head of the blockchain is lower than the height of the primary. This could be one of
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// two things:
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// 1) The witness is lagging behind
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// 2) The primary may be performing a lunatic attack with a height and time in the future
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case provider.ErrHeightTooHigh:
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// The light client now asks for the latest header that the witness has
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var isTargetHeight bool
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isTargetHeight, lightBlock, err = c.getTargetBlockOrLatest(ctx, h.Height, witness)
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if err != nil {
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errc <- err
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return
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}
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// if the witness caught up and has returned a block of the target height then we can
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// break from this switch case and continue to verify the hashes
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if isTargetHeight {
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break
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}
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// witness' last header is below the primary's header. We check the times to see if the blocks
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// have conflicting times
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if !lightBlock.Time.Before(h.Time) {
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errc <- errConflictingHeaders{Block: lightBlock, WitnessIndex: witnessIndex}
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return
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}
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// the witness is behind. We wait for a period WAITING = 2 * DRIFT + LAG.
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// This should give the witness ample time if it is a participating member
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// of consensus to produce a block that has a time that is after the primary's
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// block time. If not the witness is too far behind and the light client removes it
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time.Sleep(2*c.maxClockDrift + c.maxBlockLag)
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isTargetHeight, lightBlock, err = c.getTargetBlockOrLatest(ctx, h.Height, witness)
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if err != nil {
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errc <- errBadWitness{Reason: err, WitnessIndex: witnessIndex}
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return
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}
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if isTargetHeight {
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break
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}
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// the witness still doesn't have a block at the height of the primary.
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// Check if there is a conflicting time
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if !lightBlock.Time.Before(h.Time) {
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errc <- errConflictingHeaders{Block: lightBlock, WitnessIndex: witnessIndex}
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return
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}
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// Following this request response procedure, the witness has been unable to produce a block
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// that can somehow conflict with the primary's block. We thus conclude that the witness
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// is too far behind and thus we return a no response error.
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//
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// NOTE: If the clock drift / lag has been miscalibrated it is feasible that the light client has
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// drifted too far ahead for any witness to be able provide a comparable block and thus may allow
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// for a malicious primary to attack it
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errc <- provider.ErrNoResponse
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return
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default:
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// all other errors (i.e. invalid block, closed connection or unreliable provider) we mark the
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// witness as bad and remove it
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errc <- errBadWitness{Reason: err, WitnessIndex: witnessIndex}
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return
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}
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if !bytes.Equal(h.Hash(), lightBlock.Hash()) {
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errc <- errConflictingHeaders{Block: lightBlock, WitnessIndex: witnessIndex}
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}
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c.logger.Debug("matching header received by witness", "height", h.Height, "witness", witnessIndex)
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errc <- nil
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}
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// sendEvidence sends evidence to a provider on a best effort basis.
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func (c *Client) sendEvidence(ctx context.Context, ev *types.LightClientAttackEvidence, receiver provider.Provider) {
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err := receiver.ReportEvidence(ctx, ev)
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if err != nil {
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c.logger.Error("failed to report evidence to provider", "ev", ev, "provider", receiver)
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}
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}
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// handleConflictingHeaders handles the primary style of attack, which is where a primary and witness have
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// two headers of the same height but with different hashes
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func (c *Client) handleConflictingHeaders(
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ctx context.Context,
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primaryTrace []*types.LightBlock,
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challendingBlock *types.LightBlock,
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witnessIndex int,
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now time.Time,
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) error {
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supportingWitness := c.witnesses[witnessIndex]
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witnessTrace, primaryBlock, err := c.examineConflictingHeaderAgainstTrace(
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ctx,
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primaryTrace,
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challendingBlock,
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supportingWitness,
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now,
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)
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if err != nil {
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c.logger.Info("error validating witness's divergent header", "witness", supportingWitness, "err", err)
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return nil
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}
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// We are suspecting that the primary is faulty, hence we hold the witness as the source of truth
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// and generate evidence against the primary that we can send to the witness
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commonBlock, trustedBlock := witnessTrace[0], witnessTrace[len(witnessTrace)-1]
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evidenceAgainstPrimary := newLightClientAttackEvidence(primaryBlock, trustedBlock, commonBlock)
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c.logger.Error("ATTEMPTED ATTACK DETECTED. Sending evidence againt primary by witness", "ev", evidenceAgainstPrimary,
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"primary", c.primary, "witness", supportingWitness)
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c.sendEvidence(ctx, evidenceAgainstPrimary, supportingWitness)
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if primaryBlock.Commit.Round != witnessTrace[len(witnessTrace)-1].Commit.Round {
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c.logger.Info("The light client has detected, and prevented, an attempted amnesia attack." +
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" We think this attack is pretty unlikely, so if you see it, that's interesting to us." +
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" Can you let us know by opening an issue through https://github.com/tendermint/tendermint/issues/new?")
