2019-02-27 10:54:23 +00:00
# [WIP] SimpleSerialiZe (SSZ)
This is a **work in progress** describing typing, serialisation and Merkleisation of Ethereum 2.0 objects.
## Table of contents
- [Constants ](#constants )
- [Types ](#types )
- [Primitive types ](#primitive-types )
- [Composite types ](#composite-types )
- [Notation ](#notation )
- [Aliases ](#aliases )
- [Serialization ](#serialization )
- [`uintN` ](#uintn )
- [`bool` ](#bool )
- [Containers ](#containers )
- [Tuples ](#tuples )
- [Lists ](#lists )
- [Deserialization ](#deserialization )
- [Merkleization ](#merkleization )
- [Signed containers ](#signed-containers )
- [Implementations ](#implementations )
2018-09-23 23:52:38 +00:00
## Constants
2019-02-27 10:54:23 +00:00
| Name | Value | Definition |
|-|:-:|-|
| `LENGTH_BYTES` | 4 | Number of bytes used for the length added before a variable-length serialized object. |
2018-09-23 23:52:38 +00:00
2019-02-27 10:54:23 +00:00
## Types
2018-09-23 23:52:38 +00:00
2019-02-27 10:54:23 +00:00
### Primitive types
2018-09-23 23:52:38 +00:00
2019-02-27 10:54:23 +00:00
* `uintN` : `N` -bit unsigned integer (where `N in [8, 16, 32, 64, 128, 256]` )
* `bool` : 1-bit unsigned integer
2018-11-27 07:45:04 +00:00
2019-02-27 10:54:23 +00:00
### Composite types
2018-11-27 07:45:04 +00:00
2019-02-27 10:54:23 +00:00
* **Containes**: ordered heterogenous collection of values
* **Tuple**: ordered fixed-size homogeneous collection of values
* **List**: ordered variable-size homogenous collection of values
2018-09-23 23:52:38 +00:00
2019-02-27 10:54:23 +00:00
### Notation
2018-09-23 23:52:38 +00:00
2019-02-27 10:54:23 +00:00
* **Containes**: key-pair notation `{}` , e.g. `{'key1': uint64, 'key2': bool}`
* **Tuple**: angle-braket notation `[]` , e.g. `uint64[]`
* **List**: angle-braket notation `[N]` , e.g. `uint64[N]`
2018-09-23 23:52:38 +00:00
2019-02-27 10:54:23 +00:00
### Aliases
2018-10-02 13:33:11 +00:00
2019-02-27 10:54:23 +00:00
For convenience we alias:
2018-10-02 13:33:11 +00:00
2019-02-27 10:54:23 +00:00
* `byte` to `uint8`
* `bytes` to `byte[]`
* `bytesN` to `byte[N]`
* `bit` to `bool`
2018-10-02 22:17:29 +00:00
2019-02-27 10:54:23 +00:00
## Serialization
2018-10-02 13:42:25 +00:00
2019-02-27 10:54:23 +00:00
We reccursively define a `serialize` function. In the code below `value` refers to a value of the specified type.
2019-02-11 14:49:11 +00:00
2019-02-27 10:54:23 +00:00
### `uintN`
2018-09-23 23:52:38 +00:00
```python
2019-02-27 10:54:23 +00:00
assert N in [8, 16, 32, 64, 128, 256]
return value.to_bytes(N / 8, 'little')
2018-09-23 23:52:38 +00:00
```
2019-02-27 10:54:23 +00:00
### `bool`
2018-10-26 13:22:28 +00:00
```python
2019-02-27 10:54:23 +00:00
assert value in (True, False)
return b'\x01' if value is True else b'\x00'
2018-09-23 23:52:38 +00:00
```
2019-02-27 10:54:23 +00:00
### Containers
2018-10-26 13:22:28 +00:00
```python
2019-02-27 10:54:23 +00:00
serialized_elements = [serialize(element) for element in value]
serialized_bytes = reduce(lambda x, y: x + y, serialized_elements)
assert len(serialized_bytes) < 2 * * 32
serialized_length = len(serialized_bytes).to_bytes(LENGTH_BYTES, 'little')
return serialized_length + serialized_bytes
2018-10-03 05:08:20 +00:00
```
2019-02-27 10:54:23 +00:00
### Tuples
2019-02-01 00:03:23 +00:00
2019-02-01 14:31:00 +00:00
```python
2019-02-27 10:54:23 +00:00
serialized_elements = [serialize(element) for element in value]
serialized_bytes = reduce(lambda x, y: x + y, serialized_elements)
return serialized_bytes
2019-02-01 00:03:23 +00:00
```
2018-11-15 13:12:34 +00:00
2019-02-27 10:54:23 +00:00
### Lists
2018-11-15 13:12:34 +00:00
```python
2019-02-27 10:54:23 +00:00
serialized_elements = [serialize(element) for element in value]
serialized_bytes = reduce(lambda x, y: x + y, serialized_elements)
assert len(serialized_elements) < 2 * * 32
serialized_length = len(serialized_elements).to_bytes(LENGTH_BYTES, 'little')
return serialized_length + serialized_bytes
2018-11-15 13:12:34 +00:00
```
2019-02-27 10:54:23 +00:00
## Deserialization
2018-11-15 13:12:34 +00:00
2019-02-27 10:54:23 +00:00
Given a type, serialisation is an injective function from objects of that type to byte strings. That is, deserialisation—the inverse function—is well-defined.
