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Draft of Light Client Supervisor for discussion

TODOs

This specification in done in parallel with updates on the verification specification. So some hyperlinks have to be placed to the correct files eventually.

Light Client Sequential Supervisor

The light client implements a read operation of a header from the blockchain, by communicating with full nodes, a so-called primary and several so-called witnesses. As some full nodes may be faulty, this functionality must be implemented in a fault-tolerant way.

In the Tendermint blockchain, the validator set may change with every new block. The staking and unbonding mechanism induces a security model: starting at time Time of the header, more than two-thirds of the next validators of a new block are correct for the duration of TrustedPeriod.

Light Client Verification implements the fault-tolerant read operation designed for this security model. That is, it is safe if the model assumptions are satisfied and makes progress if it communicates to a correct primary.

However, if the security model is violated, faulty peers (that have been validators at some point in the past) may launch attacks on the Tendermint network, and on the light client. These attacks as well as an axiomatization of blocks in general are defined in a document that contains the definitions that are currently in detection.md.

If there is a light client attack (but no successful attack on the network), the safety of the verification step may be violated (as we operate outside its basic assumption). The light client also contains a defense mechanism against light clients attacks, called detection.

Light Client Detection implements a cross check of the result of the verification step. If there is a light client attack, and the light client is connected to a correct peer, the light client as a whole is safe, that is, it will not operate on invalid blocks. However, in this case it cannot successfully read, as inconsistent blocks are in the system. However, in this case the detection performs a distributed computation that results in so-called evidence. Evidence can be used to prove to a correct full node that there has been a light client attack.

Light Client Evidence Accountability is a protocol run on a full node to check whether submitted evidence indeed proves the existence of a light client attack. Further, from the evidence and its own knowledge about the blockchain, the full node computes a set of bonded full nodes (that at some point had more than one third of the voting power) that participated in the attack that will be reported via ABCI to the application.

In this document we specify

Initialization of the Light Client
The interaction of verification and detection

The details of these two protocols are captured in their own documents, as is the accountability protocol.

Another related line is IBC attack detection and submission at the relayer, as well as attack verification at the IBC handler. This will call for yet another spec.

Status

This document is work in progress. In order to develop the specification step-by-step, it assumes certain details of verification and detection that are not specified in the respective current versions yet. This inconsistencies will be addresses over several upcoming PRs.

Part I - Tendermint Blockchain

See verification spec

Part II - Sequential Problem Definition

[LC-SEQ-INIT-LIVE.1]

Upon initialization, the light client gets as input a header of the blockchain, or the genesis file of the blockchain, and eventually stores a header of the blockchain.

[LC-SEQ-LIVE.1]

The light client gets a sequence of heights as inputs. For each input height targetHeight, it eventually stores the header of height targetHeight.

[LC-SEQ-SAFE.1]

The light client never stores a header which is not in the blockchain.

Part III - Light Client as Distributed System

Computational Model

The light client communicates with remote processes only via the verification and the detection protocols. The respective assumptions are given there.

Distributed Problem Statement

Two Kinds of Liveness

In case of light client attacks, the sequential problem statement cannot always be satisfied. The lightclient cannot decide which block is from the chain and which is not. As a result, the light client just creates evidence, submits it, and terminates. For the liveness property, we thus add the possibility that instead of adding a lightblock, we also might terminate in case there is an attack.

[LC-DIST-TERM.1]

The light client either runs forever or it terminates on attack.

Design choices

[LC-DIST-STORE.1]

The light client has a local data structure called LightStore that contains light blocks (that contain a header).

The light store exposes functions to query and update it. They are specified here.

TODO: reference light store invariant [LCV-INV-LS-ROOT.2] once verification is merged

[LC-DIST-SAFE.1]

It is always the case that every header in LightStore was generated by an instance of Tendermint consensus.

[LC-DIST-LIVE.1]

Whenever the light client gets a new height h as input,

and there is no light client attack up to height h, then the lightclient eventually puts the lightblock of height h in the lightstore and wait for another input.
otherwise, that is, if there is a light client attack on height h, then the light client must perform one of the following:
- it terminates on attack.
- it eventually puts the lightblock of height h in the lightstore and wait for another input.

Observe that the "existence of a lightclient attack" just means that some node has generated a conflicting block. It does not necessarily mean that a (faulty) peer sends such a block to "our" lightclient. Thus, even if there is an attack somewhere in the system, our lightclient might still continue to operate normally.

