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package lite
import (
"bytes"
"fmt"
"time"
"github.com/pkg/errors"
"github.com/tendermint/tendermint/libs/log"
tmmath "github.com/tendermint/tendermint/libs/math"
tmrand "github.com/tendermint/tendermint/libs/rand"
"github.com/tendermint/tendermint/lite2/provider"
"github.com/tendermint/tendermint/lite2/store"
"github.com/tendermint/tendermint/types"
)
// TrustOptions are the trust parameters needed when a new light client
// connects to the network or when an existing light client that has been
// offline for longer than the trusting period connects to the network.
//
// The expectation is the user will get this information from a trusted source
// like a validator, a friend, or a secure website. A more user friendly
// solution with trust tradeoffs is that we establish an https based protocol
// with a default end point that populates this information. Also an on-chain
// registry of roots-of-trust (e.g. on the Cosmos Hub) seems likely in the
// future.
type TrustOptions struct {
// tp: trusting period.
//
// Should be significantly less than the unbonding period (e.g. unbonding
// period = 3 weeks, trusting period = 2 weeks).
//
// More specifically, trusting period + time needed to check headers + time
// needed to report and punish misbehavior should be less than the unbonding
// period.
Period time.Duration
// Header's Height and Hash must both be provided to force the trusting of a
// particular header.
Height int64
Hash []byte
}
type mode byte
const (
sequential mode = iota + 1
skipping
)
// Option sets a parameter for the light client.
type Option func(*Client)
// SequentialVerification option configures the light client to sequentially
// check the headers. Note this is much slower than SkippingVerification,
// albeit more secure.
func SequentialVerification() Option {
return func(c *Client) {
c.verificationMode = sequential
}
}
// SkippingVerification option configures the light client to skip headers as
// long as {trustLevel} of the old validator set signed the new header. The
// bisection algorithm from the specification is used for finding the minimal
// "trust path".
//
// trustLevel - fraction of the old validator set (in terms of voting power),
// which must sign the new header in order for us to trust it. NOTE this only
// applies to non-adjusted headers. For adjusted headers, sequential
// verification is used.
func SkippingVerification(trustLevel tmmath.Fraction) Option {
if err := ValidateTrustLevel(trustLevel); err != nil {
panic(err)
}
return func(c *Client) {
c.verificationMode = skipping
c.trustLevel = trustLevel
}
}
// AlternativeSources option can be used to supply alternative providers, which
// will be used for cross-checking the primary provider.
func AlternativeSources(providers []provider.Provider) Option {
return func(c *Client) {
c.alternatives = providers
}
}
// Client represents a light client, connected to a single chain, which gets
// headers from a primary provider, verifies them either sequentially or by
// skipping some and stores them in a trusted store (usually, a local FS).
//
// Default verification: SkippingVerification(DefaultTrustLevel)
type Client struct {
chainID string
trustingPeriod time.Duration // see TrustOptions.Period
verificationMode mode
trustLevel tmmath.Fraction
// Primary provider of new headers.
primary provider.Provider
// Alternative providers for checking the primary for misbehavior by
// comparing data.
alternatives []provider.Provider
// Where trusted headers are stored.
trustedStore store.Store
// Highest trusted header from the store (height=H).
trustedHeader *types.SignedHeader
// Highest next validator set from the store (height=H+1).
trustedNextVals *types.ValidatorSet
logger log.Logger
}
// NewClient returns a new light client. It returns an error if it fails to
// obtain the header & vals from the primary or they are invalid (e.g. trust
// hash does not match with the one from the header).
//
// See all Option(s) for the additional configuration.
func NewClient(
chainID string,
trustOptions TrustOptions,
primary provider.Provider,
trustedStore store.Store,
options ...Option) (*Client, error) {
c := &Client{
chainID: chainID,
trustingPeriod: trustOptions.Period,
verificationMode: skipping,
trustLevel: DefaultTrustLevel,
primary: primary,
trustedStore: trustedStore,
logger: log.NewNopLogger(),
}
for _, o := range options {
o(c)
}
if err := c.initializeWithTrustOptions(trustOptions); err != nil {
return nil, err
}
return c, nil
}
func (c *Client) initializeWithTrustOptions(options TrustOptions) error {
// 1) Fetch and verify the header.
h, err := c.primary.SignedHeader(options.Height)
if err != nil {
return err
}
// NOTE: Verify func will check if it's expired or not.
if err := h.ValidateBasic(c.chainID); err != nil {
return errors.Wrap(err, "ValidateBasic failed")
}
if !bytes.Equal(h.Hash(), options.Hash) {
return errors.Errorf("expected header's hash %X, but got %X", options.Hash, h.Hash())
}
// 2) Fetch and verify the next vals.
vals, err := c.primary.ValidatorSet(options.Height + 1)
if err != nil {
return err
}
// 3) Persist both of them and continue.
return c.updateTrustedHeaderAndVals(h, vals)
}
// SetLogger sets a logger.
func (c *Client) SetLogger(l log.Logger) {
c.logger = l
}
// TrustedHeader returns a trusted header at the given height (0 - the latest)
// or nil if no such header exist.
