# Tendermint 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`, `Vote`, `BlockID`, `Signature`, and `Evidence`. ## Block A block consists of a header, a list of transactions, a list of votes (the commit), and a list of evidence if malfeasance (ie. signing conflicting votes). ``` type Block struct { Header Header Txs [][]byte LastCommit []Vote Evidence []Evidence } ``` ## Header 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 { // block metadata Version string // Version string ChainID string // ID of the chain Height int64 // current block height Time int64 // UNIX time, in millisconds // current block NumTxs int64 // Number of txs in this block TxHash []byte // SimpleMerkle of the block.Txs LastCommitHash []byte // SimpleMerkle of the block.LastCommit // previous block TotalTxs int64 // prevBlock.TotalTxs + block.NumTxs LastBlockID BlockID // BlockID of prevBlock // application ResultsHash []byte // SimpleMerkle of []abci.Result from prevBlock AppHash []byte // Arbitrary state digest ValidatorsHash []byte // SimpleMerkle of the ValidatorSet ConsensusParamsHash []byte // SimpleMerkle of the ConsensusParams // consensus Proposer []byte // Address of the block proposer EvidenceHash []byte // SimpleMerkle of []Evidence } ``` Further details on each of this fields is taken up below. ## BlockID The `BlockID` contains two distinct Merkle roots of the block. The first, used as the block's main hash, is the Merkle root of all the fields in the header. The second, used for secure gossipping of the block during consensus, is the Merkle root of the complete serialized block cut into parts. The `BlockID` includes these two hashes, as well as the number of parts. ``` type BlockID struct { Hash []byte Parts PartsHeader } type PartsHeader struct { Hash []byte Total int32 } ``` ## 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 { Timestamp int64 Address []byte Index int Height int64 Round int Type int8 BlockID BlockID Signature Signature } ``` There are two types of votes: a prevote has `vote.Type == 1` and a precommit has `vote.Type == 2`. ## Signature Tendermint allows for multiple signature schemes to be used by prepending a single type-byte to the signature bytes. Different signatures may also come with fixed or variable lengths. Currently, Tendermint supports Ed25519 and Secp256k1. ### ED25519 An ED25519 signature has `Type == 0x1`. It looks like: ``` // Implements Signature type Ed25519Signature struct { Type int8 = 0x1 Signature [64]byte } ``` where `Signature` is the 64 byte signature. ### Secp256k1 A `Secp256k1` signature has `Type == 0x2`. It looks like: ``` // Implements Signature type Secp256k1Signature struct { Type int8 = 0x2 Signature []byte } ``` where `Signature` is the DER encoded signature, ie: ``` 0x30 <0x02> 0x2 . ``` ## Evidence TODO # 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. Elements of an object are accessed as expected, ie. `block.Header`. See [here](state.md) for the definition of `state`. ## Header A Header is valid if its corresponding fields are valid. ### Version Arbitrary string. ### ChainID Arbitrary constant string. ### Height ``` block.Header.Height > 0 block.Header.Height == prevBlock.Header.Height + 1 ``` The height is an incrementing integer. The first block has `block.Header.Height == 1`. ### Time The median of the timestamps of the valid votes in the block.LastCommit. Corresponds to the number of nanoseconds, with millisecond resolution, since January 1, 1970. Note the timestamp in a vote must be greater by at least one millisecond than that of the block being voted on. ### NumTxs ``` block.Header.NumTxs == len(block.Txs) ``` Number of transactions included in the block. ### TxHash ``` block.Header.TxHash == SimpleMerkleRoot(block.Txs) ``` Simple Merkle root of the transactions in the block. ### LastCommitHash ``` block.Header.LastCommitHash == SimpleMerkleRoot(block.LastCommit) ``` Simple Merkle root of the votes included in the block. These are the votes that committed the previous block. The first block has `block.Header.LastCommitHash == []byte{}` ### 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 ``` prevBlockParts := MakeParts(prevBlock, state.LastConsensusParams.BlockGossip.BlockPartSize) block.Header.LastBlockID == BlockID { Hash: SimpleMerkleRoot(prevBlock.Header), PartsHeader{ Hash: SimpleMerkleRoot(prevBlockParts), Total: len(prevBlockParts), }, } ``` Previous block's BlockID. Note it depends on the ConsensusParams, which are held in the `state` and may be updated by the application. The first block has `block.Header.LastBlockID == BlockID{}`. ### ResultsHash ``` block.ResultsHash == SimpleMerkleRoot(state.LastResults) ``` Simple Merkle root of the results of the transactions in the previous block. The first block has `block.Header.ResultsHash == []byte{}`. ### AppHash ``` block.AppHash == state.AppHash ``` Arbitrary byte array returned by the application after executing and commiting the previous block. The first block has `block.Header.AppHash == []byte{}`. ### ValidatorsHash ``` block.ValidatorsHash == SimpleMerkleRoot(state.Validators) ``` Simple Merkle root of the current validator set that is committing the block. This can be used to validate the `LastCommit` included in the next block. May be updated by the application. ### ConsensusParamsHash ``` block.ConsensusParamsHash == SimpleMerkleRoot(state.ConsensusParams) ``` Simple Merkle root of the consensus parameters. May be updated by the application. ### Proposer ``` block.Header.Proposer in state.Validators ``` Original proposer of the block. Must be a current validator. NOTE: this field can only be further verified by real-time participants in the consensus. This is because the same block can be proposed in multiple rounds for the same height and we do not track the initial round the block was proposed. ### EvidenceHash ``` block.EvidenceHash == SimpleMerkleRoot(block.Evidence) ``` Simple Merkle root of the evidence of Byzantine behaviour included in this block. ## 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 } ``` 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) ``` 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 TMBIN. For signing, votes are encoded in JSON, and the ChainID is included, in the form of the `CanonicalSignBytes`. We define a method `Verify` that returns `true` if the signature verifies against the pubkey for the CanonicalSignBytes using the given ChainID: ``` func (v Vote) Verify(chainID string, pubKey PubKey) bool { return pubKey.Verify(v.Signature, CanonicalSignBytes(chainID, v)) } ``` where `pubKey.Verify` performs the approprioate digital signature verification of the `pubKey` against the given signature and message bytes. ## Evidence ``` ``` Every piece of evidence contains two conflicting votes from a single validator that was active at the height indicated in the votes. The votes must not be too old. # Execution Once a block is validated, it can be executed against the state. The state follows the recursive equation: ``` app = NewABCIApp state(1) = InitialState state(h+1) <- Execute(state(h), app, block(h)) ```