2018-11-15 13:12:34 +00:00
2019-02-27 10:54:23 +00:00
## Merkleization
2018-11-15 13:12:34 +00:00
2019-02-27 10:54:23 +00:00
We first define helper functions:
2018-11-15 13:12:34 +00:00
2019-02-27 10:54:23 +00:00
* `pack` : Given ordered objects of the same basic type, serialise them, pack them into 32-byte chunks, right-pad the last chunk with zero bytes, and return the chunks.
* `merkleise` : Given ordered 32-byte chunks, right-pad them with zero chunks to the closest power of two, Merkleize the chunks, and return the root.
* `mix_in_length` : Given a Merkle root `r` and a length `l` (32-byte little-endian serialisation) return `hash(r + l)` .
2018-11-15 13:12:34 +00:00
2019-02-27 10:54:23 +00:00
Let `o` be an object. We now define object Merkleization `hash_tree_root(o)` recursively:
2019-02-16 21:44:27 +00:00
2019-02-27 10:54:23 +00:00
* `merkleize(pack(o))` if `o` is a basic object or a tuple of basic objects
* `mix_in_length(merkleize(pack(o)), len(o))` if `o` is a list of basic objects
* `merkleize([hash_tree_root(element) for element in o])` if `o` is a tuple of composite objects or a container
* `mix_in_length(merkleize([hash_tree_root(element) for element in o]), len(o))` if `o` is a list of composite objects
2019-02-16 21:44:27 +00:00
2019-02-27 10:54:23 +00:00
## Signed containers
2019-02-16 21:44:27 +00:00
2019-02-27 10:54:23 +00:00
Let `container` be a self-signed container object. The convention is that the signature (e.g. a `bytes96` BLS12-381 signature) be the last field of `container` . Further, the signed message for `container` is `signed_root(container) = hash_tree_root(truncate_last(container))` where `truncate_last` truncates the last element of `container` .
2019-02-16 21:44:27 +00:00
2018-09-23 23:52:38 +00:00
## Implementations
2019-02-27 10:54:23 +00:00
| Language | Implementation | Description |
|:-:|-|-|
| Python | [ https://github.com/ethereum/py-ssz ](https://github.com/ethereum/py-ssz ) | Python implementation of SSZ |
| Rust | [ https://github.com/sigp/lighthouse/tree/master/beacon_chain/utils/ssz ](https://github.com/sigp/lighthouse/tree/master/beacon_chain/utils/ssz ) | Lighthouse (Rust Ethereum 2.0 Node) maintained SSZ |
| Nim | [ https://github.com/status-im/nim-beacon-chain/blob/master/beacon_chain/ssz.nim ](https://github.com/status-im/nim-beacon-chain/blob/master/beacon_chain/ssz.nim ) | Nim Implementation maintained SSZ |
| Rust | [ https://github.com/paritytech/shasper/tree/master/util/ssz ](https://github.com/paritytech/shasper/tree/master/util/ssz ) | Shasper implementation of SSZ maintained by ParityTech |
| Javascript | [ https://github.com/ChainSafeSystems/ssz-js/blob/master/src/index.js ](https://github.com/ChainSafeSystems/ssz-js/blob/master/src/index.js ) | Javascript Implementation maintained SSZ |
| Java | [ https://www.github.com/ConsenSys/cava/tree/master/ssz ](https://www.github.com/ConsenSys/cava/tree/master/ssz ) | SSZ Java library part of the Cava suite |
| Go | [ https://github.com/prysmaticlabs/prysm/tree/master/shared/ssz ](https://github.com/prysmaticlabs/prysm/tree/master/shared/ssz ) | Go implementation of SSZ mantained by Prysmatic Labs |
| Swift | [ https://github.com/yeeth/SimpleSerialize.swift ](https://github.com/yeeth/SimpleSerialize.swift ) | Swift implementation maintained SSZ |
| C# | [ https://github.com/codingupastorm/csharp-ssz ](https://github.com/codingupastorm/csharp-ssz ) | C# implementation maintained SSZ |
| C++ | [ https://github.com/NAKsir-melody/cpp_ssz ](https://github.com/NAKsir-melody/cpp_ssz ) | C++ implementation maintained SSZ |