// TODO: mention how many headers will be kept by the light client.
// .
// height must be >= 0.
//
// It returns an error if:
// - the header expired (ErrOldHeaderExpired). In that case, update your
// client to more recent height;
// - there are some issues with the trusted store, although that should not
// happen normally.
func (c *Client) TrustedHeader(height int64, now time.Time) (*types.SignedHeader, error) {
if height < 0 {
return nil, errors.New("negative height")
}
if height == 0 {
var err error
height, err = c.LastTrustedHeight()
if err != nil {
return nil, err
}
}
h, err := c.trustedStore.SignedHeader(height)
if err != nil {
return nil, err
}
// Ensure header can still be trusted.
expirationTime := h.Time.Add(c.trustingPeriod)
if !expirationTime.After(now) {
return nil, ErrOldHeaderExpired{expirationTime, now}
}
return h, nil
}
// LastTrustedHeight returns a last trusted height.
func (c *Client) LastTrustedHeight() (int64, error) {
return c.trustedStore.LastSignedHeaderHeight()
}
// ChainID returns the chain ID.
func (c *Client) ChainID() string {
return c.chainID
}
// VerifyHeaderAtHeight fetches the header and validators at the given height
// and calls VerifyHeader.
//
// If the trusted header is more recent than one here, an error is returned.
func (c *Client) VerifyHeaderAtHeight(height int64, now time.Time) error {
if c.trustedHeader.Height >= height {
return errors.Errorf("height #%d is already trusted (last: #%d)", height, c.trustedHeader.Height)
}
// Request the header and the vals.
newHeader, newVals, err := c.fetchHeaderAndValsAtHeight(height)
if err != nil {
return err
}
return c.VerifyHeader(newHeader, newVals, now)
}
// VerifyHeader verifies new header against the trusted state.
//
// SequentialVerification: verifies that 2/3 of the trusted validator set has
// signed the new header. If the headers are not adjacent, **all** intermediate
// headers will be requested.
//
// SkippingVerification(trustLevel): verifies that {trustLevel} of the trusted
// validator set has signed the new header. If it's not the case and the
// headers are not adjacent, bisection is performed and necessary (not all)
// intermediate headers will be requested. See the specification for the
// algorithm.
//
// If the trusted header is more recent than one here, an error is returned.
func (c *Client) VerifyHeader(newHeader *types.SignedHeader, newVals *types.ValidatorSet, now time.Time) error {
if c.trustedHeader.Height >= newHeader.Height {
return errors.Errorf("height #%d is already trusted (last: #%d)", newHeader.Height, c.trustedHeader.Height)
}
if len(c.alternatives) > 0 {
if err := c.compareNewHeaderWithRandomAlternative(newHeader); err != nil {
return err
}
}
var err error
switch c.verificationMode {
case sequential:
err = c.sequence(newHeader, newVals, now)
case skipping:
err = c.bisection(c.trustedHeader, c.trustedNextVals, newHeader, newVals, now)
default:
panic(fmt.Sprintf("Unknown verification mode: %b", c.verificationMode))
}
if err != nil {
return err
}
// Update trusted header and vals.
nextVals, err := c.primary.ValidatorSet(newHeader.Height + 1)
if err != nil {
return err
}
return c.updateTrustedHeaderAndVals(newHeader, nextVals)
}
func (c *Client) sequence(newHeader *types.SignedHeader, newVals *types.ValidatorSet, now time.Time) error {
// 1) Verify any intermediate headers.
var (
interimHeader *types.SignedHeader
nextVals *types.ValidatorSet
err error
)
for height := c.trustedHeader.Height + 1; height < newHeader.Height; height++ {
interimHeader, err = c.primary.SignedHeader(height)
if err != nil {
return errors.Wrapf(err, "failed to obtain the header #%d", height)
}
err = Verify(c.chainID, c.trustedHeader, c.trustedNextVals, interimHeader, c.trustedNextVals,
c.trustingPeriod, now, c.trustLevel)
if err != nil {
return errors.Wrapf(err, "failed to verify the header #%d", height)
}
// Update trusted header and vals.
