# Tendermint Encoding

## Amino

Tendermint uses the Protobuf3 derivative [Amino](https://github.com/tendermint/go-amino) for all data structures.
Think of Amino as an object-oriented Protobuf3 with native JSON support.
The goal of the Amino encoding protocol is to bring parity between application
logic objects and persistence objects.

Please see the [Amino
specification](https://github.com/tendermint/go-amino#amino-encoding-for-go) for
more details.

Notably, every object that satisfies an interface (eg. a particular kind of p2p message,
or a particular kind of pubkey) is registered with a global name, the hash of
which is included in the object's encoding as the so-called "prefix bytes".

We define the `func AminoEncode(obj interface{}) []byte` function to take an
arbitrary object and return the Amino encoded bytes.

## Byte Arrays

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

For details on varints, see the [protobuf
spec](https://developers.google.com/protocol-buffers/docs/encoding#varints).

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.

## Public Key Cryptography

Tendermint uses Amino to distinguish between different types of private keys,
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, as they can be
derrived the same way as the others using Amino.

All registered objects are encoded by Amino using a 4-byte PrefixBytes that
uniquely identifies the object and includes information about its underlying
type. For details on how PrefixBytes are computed, see the [Amino
spec](https://github.com/tendermint/go-amino#computing-the-prefix-and-disambiguation-bytes).

In what follows, we provide the type names and prefix bytes directly.
Notice that when encoding byte-arrays, the length of the byte-array is appended
to the PrefixBytes. Thus the encoding of a byte array becomes `<PrefixBytes>
<Length> <ByteArray>`

NOTE: the remainder of this section on Public Key Cryptography can be generated
from [this script](https://github.com/tendermint/tendermint/blob/master/docs/spec/scripts/crypto.go)

### PubKeyEd25519

```
// Name: tendermint/PubKeyEd25519
// PrefixBytes: 0x1624DE62
// Length: 0x20
// Notes: raw 32-byte Ed25519 pubkey
type PubKeyEd25519 [32]byte

func (pubkey PubKeyEd25519) Address() []byte {
      // NOTE: hash of the Amino encoded bytes!
        return RIPEMD160(AminoEncode(pubkey))
}
```

For example, the 32-byte Ed25519 pubkey
`CCACD52F9B29D04393F01CD9AF6535455668115641F3D8BAEFD2295F24BAF60E` would be
encoded as
`1624DE6220CCACD52F9B29D04393F01CD9AF6535455668115641F3D8BAEFD2295F24BAF60E`.

The address would then be
`RIPEMD160(0x1624DE6220CCACD52F9B29D04393F01CD9AF6535455668115641F3D8BAEFD2295F24BAF60E)`
or `430FF75BAF1EC4B0D51BB3EEC2955479D0071605`

### SignatureEd25519

```
// Name: tendermint/SignatureKeyEd25519
// PrefixBytes: 0x3DA1DB2A
// Length: 0x40
// Notes: raw 64-byte Ed25519 signature
type SignatureEd25519 [64]byte
```

For example, the 64-byte Ed25519 signature
`1B6034A8ED149D3C94FDA13EC03B26CC0FB264D9B0E47D3FA3DEF9FCDE658E49C80B35F9BE74949356401B15B18FB817D6E54495AD1C4A8401B248466CB0DB0B`
would be encoded as
`3DA1DB2A401B6034A8ED149D3C94FDA13EC03B26CC0FB264D9B0E47D3FA3DEF9FCDE658E49C80B35F9BE74949356401B15B18FB817D6E54495AD1C4A8401B248466CB0DB0B`

### PrivKeyEd25519

```
// Name: tendermint/PrivKeyEd25519
// Notes: raw 32-byte priv key concatenated to raw 32-byte pub key
type PrivKeyEd25519 [64]byte
```

### PubKeySecp256k1

```
// Name: tendermint/PubKeySecp256k1
// PrefixBytes: 0xEB5AE982
// Length: 0x21
// Notes: OpenSSL compressed pubkey prefixed with 0x02 or 0x03
type PubKeySecp256k1 [33]byte

func (pubkey PubKeySecp256k1) Address() []byte {
      // NOTE: hash of the raw pubkey bytes (not Amino encoded!).
        // Compatible with Bitcoin addresses.
          return RIPEMD160(SHA256(pubkey[:]))
}
```

For example, the 33-byte Secp256k1 pubkey
`020BD40F225A57ED383B440CF073BC5539D0341F5767D2BF2D78406D00475A2EE9` would be
encoded as
`EB5AE98221020BD40F225A57ED383B440CF073BC5539D0341F5767D2BF2D78406D00475A2EE9`

The address would then be
`RIPEMD160(SHA256(0x020BD40F225A57ED383B440CF073BC5539D0341F5767D2BF2D78406D00475A2EE9))`
or `0AE5BEE929ABE51BAD345DB925EEA652680783FC`

