# Tendermint Specification

This is a markdown specification of the Tendermint blockchain.
It defines the base data structures used in the blockchain and how they are validated.

XXX: this spec is a work in progress and not yet complete - see github
[isses](https://github.com/tendermint/tendermint/issues) and
[pull requests](https://github.com/tendermint/tendermint/pulls)
for more details.

If you find discrepancies between the spec and the code that
do not have an associated issue or pull request on github,
please submit them to our [bug bounty](https://tendermint.com/security)!

## Contents

- [Overview](#overview)
- [Encoding and Digests](encoding.md)
- [Blockchain](blockchain.md)
- [State](state.md)
- [Consensus](consensus.md)
- [P2P](p2p/node.md)

## Overview

Tendermint provides Byzantine Fault Tolerant State Machine Replication using
hash-linked batches of transactions. Such transaction batches are called "blocks".
Hence Tendermint defines a "blockchain".

Each block in Tendermint has a unique index - its Height.
A block at `Height == H` can only be committed *after* the
block at `Height == H-1`.
Each block is committed by a known set of weighted Validators.
Membership and weighting within this set may change over time.
Tendermint guarantees the safety and liveness of the blockchain
so long as less than 1/3 of the total weight of the Validator set
is malicious.

A commit in Tendermint is a set of signed messages from more than 2/3 of
the total weight of the current Validator set. Validators take turns proposing
blocks and voting on them. Once enough votes are received, the block is considered
committed. These votes are included in the *next* block as proof that the previous block
was committed - they cannot be included in the current block, as that block has already been
created.

Once a block is committed, it can be executed against an application.
The application returns results for each of the transactions in the block.
The application can also return changes to be made to the validator set,
as well as a cryptographic digest of its latest state.

Tendermint is designed to enable efficient verification and authentication
of the latest state of the blockchain. To achieve this, it embeds
cryptographic commitments to certain information in the block "header".
This information includes the contents of the block (eg. the transactions),
the validator set committing the block, as well as the various results returned by the application.
Note, however, that block execution only occurs *after* a block is committed.
Thus, application results can only be included in the *next* block.

Also note that information like the transaction results and the validator set are never
directly included in the block - only their cryptographic digests (Merkle roots) are.
Hence, verification of a block requires a separate data structure to store this information.
We call this the `State`. Block verification also requires access to the previous block.

## TODO

- Light Client
- P2P
- Reactor protocols (consensus, mempool, blockchain, pex)