Solving the sequential specification

[LC-DIST-SAFE.1] is guaranteed by the detector; in particular it follows from [LCD-DIST-INV-STORE.1] [LCD-DIST-LIVE.1]

Part IV - Light Client Supervisor Protocol

We provide a specification for a sequential Light Client Supervisor. The local code for verification is presented by a sequential function Sequential-Supervisor to highlight the control flow of this functionality. Each lightblock is first verified with a primary, and then cross-checked with secondaries, and if all goes well, the lightblock is added (with the attribute "trusted") to the lightstore. Intermiate lightblocks that were used to verify the target block but were not cross-checked are stored as "verified"

We note that if a different concurrency model is considered for an implementation, the semantics of the lightstore might change: In a concurrent implementation, we might do verification for some height h, add the lightblock to the lightstore, and start concurrent threads that

do verification for the next height h' != h

do cross-checking for height h. If we find an attack, we remove h from the lightstore.

the user might already start to use h

Thus, this concurrency model changes the semantics of the lightstore (not all lightblocks that are read by the user are trusted; they may be removed if we find a problem). Whether this is desirable, and whether the gain in performance is worth it, we keep for future versions/discussion of lightclient protocols.

Definitions

Peers

[LC-DATA-PEERS.1]:

A fixed set of full nodes is provided in the configuration upon initialization. Initially this set is partitioned into

one full node that is the primary (singleton set),
a set Secondaries (of fixed size, e.g., 3),
a set FullNodes; it excludes primary and Secondaries nodes.
A set FaultyNodes of nodes that the light client suspects of being faulty; it is initially empty

[LC-INV-NODES.1]:

The detector shall maintain the following invariants:

FullNodes \intersect Secondaries = {}
FullNodes \intersect FaultyNodes = {}
Secondaries \intersect FaultyNodes = {}

and the following transition invariant

FullNodes' \union Secondaries' \union FaultyNodes' = FullNodes \union Secondaries \union FaultyNodes

[LC-FUNC-REPLACE-PRIMARY.1]:

Replace_Primary(root-of-trust LightBlock)

Implementation remark
- the primary is replaced by a secondary
- to maintain a constant size of secondaries, need to
  - pick a new secondary nsec while ensuring [LC-INV-ROOT-AGREED.1]
  - that is, we need to ensure that root-of-trust = FetchLightBlock(nsec, root-of-trust.Header.Height)
Expected precondition
- FullNodes is nonempty
Expected postcondition
- primary is moved to FaultyNodes
- a secondary s is moved from Secondaries to primary
Error condition
- if precondition is violated

[LC-FUNC-REPLACE-SECONDARY.1]:

Replace_Secondary(addr Address, root-of-trust LightBlock)

Implementation remark
- maintain [LC-INV-ROOT-AGREED.1], that is, ensure root-of-trust = FetchLightBlock(nsec, root-of-trust.Header.Height)
Expected precondition
- FullNodes is nonempty
Expected postcondition
- addr is moved from Secondaries to FaultyNodes
- an address nsec is moved from FullNodes to Secondaries
Error condition
- if precondition is violated

Data Types

The core data structure of the protocol is the LightBlock.

[LC-DATA-LIGHTBLOCK.1]

type LightBlock struct {
                Header          Header
                Commit          Commit
                Validators      ValidatorSet
                NextValidators  ValidatorSet
                Provider        PeerID
}

[LC-DATA-LIGHTSTORE.1]

LightBlocks are stored in a structure which stores all LightBlock from initialization or received from peers.

type LightStore struct {
        ...
}

We use the functions that the LightStore exposes, which are defined in the verification specification.

Inputs

The lightclient is initialized with LCInitData

[LC-DATA-INIT.1]

type LCInitData struct {
    lightBlock     LightBlock
    genesisDoc     GenesisDoc
}

where only one of the components must be provided. GenesisDoc is defined in the Tendermint Types.

[LC-DATA-GENESIS.1]

type GenesisDoc struct {
    GenesisTime     time.Time                `json:"genesis_time"`
    ChainID         string                   `json:"chain_id"`
    InitialHeight   int64                    `json:"initial_height"`
    ConsensusParams *tmproto.ConsensusParams `json:"consensus_params,omitempty"`
    Validators      []GenesisValidator       `json:"validators,omitempty"`
    AppHash         tmbytes.HexBytes         `json:"app_hash"`
    AppState        json.RawMessage          `json:"app_state,omitempty"`
}

We use the following function makeblock so that we create a lightblock from the genesis file in order to do verification based on the data from the genesis file using the same verification function we use in normal operation.