### SignatureSecp256k1

```
// Name: tendermint/SignatureKeySecp256k1
// PrefixBytes: 0x16E1FEEA
// Length: Variable
// Encoding prefix: Variable
// Notes: raw bytes of the Secp256k1 signature
type SignatureSecp256k1 []byte
```

For example, the Secp256k1 signature
`304402201CD4B8C764D2FD8AF23ECFE6666CA8A53886D47754D951295D2D311E1FEA33BF02201E0F906BB1CF2C30EAACFFB032A7129358AFF96B9F79B06ACFFB18AC90C2ADD7`
would be encoded as
`16E1FEEA46304402201CD4B8C764D2FD8AF23ECFE6666CA8A53886D47754D951295D2D311E1FEA33BF02201E0F906BB1CF2C30EAACFFB032A7129358AFF96B9F79B06ACFFB18AC90C2ADD7`

### PrivKeySecp256k1

```
// Name: tendermint/PrivKeySecp256k1
// Notes: raw 32-byte priv key
type PrivKeySecp256k1 [32]byte
```

## Other Common Types

### BitArray

The BitArray is used in block headers and some consensus messages to signal
whether or not something was done by each validator. BitArray is represented
with a struct containing the number of bits (`Bits`) and the bit-array itself
encoded in base64 (`Elems`).

```go
type BitArray struct {
    Bits  int
    Elems []uint64
}
```

This type is easily encoded directly by Amino.

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 in the larger set (`Index`), the actual
underlying data of the part (`Bytes`), and a simple Merkle proof that the part is contained in
the larger set (`Proof`).

```go
type Part struct {
    Index int
    Bytes byte[]
    Proof byte[]
}
```

### MakeParts

Encode an object using Amino and slice it into parts.

```go
func MakeParts(obj interface{}, partSize int) []Part
```

## Merkle Trees

Simple Merkle trees are used in numerous places in Tendermint to compute a cryptographic digest of a data structure.

RIPEMD160 is always used as the hashing function.

### Simple Merkle Root

The function `SimpleMerkleRoot` is a simple recursive function defined as follows:

```go
func SimpleMerkleRoot(hashes [][]byte) []byte{
    switch len(hashes) {
    case 0:
        return nil
    case 1:
        return hashes[0]
    default:
        left := SimpleMerkleRoot(hashes[:(len(hashes)+1)/2])
        right := SimpleMerkleRoot(hashes[(len(hashes)+1)/2:])
        return SimpleConcatHash(left, right)
    }
}

func SimpleConcatHash(left, right []byte) []byte{
    left = encodeByteSlice(left)
    right = encodeByteSlice(right)
    return RIPEMD160 (append(left, right))
}
```

Note that the leaves are Amino encoded as byte-arrays (ie. simple Uvarint length
prefix) before being concatenated together and hashed.

Note: we will abuse notion and invoke `SimpleMerkleRoot` with arguments of type `struct` or type `[]struct`.
For `struct` arguments, we compute a `[][]byte` by sorting elements of the `struct` according to
field name and then hashing them.
For `[]struct` arguments, we compute a `[][]byte` by hashing the individual `struct` elements.

### Simple Merkle Proof

Proof that a leaf is in a Merkle tree consists of a simple structure:


```
type SimpleProof struct {
        Aunts [][]byte
}
```

Which is verified using the following:

```
func (proof SimpleProof) Verify(index, total int, leafHash, rootHash []byte) bool {
	computedHash := computeHashFromAunts(index, total, 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 := (total + 1) / 2
	if index < numLeft {
		leftHash := computeHashFromAunts(index, numLeft, leafHash, innerHashes[:len(innerHashes)-1])
		assert(leftHash != nil)
		return SimpleHashFromTwoHashes(leftHash, innerHashes[len(innerHashes)-1])
	}
	rightHash := computeHashFromAunts(index-numLeft, total-numLeft, leafHash, innerHashes[:len(innerHashes)-1])
	assert(rightHash != nil)
	return SimpleHashFromTwoHashes(innerHashes[len(innerHashes)-1], rightHash)
}
```

## JSON

### Amino

TODO: improve this

Amino also supports JSON encoding - registered types are simply encoded as:

```
{
  "type": "<DisfixBytes>",
  "value": <JSON>
}
```

For instance, an ED25519 PubKey would look like:

```
{
  "type": "AC26791624DE60",
  "value": "uZ4h63OFWuQ36ZZ4Bd6NF+/w9fWUwrOncrQsackrsTk="
}
```

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


### Signed Messages

Signed messages (eg. votes, proposals) in the consensus are encoded using Amino-JSON, rather than in the standard binary format.

When signing, the elements of a message are sorted by key and the sorted message is embedded in an
outer JSON that includes a `chain_id` field.
We call this encoding the CanonicalSignBytes. For instance, CanonicalSignBytes for a vote would look
like:

```json
{"chain_id":"my-chain-id","vote":{"block_id":{"hash":DEADBEEF,"parts":{"hash":BEEFDEAD,"total":3}},"height":3,"round":2,"timestamp":1234567890, "type":2}
```

Note how the fields within each level are sorted.