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}
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// This may not be valid because the witness itself is at fault. So now we reverse it, examining the
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// trace provided by the witness and holding the primary as the source of truth. Note: primary may not
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// respond but this is okay as we will halt anyway.
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primaryTrace, witnessBlock, err := c.examineConflictingHeaderAgainstTrace(
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ctx,
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witnessTrace,
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primaryBlock,
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c.primary,
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now,
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)
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if err != nil {
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c.logger.Info("Error validating primary's divergent header", "primary", c.primary, "err", err)
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return ErrLightClientAttack
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}
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// We now use the primary trace to create evidence against the witness and send it to the primary
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commonBlock, trustedBlock = primaryTrace[0], primaryTrace[len(primaryTrace)-1]
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evidenceAgainstWitness := newLightClientAttackEvidence(witnessBlock, trustedBlock, commonBlock)
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c.logger.Error("Sending evidence against witness by primary", "ev", evidenceAgainstWitness,
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"primary", c.primary, "witness", supportingWitness)
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c.sendEvidence(ctx, evidenceAgainstWitness, c.primary)
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// We return the error and don't process anymore witnesses
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return ErrLightClientAttack
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}
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// examineConflictingHeaderAgainstTrace takes a trace from one provider and a divergent header that
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// it has received from another and preforms verifySkipping at the heights of each of the intermediate
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// headers in the trace until it reaches the divergentHeader. 1 of 2 things can happen.
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//
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// 1. The light client verifies a header that is different to the intermediate header in the trace. This
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// is the bifurcation point and the light client can create evidence from it
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// 2. The source stops responding, doesn't have the block or sends an invalid header in which case we
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// return the error and remove the witness
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//
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// CONTRACT:
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// 1. Trace can not be empty len(trace) > 0
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// 2. The last block in the trace can not be of a lower height than the target block
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// trace[len(trace)-1].Height >= targetBlock.Height
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// 3. The
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func (c *Client) examineConflictingHeaderAgainstTrace(
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ctx context.Context,
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trace []*types.LightBlock,
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targetBlock *types.LightBlock,
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source provider.Provider, now time.Time,
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) ([]*types.LightBlock, *types.LightBlock, error) {
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var (
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previouslyVerifiedBlock, sourceBlock *types.LightBlock
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sourceTrace []*types.LightBlock
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err error
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)
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if targetBlock.Height < trace[0].Height {
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return nil, nil, fmt.Errorf("target block has a height lower than the trusted height (%d < %d)",
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targetBlock.Height, trace[0].Height)
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}
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for idx, traceBlock := range trace {
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// this case only happens in a forward lunatic attack. We treat the block with the
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// height directly after the targetBlock as the divergent block
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if traceBlock.Height > targetBlock.Height {
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// sanity check that the time of the traceBlock is indeed less than that of the targetBlock. If the trace
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// was correctly verified we should expect monotonically increasing time. This means that if the block at
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// the end of the trace has a lesser time than the target block then all blocks in the trace should have a
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// lesser time
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if traceBlock.Time.After(targetBlock.Time) {
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return nil, nil,
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errors.New("sanity check failed: expected traceblock to have a lesser time than the target block")
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}
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// before sending back the divergent block and trace we need to ensure we have verified
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// the final gap between the previouslyVerifiedBlock and the targetBlock
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if previouslyVerifiedBlock.Height != targetBlock.Height {
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sourceTrace, err = c.verifySkipping(ctx, source, previouslyVerifiedBlock, targetBlock, now)
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if err != nil {
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return nil, nil, fmt.Errorf("verifySkipping of conflicting header failed: %w", err)
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}
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}
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return sourceTrace, traceBlock, nil
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}
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// get the corresponding block from the source to verify and match up against the traceBlock
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if traceBlock.Height == targetBlock.Height {
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sourceBlock = targetBlock
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} else {
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sourceBlock, err = source.LightBlock(ctx, traceBlock.Height)
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if err != nil {
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return nil, nil, fmt.Errorf("failed to examine trace: %w", err)
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}
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}
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// The first block in the trace MUST be the same to the light block that the source produces
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// else we cannot continue with verification.
