tendermint/spec/core/encoding.md

Encoding

Protocol Buffers

Tendermint uses Protocol Buffers, specifically proto3, for all data structures.

Please see the Proto3 language guide for more details.

Byte Arrays

The encoding of a byte array is simply the raw-bytes prefixed with the length of the array as a UVarint (what proto calls a Varint).

For details on varints, see the protobuf spec.

For example, the byte-array [0xA, 0xB] would be encoded as 0x020A0B, while a byte-array containing 300 entires beginning with [0xA, 0xB, ...] would be encoded as 0xAC020A0B... where 0xAC02 is the UVarint encoding of 300.

Hashing

Tendermint uses SHA256 as its hash function. Objects are always serialized before being hashed. So SHA256(obj) is short for SHA256(ProtoEncoding(obj)).

Public Key Cryptography

Tendermint uses Protobuf Oneof to distinguish between different types public keys, and signatures. Additionally, for each public key, Tendermint defines an Address function that can be used as a more compact identifier in place of the public key. Here we list the concrete types, their names, and prefix bytes for public keys and signatures, as well as the address schemes for each PubKey. Note for brevity we don't include details of the private keys beyond their type and name.

Key Types

Each type specifies it's own pubkey, address, and signature format.

Ed25519

The address is the first 20-bytes of the SHA256 hash of the raw 32-byte public key:

address = SHA256(pubkey)[:20]

The signature is the raw 64-byte ED25519 signature.

Tendermint adopted zip215 for verification of ed25519 signatures.

Note: This change will be released in the next major release of Tendermint-Go (0.35).

Secp256k1

The address is the first 20-bytes of the SHA256 hash of the raw 32-byte public key:

address = SHA256(pubkey)[:20]

Other Common Types

BitArray

The BitArray is used in some consensus messages to represent votes received from validators, or parts received in a block. It is represented with a struct containing the number of bits (Bits) and the bit-array itself encoded in base64 (Elems).

Name	Type
bits	int64
elems	slice of int64 ([]int64)

Note BitArray receives a special JSON encoding in the form of x and _ representing 1 and 0. Ie. the BitArray 10110 would be JSON encoded as "x_xx_"

Part

Part is used to break up blocks into pieces that can be gossiped in parallel and securely verified using a Merkle tree of the parts.

Part contains the index of the part (Index), the actual underlying data of the part (Bytes), and a Merkle proof that the part is contained in the set (Proof).

Name	Type
index	uint32
bytes	slice of bytes ([]byte)
proof	proof

See details of SimpleProof, below.

MakeParts

Encode an object using Protobuf and slice it into parts. Tendermint uses a part size of 65536 bytes, and allows a maximum of 1601 parts (see types.MaxBlockPartsCount). This corresponds to the hard-coded block size limit of 100MB.

func MakeParts(block Block) []Part

Merkle Trees

For an overview of Merkle trees, see wikipedia

We use the RFC 6962 specification of a merkle tree, with sha256 as the hash function. Merkle trees are used throughout Tendermint to compute a cryptographic digest of a data structure. The differences between RFC 6962 and the simplest form a merkle tree are that:

1. leaf nodes and inner nodes have different hashes. This is for "second pre-image resistance", to prevent the proof to an inner node being valid as the proof of a leaf. The leaf nodes are SHA256(0x00 || leaf_data), and inner nodes are SHA256(0x01 || left_hash || right_hash).

2. When the number of items isn't a power of two, the left half of the tree is as big as it could be. (The largest power of two less than the number of items) This allows new leaves to be added with less recomputation. For example:

   Simple Tree with 6 items           Simple Tree with 7 items

              *                                  *
             / \                                / \
           /     \                            /     \
         /         \                        /         \
       /             \                    /             \
      *               *                  *               *
     / \             / \                / \             / \
    /   \           /   \              /   \           /   \
   /     \         /     \            /     \         /     \
  *       *       h4     h5          *       *       *       h6
 / \     / \                        / \     / \     / \
h0  h1  h2 h3                      h0  h1  h2  h3  h4  h5

MerkleRoot

The function MerkleRoot is a simple recursive function defined as follows:

// SHA256([]byte{})
func emptyHash() []byte {
    return tmhash.Sum([]byte{})
}

// SHA256(0x00 || leaf)
func leafHash(leaf []byte) []byte {
 return tmhash.Sum(append(0x00, leaf...))
}

