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142 lines
5.8 KiB
Markdown
142 lines
5.8 KiB
Markdown
# SimpleSerialiZe (SSZ)
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This is a **work in progress** describing typing, serialization and Merkleization of Ethereum 2.0 objects.
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## Table of contents
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- [Constants](#constants)
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- [Typing](#typing)
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- [Basic types](#basic-types)
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- [Composite types](#composite-types)
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- [Notation](#notation)
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- [Aliases](#aliases)
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- [Serialization](#serialization)
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- [`uintN`](#uintn)
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- [`bool`](#bool)
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- [Containers](#containers)
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- [Tuples](#tuples)
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- [Lists](#lists)
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- [Deserialization](#deserialization)
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- [Merkleization](#merkleization)
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- [Self-signed containers](#self-signed-containers)
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- [Implementations](#implementations)
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## Constants
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| Name | Value | Description |
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| `LENGTH_BYTES` | `4` | Number of bytes for the length of variable-length serialized objects. |
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| `MAX_LENGTH` | `2**(8 * LENGTH_BYTES)` | Maximum serialization length. |
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## Typing
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#### Basic types
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* `uintN`: `N`-bit unsigned integer (where `N in [8, 16, 32, 64, 128, 256]`)
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* `bool`: 1-bit unsigned integer
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#### Composite types
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* **container**: ordered heterogenous collection of values
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* **tuple**: ordered fixed-length homogeneous collection of values
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* **list**: ordered variable-length homogenous collection of values
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#### Notation
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* **container**: key-pair curly braket notation `{}` (e.g. `{'key1': uint64, 'key2': bool}`)
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* **tuple**: angle braket notation `[N]` (e.g. `uint64[N]`)
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* **list**: angle braket notation `[]` (e.g. `uint64[]`)
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#### Aliases
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For convenience we alias:
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* `byte` to `uint8`
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* `bytes` to `byte[]`
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* `bytesN` to `byte[N]`
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* `bit` to `bool`
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## Serialization
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We reccursively define the `serialize` function which consumes an object `object` (of the type specified) and returns a byte string of type `bytes`.
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#### `uintN`
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```python
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assert N in [8, 16, 32, 64, 128, 256]
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return object.to_bytes(N // 8, 'little')
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```
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#### `bool`
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```python
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assert object in (True, False)
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return b'\x01' if object is True else b'\x00'
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```
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#### Containers
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```python
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serialized_elements = [serialize(element) for element in object]
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serialized_bytes = reduce(lambda x, y: x + y, serialized_elements)
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assert len(serialized_bytes) < MAX_LENGTH
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serialized_length = len(serialized_bytes).to_bytes(LENGTH_BYTES, 'little')
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return serialized_length + serialized_bytes
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```
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#### Tuples
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```python
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serialized_elements = [serialize(element) for element in object]
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serialized_bytes = reduce(lambda x, y: x + y, serialized_elements)
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return serialized_bytes
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```
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#### Lists
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```python
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serialized_elements = [serialize(element) for element in object]
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serialized_bytes = reduce(lambda x, y: x + y, serialized_elements)
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assert len(serialized_elements) < MAX_LENGTH
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serialized_length = len(serialized_elements).to_bytes(LENGTH_BYTES, 'little')
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return serialized_length + serialized_bytes
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```
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## Deserialization
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Given a type, serialization is an injective function from objects of that type to byte strings. That is, deserialization—the inverse function—is well-defined.
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## Merkleization
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We first define helper functions:
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* `pack`: Given ordered objects of the same basic type, serialize them, pack them into 32-byte chunks, right-pad the last chunk with zero bytes, and return the chunks.
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* `merkleize`: Given ordered 32-byte chunks, right-pad them with zero chunks to the next power of two, Merkleize the chunks, and return the root.
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* `mix_in_length`: Given a Merkle root `root` and a length `length` (32-byte little-endian serialization) return `hash(root + length)`.
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Let `object` be an object. We now define object Merkleization `hash_tree_root(object)` recursively:
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* `merkleize(pack(object))` if `object` is a basic object or a tuple of basic objects
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* `mix_in_length(merkleize(pack(object)), len(object))` if `object` is a list of basic objects
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* `merkleize([hash_tree_root(element) for element in object])` if `object` is a tuple of composite objects or a container
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* `mix_in_length(merkleize([hash_tree_root(element) for element in object]), len(object))` if `object` is a list of composite objects
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## Self-signed containers
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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`.
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## Implementations
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| Language | Project | Maintainer | Implementation |
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| Python | Ethereum 2.0 | Ethereum Foundation | [https://github.com/ethereum/py-ssz](https://github.com/ethereum/py-ssz) |
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| Rust | Lighthouse | Sigma Prime | [https://github.com/sigp/lighthouse/tree/master/beacon_chain/utils/ssz](https://github.com/sigp/lighthouse/tree/master/beacon_chain/utils/ssz) |
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| Nim | Nimbus | Status | [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) |
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| Rust | Shasper | ParityTech | [https://github.com/paritytech/shasper/tree/master/util/ssz](https://github.com/paritytech/shasper/tree/master/util/ssz) |
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| Javascript | Lodestart | Chain Safe Systems | [https://github.com/ChainSafeSystems/ssz-js/blob/master/src/index.js](https://github.com/ChainSafeSystems/ssz-js/blob/master/src/index.js) |
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| Java | Cava | ConsenSys | [https://www.github.com/ConsenSys/cava/tree/master/ssz](https://www.github.com/ConsenSys/cava/tree/master/ssz) |
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| Go | Prysm | Prysmatic Labs | [https://github.com/prysmaticlabs/prysm/tree/master/shared/ssz](https://github.com/prysmaticlabs/prysm/tree/master/shared/ssz) |
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| Swift | Yeeth | Dean Eigenmann | [https://github.com/yeeth/SimpleSerialize.swift](https://github.com/yeeth/SimpleSerialize.swift) |
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| C# | | Jordan Andrews | [https://github.com/codingupastorm/csharp-ssz](https://github.com/codingupastorm/csharp-ssz) |
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| C++ | | | [https://github.com/NAKsir-melody/cpp_ssz](https://github.com/NAKsir-melody/cpp_ssz) |
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