# Blockchain

Here we describe the data structures in the Tendermint blockchain and the rules for validating them.

# Data Structures

The Tendermint blockchains consists of a short list of basic data types:

Block
Header
Version
BlockID
Time
Data (for transactions)
Commit and Vote
EvidenceData and Evidence

# Block

A block consists of a header, transactions, votes (the commit), and a list of evidence of malfeasance (ie. signing conflicting votes).

type Block struct {
    Header      Header
    Txs         Data
    Evidence    EvidenceData
    LastCommit  Commit
}

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Note the LastCommit is the set of votes that committed the last block.

A block header contains metadata about the block and about the consensus, as well as commitments to the data in the current block, the previous block, and the results returned by the application:

type Header struct {
	// basic block info
	Version  Version
	ChainID  string
	Height   int64
	Time     Time
	NumTxs   int64
	TotalTxs int64

	// prev block info
	LastBlockID BlockID

	// hashes of block data
	LastCommitHash []byte // commit from validators from the last block
	DataHash       []byte // MerkleRoot of transaction hashes

	// hashes from the app output from the prev block
	ValidatorsHash     []byte // validators for the current block
	NextValidatorsHash []byte // validators for the next block
	ConsensusHash      []byte // consensus params for current block
	AppHash            []byte // state after txs from the previous block
	LastResultsHash    []byte // root hash of all results from the txs from the previous block

	// consensus info
	EvidenceHash    []byte // evidence included in the block
	ProposerAddress []byte // original proposer of the block

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Further details on each of these fields is described below.

# Version

The Version contains the protocol version for the blockchain and the application as two uint64 values:

type Version struct {
    Block   uint64
    App     uint64
}

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# BlockID

The BlockID contains two distinct Merkle roots of the block. The first, used as the block's main hash, is the MerkleRoot of all the fields in the header (ie. MerkleRoot(header). The second, used for secure gossipping of the block during consensus, is the MerkleRoot of the complete serialized block cut into parts (ie. MerkleRoot(MakeParts(block))). The BlockID includes these two hashes, as well as the number of parts (ie. len(MakeParts(block)))

type BlockID struct {
    Hash []byte
    PartsHeader PartSetHeader
}

type PartSetHeader struct {
    Total int32
    Hash []byte
}

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See MerkleRoot for details.

# Time

Tendermint uses the Google.Protobuf.WellKnownTypes.Timestamp format, which uses two integers, one for Seconds and for Nanoseconds.

# Data

Data is just a wrapper for a list of transactions, where transactions are arbitrary byte arrays:

type Data struct {
    Txs [][]byte
}

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# Commit

Commit is a simple wrapper for a list of votes, with one vote for each validator. It also contains the relevant BlockID:

type Commit struct {
    BlockID     BlockID
    Precommits  []Vote
}

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NOTE: this will likely change to reduce the commit size by eliminating redundant information - see issue #1648.

# Vote

A vote is a signed message from a validator for a particular block. The vote includes information about the validator signing it.

type Vote struct {
	Type             byte
	Height           int64
	Round            int
	BlockID          BlockID
	Timestamp        Time
	ValidatorAddress []byte
	ValidatorIndex   int
	Signature        []byte
}

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There are two types of votes: a prevote has vote.Type == 1 and a precommit has vote.Type == 2.

# Signature

Signatures in Tendermint are raw bytes representing the underlying signature.

See the signature spec for more.

# EvidenceData

EvidenceData is a simple wrapper for a list of evidence:

type EvidenceData struct {
    Evidence []Evidence
}

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# Evidence

Evidence in Tendermint is implemented as an interface. This means any evidence is encoded using its Amino prefix. There is currently only a single type, the DuplicateVoteEvidence.

// amino name: "tendermint/DuplicateVoteEvidence"
type DuplicateVoteEvidence struct {
	PubKey PubKey
	VoteA  Vote
	VoteB  Vote
}

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See the pubkey spec for more.