// SHA256(0x01 || left || right)
func innerHash(left []byte, right []byte) []byte {
 return tmhash.Sum(append(0x01, append(left, right...)...))
}

// largest power of 2 less than k
func getSplitPoint(k int) { ... }

func MerkleRoot(items [][]byte) []byte{
 switch len(items) {
 case 0:
  return empthHash()
 case 1:
  return leafHash(items[0])
 default:
  k := getSplitPoint(len(items))
  left := MerkleRoot(items[:k])
  right := MerkleRoot(items[k:])
  return innerHash(left, right)
 }
}

Note: MerkleRoot operates on items which are arbitrary byte arrays, not necessarily hashes. For items which need to be hashed first, we introduce the Hashes function:

func Hashes(items [][]byte) [][]byte {
    return SHA256 of each item
}

Note: we will abuse notion and invoke MerkleRoot with arguments of type struct or type []struct. For struct arguments, we compute a [][]byte containing the protobuf encoding of each field in the struct, in the same order the fields appear in the struct. For []struct arguments, we compute a [][]byte by protobuf encoding the individual struct elements.

Merkle Proof

Proof that a leaf is in a Merkle tree is composed as follows:

Name	Type
total	int64
index	int64
leafHash	slice of bytes ([]byte)
aunts	Matrix of bytes ([][]byte)

Which is verified as follows:

func (proof Proof) Verify(rootHash []byte, leaf []byte) bool {
 assert(proof.LeafHash, leafHash(leaf)

 computedHash := computeHashFromAunts(proof.Index, proof.Total, proof.LeafHash, proof.Aunts)
    return computedHash == rootHash
}

func computeHashFromAunts(index, total int, leafHash []byte, innerHashes [][]byte) []byte{
 assert(index < total && index >= 0 && total > 0)

 if total == 1{
  assert(len(proof.Aunts) == 0)
  return leafHash
 }

 assert(len(innerHashes) > 0)

 numLeft := getSplitPoint(total) // largest power of 2 less than total
 if index < numLeft {
  leftHash := computeHashFromAunts(index, numLeft, leafHash, innerHashes[:len(innerHashes)-1])
  assert(leftHash != nil)
  return innerHash(leftHash, innerHashes[len(innerHashes)-1])
 }
 rightHash := computeHashFromAunts(index-numLeft, total-numLeft, leafHash, innerHashes[:len(innerHashes)-1])
 assert(rightHash != nil)
 return innerHash(innerHashes[len(innerHashes)-1], rightHash)
}

The number of aunts is limited to 100 (MaxAunts) to protect the node against DOS attacks. This limits the tree size to 2^100 leaves, which should be sufficient for any conceivable purpose.

IAVL+ Tree

Because Tendermint only uses a Simple Merkle Tree, application developers are expect to use their own Merkle tree in their applications. For example, the IAVL+ Tree - an immutable self-balancing binary tree for persisting application state is used by the Cosmos SDK

JSON

Tendermint has its own JSON encoding in order to keep backwards compatibility with the previous RPC layer.

Registered types are encoded as:

{
  "type": "<type name>",
  "value": <JSON>
}

For instance, an ED25519 PubKey would look like:

{
  "type": "tendermint/PubKeyEd25519",
  "value": "uZ4h63OFWuQ36ZZ4Bd6NF+/w9fWUwrOncrQsackrsTk="
}

Where the "value" is the base64 encoding of the raw pubkey bytes, and the "type" is the type name for Ed25519 pubkeys.

Signed Messages

Signed messages (eg. votes, proposals) in the consensus are encoded using protobuf.

When signing, the elements of a message are re-ordered so the fixed-length fields are first, making it easy to quickly check the type, height, and round. The ChainID is also appended to the end. We call this encoding the SignBytes. For instance, SignBytes for a vote is the protobuf encoding of the following struct:

message CanonicalVote {
  SignedMsgType             type      = 1;  
  sfixed64                  height    = 2;  // canonicalization requires fixed size encoding here
  sfixed64                  round     = 3;  // canonicalization requires fixed size encoding here
  CanonicalBlockID          block_id  = 4;
  google.protobuf.Timestamp timestamp = 5;
  string                    chain_id  = 6;
}

The field ordering and the fixed sized encoding for the first three fields is optimized to ease parsing of SignBytes in HSMs. It creates fixed offsets for relevant fields that need to be read in this context.

Note: All canonical messages are length prefixed.

For more details, see the signing spec. Also, see the motivating discussion in #1622.