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if idx == 0 {
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if shash, thash := sourceBlock.Hash(), traceBlock.Hash(); !bytes.Equal(shash, thash) {
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return nil, nil, fmt.Errorf("trusted block is different to the source's first block (%X = %X)",
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thash, shash)
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}
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previouslyVerifiedBlock = sourceBlock
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continue
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}
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// we check that the source provider can verify a block at the same height of the
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// intermediate height
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sourceTrace, err = c.verifySkipping(ctx, source, previouslyVerifiedBlock, sourceBlock, now)
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if err != nil {
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return nil, nil, fmt.Errorf("verifySkipping of conflicting header failed: %w", err)
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}
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// check if the headers verified by the source has diverged from the trace
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if shash, thash := sourceBlock.Hash(), traceBlock.Hash(); !bytes.Equal(shash, thash) {
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// Bifurcation point found!
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return sourceTrace, traceBlock, nil
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}
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// headers are still the same. update the previouslyVerifiedBlock
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previouslyVerifiedBlock = sourceBlock
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}
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// We have reached the end of the trace. This should never happen. This can only happen if one of the stated
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// prerequisites to this function were not met. Namely that either trace[len(trace)-1].Height < targetBlock.Height
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// or that trace[i].Hash() != targetBlock.Hash()
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return nil, nil, errNoDivergence
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}
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// getTargetBlockOrLatest gets the latest height, if it is greater than the target height then it queries
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// the target heght else it returns the latest. returns true if it successfully managed to acquire the target
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// height.
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func (c *Client) getTargetBlockOrLatest(
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ctx context.Context,
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height int64,
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witness provider.Provider,
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) (bool, *types.LightBlock, error) {
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lightBlock, err := witness.LightBlock(ctx, 0)
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if err != nil {
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return false, nil, err
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}
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if lightBlock.Height == height {
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// the witness has caught up to the height of the provider's signed header. We
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// can resume with checking the hashes.
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return true, lightBlock, nil
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}
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if lightBlock.Height > height {
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// the witness has caught up. We recursively call the function again. However in order
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// to avoud a wild goose chase where the witness sends us one header below and one header
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// above the height we set a timeout to the context
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lightBlock, err := witness.LightBlock(ctx, height)
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return true, lightBlock, err
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}
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return false, lightBlock, nil
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}
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// newLightClientAttackEvidence determines the type of attack and then forms the evidence filling out
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// all the fields such that it is ready to be sent to a full node.
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func newLightClientAttackEvidence(conflicted, trusted, common *types.LightBlock) *types.LightClientAttackEvidence {
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ev := &types.LightClientAttackEvidence{ConflictingBlock: conflicted}
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// if this is an equivocation or amnesia attack, i.e. the validator sets are the same, then we
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// return the height of the conflicting block else if it is a lunatic attack and the validator sets
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// are not the same then we send the height of the common header.
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if ev.ConflictingHeaderIsInvalid(trusted.Header) {
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ev.CommonHeight = common.Height
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ev.Timestamp = common.Time
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ev.TotalVotingPower = common.ValidatorSet.TotalVotingPower()
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} else {
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ev.CommonHeight = trusted.Height
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ev.Timestamp = trusted.Time
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ev.TotalVotingPower = trusted.ValidatorSet.TotalVotingPower()
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}
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ev.ByzantineValidators = ev.GetByzantineValidators(common.ValidatorSet, trusted.SignedHeader)
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return ev
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}
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