# Validation

Here we describe the validation rules for every element in a block. Blocks which do not satisfy these rules are considered invalid.

We abuse notation by using something that looks like Go, supplemented with English. A statement such as x == y is an assertion - if it fails, the item is invalid.

We refer to certain globally available objects: block is the block under consideration, prevBlock is the block at the previous height, and state keeps track of the validator set, the consensus parameters and other results from the application. At the point when block is the block under consideration, the current version of the state corresponds to the state after executing transactions from the prevBlock. Elements of an object are accessed as expected, ie. block.Header. See the definition of State.

# Header

A Header is valid if its corresponding fields are valid.

# Version

block.Version.Block == state.Version.Block
block.Version.App == state.Version.App

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The block version must match the state version.

# ChainID

len(block.ChainID) < 50

ChainID must be less than 50 bytes.

# Height

block.Header.Height > 0
block.Header.Height == prevBlock.Header.Height + 1

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The height is an incrementing integer. The first block has block.Header.Height == 1.

# Time

block.Header.Timestamp >= prevBlock.Header.Timestamp + state.consensusParams.Block.TimeIotaMs
block.Header.Timestamp == MedianTime(block.LastCommit, state.LastValidators)

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The block timestamp must be monotonic. It must equal the weighted median of the timestamps of the valid votes in the block.LastCommit.

Note: the timestamp of a vote must be greater by at least one millisecond than that of the block being voted on.

The timestamp of the first block must be equal to the genesis time (since there's no votes to compute the median).

if block.Header.Height == 1 {
    block.Header.Timestamp == genesisTime
}

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See the section on BFT time for more details.

# NumTxs

block.Header.NumTxs == len(block.Txs.Txs)

Number of transactions included in the block.

# TotalTxs

block.Header.TotalTxs == prevBlock.Header.TotalTxs + block.Header.NumTxs

The cumulative sum of all transactions included in this blockchain.

The first block has block.Header.TotalTxs = block.Header.NumberTxs.

# LastBlockID

LastBlockID is the previous block's BlockID:

prevBlockParts := MakeParts(prevBlock)
block.Header.LastBlockID == BlockID {
    Hash: MerkleRoot(prevBlock.Header),
    PartsHeader{
        Hash: MerkleRoot(prevBlockParts),
        Total: len(prevBlockParts),
    },
}

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The first block has block.Header.LastBlockID == BlockID{}.

# LastCommitHash

block.Header.LastCommitHash == MerkleRoot(block.LastCommit.Precommits)

MerkleRoot of the votes included in the block. These are the votes that committed the previous block.

The first block has block.Header.LastCommitHash == []byte{}

# DataHash

block.Header.DataHash == MerkleRoot(Hashes(block.Txs.Txs))

MerkleRoot of the hashes of transactions included in the block.

Note the transactions are hashed before being included in the Merkle tree, so the leaves of the Merkle tree are the hashes, not the transactions themselves. This is because transaction hashes are regularly used as identifiers for transactions.

# ValidatorsHash

block.ValidatorsHash == MerkleRoot(state.Validators)

MerkleRoot of the current validator set that is committing the block. This can be used to validate the LastCommit included in the next block. Note the validators are sorted by their address before computing the MerkleRoot.

# NextValidatorsHash

block.NextValidatorsHash == MerkleRoot(state.NextValidators)

MerkleRoot of the next validator set that will be the validator set that commits the next block. This is included so that the current validator set gets a chance to sign the next validator sets Merkle root. Note the validators are sorted by their address before computing the MerkleRoot.

# ConsensusHash

block.ConsensusHash == state.ConsensusParams.Hash()

Hash of the amino-encoding of a subset of the consensus parameters.

# AppHash

block.AppHash == state.AppHash

Arbitrary byte array returned by the application after executing and commiting the previous block. It serves as the basis for validating any merkle proofs that comes from the ABCI application and represents the state of the actual application rather than the state of the blockchain itself.

The first block has block.Header.AppHash == []byte{}.