[LC-FUNC-MAKEBLOCK.1]

func makeblock (genesisDoc GenesisDoc) (lightBlock LightBlock))

Implementation remark
- none
Expected precondition
- none
Expected postcondition
- lightBlock.Header.Height = genesisDoc.InitialHeight
- lightBlock.Header.Time = genesisDoc.GenesisTime
- lightBlock.Header.LastBlockID = nil
- lightBlock.Header.LastCommit = nil
- lightBlock.Header.Validators = genesisDoc.Validators
- lightBlock.Header.NextValidators = genesisDoc.Validators
- lightBlock.Header.Data = nil
- lightBlock.Header.AppState = genesisDoc.AppState
- lightBlock.Header.LastResult = nil
- lightBlock.Commit = nil
- lightBlock.Validators = genesisDoc.Validators
- lightBlock.NextValidators = genesisDoc.Validators
- lightBlock.Provider = nil
Error condition
- none

Configuration Parameters

[LC-INV-ROOT-AGREED.1]

In the Sequential-Supervisor, it is always the case that the primary and all secondaries agree on lightStore.Latest().

Assumptions

We have to assume that the initialization data (the lightblock or the genesis file) are consistent with the blockchain. This is subjective initialization and it cannot be checked locally.

Invariants

[LC-INV-PEERLIST.1]:

The peer list contains a primary and a secondary.

If the invariant is violated, the light client does not have enough peers to download headers from. As a result, the light client needs to terminate in case this invariant is violated.

Supervisor

Outline

The supervisor implements the functionality of the lightclient. It is initialized with a genesis file or with a lightblock the user trusts. This initialization is subjective, that is, the security of the lightclient is based on the validity of the input. If the genesis file or the lightblock deviate from the actual ones on the blockchain, the lightclient provides no guarantees.

After initialization, the supervisor awaits an input, that is, the height of the next lightblock that should be obtained. Then it downloads, verifies, and cross-checks a lightblock, and if all tests go through, the light block (and possibly other lightblocks) are added to the lightstore, which is returned in an output event to the user.

The following main loop does the interaction with the user (input, output) and calls the following two functions:

InitLightClient: it initializes the lightstore either with the provided lightblock or with the lightblock that corresponds to the first block generated by the blockchain (by the validators defined by the genesis file)
VerifyAndDetect: takes as input a lightstore and a height and returns the updated lightstore.

[LC-FUNC-SUPERVISOR.1]:

func Sequential-Supervisor (initdata LCInitData) (Error) {

    lightStore,result := InitLightClient(initData);
    if result != OK {
        return result;
    }

    loop {
        // get the next height
        nextHeight := input();
  
        lightStore,result := VerifyAndDetect(lightStore, nextHeight);
  
        if result == OK {
            output(LightStore.Get(targetHeight));
   // we only output a trusted lightblock
        }
        else {
            return result
        }
        // QUESTION: is it OK to generate output event in normal case,
        // and terminate with failure in the (light client) attack case?
    }
}

Implementation remark
- infinite loop unless a light client attack is detected
- In typical implementations (e.g., the one in Rust), there are mutliple input actions: VerifytoLatest, LatestTrusted, and GetStatus. The information can be easily obtained from the lightstore, so that we do not treat these requests explicitly here but just consider the request for a block of a given height which requires more involved computation and communication.
Expected precondition
- LCInitData contains a genesis file or a lightblock.
Expected postcondition
- if a light client attack is detected: it stops and submits evidence (in InitLightClient or VerifyAndDetect)
- otherwise: non. It runs forever.
Invariant: lightStore contains trusted lightblocks only.
Error condition
- if InitLightClient or VerifyAndDetect fails (if a attack is detected, or if [LCV-INV-TP.1] is violated)

Details of the Functions

Initialization

The light client is based on subjective initialization. It has to trust the initial data given to it by the user. It cannot do any detection of attack. So either upon initialization we obtain a lightblock and just initialize the lightstore with it. Or in case of a genesis file, we download, verify, and cross-check the first block, to initialize the lightstore with this first block. The reason is that we want to maintain [LCV-INV-TP.1] from the beginning.