if height == newHeader.Height-1 {
nextVals = newVals
} else {
nextVals, err = c.primary.ValidatorSet(height + 1)
if err != nil {
return errors.Wrapf(err, "failed to obtain the vals #%d", height+1)
}
}
err = c.updateTrustedHeaderAndVals(interimHeader, nextVals)
if err != nil {
return errors.Wrapf(err, "failed to update trusted state #%d", height)
}
}
// 2) Verify the new header.
return Verify(c.chainID, c.trustedHeader, c.trustedNextVals, newHeader, newVals, c.trustingPeriod, now, c.trustLevel)
}
func (c *Client) bisection(
lastHeader *types.SignedHeader,
lastVals *types.ValidatorSet,
newHeader *types.SignedHeader,
newVals *types.ValidatorSet,
now time.Time) error {
err := Verify(c.chainID, lastHeader, lastVals, newHeader, newVals, c.trustingPeriod, now, c.trustLevel)
switch err.(type) {
case nil:
return nil
case types.ErrTooMuchChange:
// continue bisection
default:
return errors.Wrapf(err, "failed to verify the header #%d ", newHeader.Height)
}
if newHeader.Height == c.trustedHeader.Height+1 {
// TODO: submit evidence here
return errors.Errorf("adjacent headers (#%d and #%d) that are not matching", lastHeader.Height, newHeader.Height)
}
pivot := (c.trustedHeader.Height + newHeader.Header.Height) / 2
pivotHeader, pivotVals, err := c.fetchHeaderAndValsAtHeight(pivot)
if err != nil {
return err
}
// left branch
{
err := c.bisection(lastHeader, lastVals, pivotHeader, pivotVals, now)
if err != nil {
return errors.Wrapf(err, "bisection of #%d and #%d", lastHeader.Height, pivot)
}
}
// right branch
{
nextVals, err := c.primary.ValidatorSet(pivot + 1)
if err != nil {
return errors.Wrapf(err, "failed to obtain the vals #%d", pivot+1)
}
if !bytes.Equal(pivotHeader.NextValidatorsHash, nextVals.Hash()) {
return errors.Errorf("expected next validator's hash %X, but got %X (height #%d)",
pivotHeader.NextValidatorsHash,
nextVals.Hash(),
pivot)
}
err = c.updateTrustedHeaderAndVals(pivotHeader, nextVals)
if err != nil {
return errors.Wrapf(err, "failed to update trusted state #%d", pivot)
}
err = c.bisection(pivotHeader, nextVals, newHeader, newVals, now)
if err != nil {
return errors.Wrapf(err, "bisection of #%d and #%d", pivot, newHeader.Height)
}
}
return nil
}
func (c *Client) updateTrustedHeaderAndVals(h *types.SignedHeader, vals *types.ValidatorSet) error {
if !bytes.Equal(h.NextValidatorsHash, vals.Hash()) {
return errors.Errorf("expected next validator's hash %X, but got %X", h.NextValidatorsHash, vals.Hash())
}
if err := c.trustedStore.SaveSignedHeader(h); err != nil {
return errors.Wrap(err, "failed to save trusted header")
}
if err := c.trustedStore.SaveValidatorSet(vals, h.Height+1); err != nil {
return errors.Wrap(err, "failed to save trusted vals")
}
c.trustedHeader = h
c.trustedNextVals = vals
return nil
}
func (c *Client) fetchHeaderAndValsAtHeight(height int64) (*types.SignedHeader, *types.ValidatorSet, error) {
h, err := c.primary.SignedHeader(height)
if err != nil {
return nil, nil, errors.Wrapf(err, "failed to obtain the header #%d", height)
}
vals, err := c.primary.ValidatorSet(height)
if err != nil {
return nil, nil, errors.Wrapf(err, "failed to obtain the vals #%d", height)
}
return h, vals, nil
}
func (c *Client) compareNewHeaderWithRandomAlternative(h *types.SignedHeader) error {
// 1. Pick an alternative provider.
p := c.alternatives[tmrand.Intn(len(c.alternatives))]
// 2. Fetch the header.
altHeader, err := p.SignedHeader(h.Height)
if err != nil {
return errors.Wrapf(err,
"failed to obtain header #%d from alternative provider %v", h.Height, p)
}
// 3. Compare hashes.
if !bytes.Equal(h.Hash(), altHeader.Hash()) {
// TODO: One of the providers is lying. Send the evidence to fork
// accountability server.
return errors.Errorf(
"new header hash %X does not match one from alternative provider %X",
h.Hash(), altHeader.Hash())
}
return nil
}