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124 lines
4.7 KiB
124 lines
4.7 KiB
/*
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package light provides a light client implementation.
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The concept of light clients was introduced in the Bitcoin white paper. It
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describes a watcher of distributed consensus process that only validates the
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consensus algorithm and not the state machine transactions within.
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Tendermint light clients allow bandwidth & compute-constrained devices, such as
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smartphones, low-power embedded chips, or other blockchains to efficiently
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verify the consensus of a Tendermint blockchain. This forms the basis of safe
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and efficient state synchronization for new network nodes and inter-blockchain
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communication (where a light client of one Tendermint instance runs in another
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chain's state machine).
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In a network that is expected to reliably punish validators for misbehavior by
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slashing bonded stake and where the validator set changes infrequently, clients
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can take advantage of this assumption to safely synchronize a light client
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without downloading the intervening headers.
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Light clients (and full nodes) operating in the Proof Of Stake context need a
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trusted block height from a trusted source that is no older than 1 unbonding
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window plus a configurable evidence submission synchrony bound. This is called
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weak subjectivity.
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Weak subjectivity is required in Proof of Stake blockchains because it is
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costless for an attacker to buy up voting keys that are no longer bonded and
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fork the network at some point in its prior history. See Vitalik's post at
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[Proof of Stake: How I Learned to Love Weak
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Subjectivity](https://blog.ethereum.org/2014/11/25/proof-stake-learned-love-weak-subjectivity/).
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NOTE: Tendermint provides a somewhat different (stronger) light client model
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than Bitcoin under eclipse, since the eclipsing node(s) can only fool the light
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client if they have two-thirds of the private keys from the last root-of-trust.
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# Common structures
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* SignedHeader
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SignedHeader is a block header along with a commit -- enough validator
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precommit-vote signatures to prove its validity (> 2/3 of the voting power)
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given the validator set responsible for signing that header.
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The hash of the next validator set is included and signed in the SignedHeader.
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This lets the light client keep track of arbitrary changes to the validator set,
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as every change to the validator set must be approved by inclusion in the
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header and signed in the commit.
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In the worst case, with every block changing the validators around completely,
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a light client can sync up with every block header to verify each validator set
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change on the chain. In practice, most applications will not have frequent
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drastic updates to the validator set, so the logic defined in this package for
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light client syncing is optimized to use intelligent bisection.
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# What this package provides
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This package provides three major things:
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1. Client implementation (see client.go)
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2. Pure functions to verify a new header (see verifier.go)
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3. Secure RPC proxy
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## 1. Client implementation (see client.go)
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Example usage:
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db, err := dbm.NewGoLevelDB("light-client-db", dbDir)
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if err != nil {
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// handle error
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}
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c, err := NewHTTPClient(
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chainID,
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TrustOptions{
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Period: 504 * time.Hour, // 21 days
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Height: 100,
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Hash: header.Hash(),
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},
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"http://localhost:26657",
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[]string{"http://witness1:26657"},
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dbs.New(db, ""),
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)
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if err != nil {
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// handle error
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}
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h, err := c.TrustedHeader(100)
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if err != nil {
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// handle error
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}
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fmt.Println("header", h)
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Check out other examples in example_test.go
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## 2. Pure functions to verify a new header (see verifier.go)
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Verify function verifies a new header against some trusted header. See
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https://github.com/tendermint/tendermint/blob/master/spec/light-client/verification/README.md
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for details.
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There are two methods of verification: sequential and bisection
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Sequential uses the headers hashes and the validator sets to verify each adjacent header until
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it reaches the target header.
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Bisection finds the middle header between a trusted and new header, reiterating the action until it
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verifies a header. A cache of headers requested by the primary is kept such that when a
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verification is made, and the light client tries again to verify the new header in the middle,
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the light client does not need to ask for all the same headers again.
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refer to docs/imgs/light_client_bisection_alg.png
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## 3. Secure RPC proxy
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Tendermint RPC exposes a lot of info, but a malicious node could return any
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data it wants to queries, or even to block headers, even making up fake
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signatures from non-existent validators to justify it. Secure RPC proxy serves
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as a wrapper, which verifies all the headers, using a light client connected to
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some other node.
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See
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https://github.com/tendermint/tendermint/tree/master/spec/light-client
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for the light client specification.
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*/
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package light
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