If the lightclient is initialized with a lightblock, one might think it may increase trust, when one cross-checks the initial light block. However, if a peer provides a conflicting lightblock, the question is to distinguish the case of a bogus block (upon which operation should proceed) from a light client attack (upon which operation should stop). In case of a bogus block, the lightclient might be forced to do backwards verification until the blocks are out of the trusting period, to make sure no previous validator set could have generated the bogus block, which effectively opens up a DoS attack on the lightclient without adding effective robustness.

[LC-FUNC-INIT.1]:

func InitLightClient (initData LCInitData) (LightStore, Error) {

    if LCInitData.LightBlock != nil {
        // we trust the provided initial block.
        newblock := LCInitData.LightBlock
    }
    else {
        genesisBlock := makeblock(initData.genesisDoc);

        result := NoResult;
        while result != ResultSuccess {
            current = FetchLightBlock(PeerList.primary(), genesisBlock.Header.Height + 1)
            // QUESTION: is the height with "+1" OK?

            if CANNOT_VERIFY = ValidAndVerify(genesisBlock, current) {
                Replace_Primary();
            }
            else {
                result = ResultSuccess
            }
        }
  
        // cross-check
  auxLS := new LightStore
  auxLS.Add(current)
        Evidences := AttackDetector(genesisBlock, auxLS)
        if Evidences.Empty {
            newBlock := current
        }
        else {
            // [LC-SUMBIT-EVIDENCE.1]
            submitEvidence(Evidences);
            return(nil, ErrorAttack);
        }
    }

    lightStore := new LightStore;
    lightStore.Add(newBlock);
    return (lightStore, OK);
}

Implementation remark
- none
Expected precondition
- LCInitData contains either a genesis file of a lightblock
- if genesis it passes ValidateAndComplete() see Tendermint
Expected postcondition
- lightStore initialized with trusted lightblock. It has either been cross-checked (from genesis) or it has initial trust from the user.
Error condition
- if precondition is violated
- empty peerList

Main verification and detection logic

[LC-FUNC-MAIN-VERIF-DETECT.1]:

func VerifyAndDetect (lightStore LightStore, targetHeight Height)
                     (LightStore, Result) {

    b1, r1 = lightStore.Get(targetHeight)
    if r1 == true {
        if b1.State == StateTrusted {
            // block already there and trusted
            return (lightStore, ResultSuccess)
  }
  else {
            // We have a lightblock in the store, but it has not been 
            // cross-checked by now. We do that now.
            root_of_trust, auxLS := lightstore.TraceTo(b1);
   
            // Cross-check
            Evidences := AttackDetector(root_of_trust, auxLS);
            if Evidences.Empty {
                // no attack detected, we trust the new lightblock
                lightStore.Update(auxLS.Latest(), 
                                  StateTrusted, 
                                  verfiedLS.Latest().verification-root);
                return (lightStore, OK);
            }
            else {
                // there is an attack, we exit
  submitEvidence(Evidences);
                return(lightStore, ErrorAttack);
            }
        }
    }

    // get the lightblock with maximum height smaller than targetHeight
    // would typically be the heighest, if we always move forward
    root_of_trust, r2 = lightStore.LatestPrevious(targetHeight);

    if r2 = false {
        // there is no lightblock from which we can do forward
        // (skipping) verification. Thus we have to go backwards.
        // No cross-check needed. We trust hashes. Therefore, we
        // directly return the result
        return Backwards(primary, lightStore.Lowest(), targetHeight)
    }
    else {
        // Forward verification + detection
        result := NoResult;
        while result != ResultSuccess {
            verifiedLS,result := VerifyToTarget(primary,
                                                root_of_trust,
                                                nextHeight);
            if result == ResultFailure {
                // pick new primary (promote a secondary to primary)
                Replace_Primary(root_of_trust);
            }
            else if result == ResultExpired {
                return (lightStore, result)
            }
        }

        // Cross-check
        Evidences := AttackDetector(root_of_trust, verifiedLS);
        if Evidences.Empty {
            // no attack detected, we trust the new lightblock
            verifiedLS.Update(verfiedLS.Latest(), 
                              StateTrusted, 
                              verfiedLS.Latest().verification-root);
            lightStore.store_chain(verifidLS);
            return (lightStore, OK);
        }
        else {
            // there is an attack, we exit
            return(lightStore, ErrorAttack);
        }
    }
}

Implementation remark
- none
Expected precondition
- none
Expected postcondition
- lightblock of height targetHeight (and possibly additional blocks) added to lightStore
Error condition
- an attack is detected
- [LC-DATA-PEERLIST-INV.1] is violated

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