Validate a Remote Chain Header in CosmosSDK

qizhou · May 5, 2020, 2:30am

A core technique in IBC is to act as a light client of another chain and be able to verify its headers. The sample code can be found here:

github.com

cosmos/cosmos-sdk/blob/84f1261c7b05d392dd56b4efaeb1c20195ec2cd2/x/ibc/07-tendermint/update.go#L25




// CheckValidityAndUpdateState checks if the provided header is valid and updates
// the consensus state if appropriate. It returns an error if:
// - the client or header provided are not parseable to tendermint types
// - the header is invalid
// - header height is lower than the latest client height
// - header valset commit verification fails
//
// Tendermint client validity checking uses the bisection algorithm described
// in the [Tendermint spec](https://github.com/tendermint/spec/blob/master/spec/consensus/light-client.md).
func CheckValidityAndUpdateState(
	clientState clientexported.ClientState, header clientexported.Header,
	currentTimestamp time.Time,
) (clientexported.ClientState, clientexported.ConsensusState, error) {
	tmClientState, ok := clientState.(types.ClientState)
	if !ok {
		return nil, nil, sdkerrors.Wrap(
			clienttypes.ErrInvalidClientType, "light client is not from Tendermint",
		)
	}

where the key is to call lite.verify in TM

github.com

cosmos/cosmos-sdk/blob/84f1261c7b05d392dd56b4efaeb1c20195ec2cd2/x/ibc/07-tendermint/update.go#L88




	// assert header height is newer than any we know
	if header.GetHeight() <= clientState.GetLatestHeight() {
		return sdkerrors.Wrapf(
			clienttypes.ErrInvalidHeader,
			"header height ≤ latest client state height (%d ≤ %d)", header.GetHeight(), clientState.GetLatestHeight(),
		)
	}


	// Verify next header with the last header's validatorset as trusted validatorset
	err := lite.Verify(
		clientState.GetChainID(), &clientState.LastHeader.SignedHeader,
		clientState.LastHeader.ValidatorSet, &header.SignedHeader, header.ValidatorSet,
		clientState.TrustingPeriod, currentTimestamp, clientState.MaxClockDrift, lite.DefaultTrustLevel,
	)
	if err != nil {
		return sdkerrors.Wrap(clienttypes.ErrInvalidHeader, err.Error())
	}
	return nil
}

github.com

tendermint/tendermint/blob/b7c2d7a977674e3eb62aaf4f7f5ef35f664cb091/lite2/verifier.go#L140


	// Ensure that +2/3 of new validators signed correctly.
	if err := untrustedVals.VerifyCommit(chainID, untrustedHeader.Commit.BlockID, untrustedHeader.Height,
		untrustedHeader.Commit); err != nil {
		return ErrInvalidHeader{err}
	}


	return nil
}


// Verify combines both VerifyAdjacent and VerifyNonAdjacent functions.
func Verify(
	chainID string,
	trustedHeader *types.SignedHeader, // height=X
	trustedVals *types.ValidatorSet, // height=X or height=X+1
	untrustedHeader *types.SignedHeader, // height=Y
	untrustedVals *types.ValidatorSet, // height=Y
	trustingPeriod time.Duration,
	now time.Time,
	maxClockDrift time.Duration,
	trustLevel tmmath.Fraction) error {

qizhou · May 6, 2020, 3:05am

And the part that sends a packet is here:

github.com

cosmos/cosmos-sdk/blob/c211488b0b4b0e85eec68d24350a594ebdac5c46/x/ibc/04-channel/keeper/packet.go#L119


			"packet sequence ≠ next send sequence (%d ≠ %d)", packet.GetSequence(), nextSequenceSend,
		)
	}


	nextSequenceSend++
	k.SetNextSequenceSend(ctx, packet.GetSourcePort(), packet.GetSourceChannel(), nextSequenceSend)
	k.SetPacketCommitment(ctx, packet.GetSourcePort(), packet.GetSourceChannel(), packet.GetSequence(), types.CommitPacket(packet))


	// Emit Event with Packet data along with other packet information for relayer to pick up
	// and relay to other chain
	ctx.EventManager().EmitEvents(sdk.Events{
		sdk.NewEvent(
			types.EventTypeSendPacket,
			sdk.NewAttribute(types.AttributeKeyData, string(packet.GetData())),
			sdk.NewAttribute(types.AttributeKeyTimeoutHeight, fmt.Sprintf("%d", packet.GetTimeoutHeight())),
			sdk.NewAttribute(types.AttributeKeyTimeoutTimestamp, fmt.Sprintf("%d", packet.GetTimeoutTimestamp())),
			sdk.NewAttribute(types.AttributeKeySequence, fmt.Sprintf("%d", packet.GetSequence())),
			sdk.NewAttribute(types.AttributeKeySrcPort, packet.GetSourcePort()),
			sdk.NewAttribute(types.AttributeKeySrcChannel, packet.GetSourceChannel()),
			sdk.NewAttribute(types.AttributeKeyDstPort, packet.GetDestPort()),
			sdk.NewAttribute(types.AttributeKeyDstChannel, packet.GetDestChannel()),

And receive part:

github.com

cosmos/cosmos-sdk/blob/c211488b0b4b0e85eec68d24350a594ebdac5c46/x/ibc/04-channel/keeper/packet.go#L139


			sdk.NewAttribute(types.AttributeKeyDstChannel, packet.GetDestChannel()),
		),
	})


	k.Logger(ctx).Info(fmt.Sprintf("packet sent: %v", packet))
	return nil
}


// RecvPacket is called by a module in order to receive & process an IBC packet
// sent on the corresponding channel end on the counterparty chain.
func (k Keeper) RecvPacket(
	ctx sdk.Context,
	packet exported.PacketI,
	proof commitmentexported.Proof,
	proofHeight uint64,
) (exported.PacketI, error) {
	channel, found := k.GetChannel(ctx, packet.GetDestPort(), packet.GetDestChannel())
	if !found {
		return nil, sdkerrors.Wrap(types.ErrChannelNotFound, packet.GetDestChannel())
	}

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Validate a Remote Chain Header in CosmosSDK

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