# LastResultsHash

block.ResultsHash == MerkleRoot(state.LastResults)

MerkleRoot of the results of the transactions in the previous block.

The first block has block.Header.ResultsHash == []byte{}.

# EvidenceHash

block.EvidenceHash == MerkleRoot(block.Evidence)

MerkleRoot of the evidence of Byzantine behaviour included in this block.

# ProposerAddress

block.Header.ProposerAddress in state.Validators

Address of the original proposer of the block. Must be a current validator.

# Txs

Arbitrary length array of arbitrary length byte-arrays.

# LastCommit

The first height is an exception - it requires the LastCommit to be empty:

if block.Header.Height == 1 {
    len(b.LastCommit) == 0
}

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Otherwise, we require:

len(block.LastCommit) == len(state.LastValidators)
talliedVotingPower := 0
for i, vote := range block.LastCommit{
    if vote == nil{
        continue
    }
    vote.Type == 2
    vote.Height == block.LastCommit.Height()
    vote.Round == block.LastCommit.Round()
    vote.BlockID == block.LastBlockID

    val := state.LastValidators[i]
    vote.Verify(block.ChainID, val.PubKey) == true

    talliedVotingPower += val.VotingPower
}

talliedVotingPower > (2/3) * TotalVotingPower(state.LastValidators)

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Includes one (possibly nil) vote for every current validator. Non-nil votes must be Precommits. All votes must be for the same height and round. All votes must be for the previous block. All votes must have a valid signature from the corresponding validator. The sum total of the voting power of the validators that voted must be greater than 2/3 of the total voting power of the complete validator set.

# Vote

A vote is a signed message broadcast in the consensus for a particular block at a particular height and round. When stored in the blockchain or propagated over the network, votes are encoded in Amino. For signing, votes are represented via CanonicalVote and also encoded using amino (protobuf compatible) via Vote.SignBytes which includes the ChainID, and uses a different ordering of the fields.

We define a method Verify that returns true if the signature verifies against the pubkey for the SignBytes using the given ChainID:

func (vote *Vote) Verify(chainID string, pubKey crypto.PubKey) error {
	if !bytes.Equal(pubKey.Address(), vote.ValidatorAddress) {
		return ErrVoteInvalidValidatorAddress
	}

	if !pubKey.VerifyBytes(vote.SignBytes(chainID), vote.Signature) {
		return ErrVoteInvalidSignature
	}
	return nil
}

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where pubKey.Verify performs the appropriate digital signature verification of the pubKey against the given signature and message bytes.

# Evidence

There is currently only one kind of evidence, DuplicateVoteEvidence.

DuplicateVoteEvidence ev is valid if

ev.VoteA and ev.VoteB can be verified with ev.PubKey
ev.VoteA and ev.VoteB have the same Height, Round, Address, Index, Type
ev.VoteA.BlockID != ev.VoteB.BlockID
(block.Height - ev.VoteA.Height) < MAX_EVIDENCE_AGE

# Execution

Once a block is validated, it can be executed against the state.

The state follows this recursive equation:

state(1) = InitialState
state(h+1) <- Execute(state(h), ABCIApp, block(h))

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where InitialState includes the initial consensus parameters and validator set, and ABCIApp is an ABCI application that can return results and changes to the validator set (TODO). Execute is defined as:

Execute(s State, app ABCIApp, block Block) State {
    // Fuction ApplyBlock executes block of transactions against the app and returns the new root hash of the app state,
    // modifications to the validator set and the changes of the consensus parameters.
    AppHash, ValidatorChanges, ConsensusParamChanges := app.ApplyBlock(block)

    return State{
        LastResults: abciResponses.DeliverTxResults,
        AppHash: AppHash,
        LastValidators: state.Validators,
        Validators: state.NextValidators,
        NextValidators: UpdateValidators(state.NextValidators, ValidatorChanges),
        ConsensusParams: UpdateConsensusParams(state.ConsensusParams, ConsensusParamChanges),
    }
}

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