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[WIP] SimpleSerialize (SSZ) Spec

This is the work in progress document to describe SimpleSerialize, the current selected serialization method for Ethereum 2.0 using the Beacon Chain.

This document specifies the general information for serializing and deserializing objects and data types.

ToC

About

SimpleSerialize was first proposed by Vitalik Buterin as the serialization protocol for use in the Ethereum 2.0 Beacon Chain.

The core feature of ssz is the simplicity of the serialization with low overhead.

Terminology

Term Definition
little Little endian.
byte_order Specifies endianness: big endian or little endian.
len Length/number of bytes.
to_bytes Convert to bytes. Should take parameters size and byte_order.
from_bytes Convert from bytes to object. Should take bytes and byte_order.
value The value to serialize.
rawbytes Raw serialized bytes.

Constants

Constant Value Definition
LENGTH_BYTES 4 Number of bytes used for the length added before a variable-length serialized object.
SSZ_CHUNK_SIZE 128 Number of bytes for the chunk size of the Merkle tree leaf.

Overview

Serialize/Encode

uint

uint Type Usage
uintN Type of N bits unsigned integer, where N % 8 == 0.

Convert directly to bytes the size of the int. (e.g. uint16 = 2 bytes)

All integers are serialized as little endian.

Check to perform Code
Size is a byte integer int_size % 8 == 0 
assert(int_size % 8 == 0)
buffer_size = int_size / 8
return value.to_bytes(buffer_size, 'little')

Bool

Convert directly to a single 0x00 or 0x01 byte.

Check to perform Code
Value is boolean value in (True, False) 
assert(value in (True, False))
return b'\x01' if value is True else b'\x00'

Bytes

Bytes Type Usage
bytesN Explicit length N bytes data.
bytes Bytes data with arbitrary length.
bytesN
Checks to perform Code
Length in bytes is correct for bytesN len(value) == N 
assert(len(value) == N)

return value
bytes

For general bytes type:

  1. Get the length/number of bytes; Encode into a 4-byte integer.
  2. Append the value to the length and return: [ length_bytes ] + [ value_bytes ]
Check to perform Code
Length of bytes can fit into 4 bytes len(value) < 2**32 
assert(len(value) < 2**32)
byte_length = (len(value)).to_bytes(LENGTH_BYTES, 'little')
return byte_length + value

List/Vectors

Lists are a collection of elements of the same homogeneous type.

Check to perform Code
Length of serialized list fits into 4 bytes len(serialized) < 2**32
  1. Get the number of raw bytes to serialize: it is len(list) * sizeof(element).
    • Encode that as a 4-byte little endian uint32.
  2. Append the elements in a packed manner.
  • Note on efficiency: consider using a container that does not need to iterate over all elements to get its length. For example Python lists, C++ vectors or Rust Vec.

Example in Python


serialized_list_string = b''

for item in value:
   serialized_list_string += serialize(item)

assert(len(serialized_list_string) < 2**32)

serialized_len = (len(serialized_list_string).to_bytes(LENGTH_BYTES, 'little'))

return serialized_len + serialized_list_string

Container

A container represents a heterogenous, associative collection of key-value pairs. Each pair is referred to as a field. To get the value for a given field, you supply the key which is a symbol unique to the container referred to as the field's name. The container data type is analogous to the struct type found in many languages like C or Go.

To serialize a container, obtain the list of its field's names in the specified order. For each field name in this list, obtain the corresponding value and serialize it. Tightly pack the complete set of serialized values in the same order as the field names into a buffer. Calculate the size of this buffer of serialized bytes and encode as a 4-byte little endian uint32. Prepend the encoded length to the buffer. The result of this concatenation is the final serialized value of the container.

Check to perform Code
Length of serialized fields fits into 4 bytes len(serialized) < 2**32

To serialize:

  1. Get the list of the container's fields.

  2. For each name in the list, obtain the corresponding value from the container and serialize it. Place this serialized value into a buffer. The serialized values should be tightly packed.

  3. Get the number of raw bytes in the serialized buffer. Encode that number as a 4-byte little endian uint32.

  4. Prepend the length to the serialized buffer.

Example in Python

def get_field_names(typ):
    return typ.fields.keys()

def get_value_for_field_name(value, field_name):
    return getattr(value, field_name)

def get_type_for_field_name(typ, field_name):
    return typ.fields[field_name]

serialized_buffer = b''

typ = type(value)
for field_name in get_field_names(typ):
    field_value = get_value_for_field_name(value, field_name)
    field_type = get_type_for_field_name(typ, field_name)
    serialized_buffer += serialize(field_value, field_type)

assert(len(serialized_buffer) < 2**32)

serialized_len = (len(serialized_buffer).to_bytes(LENGTH_BYTES, 'little'))

return serialized_len + serialized_buffer

Deserialize/Decode

The decoding requires knowledge of the type of the item to be decoded. When performing decoding on an entire serialized string, it also requires knowledge of the order in which the objects have been serialized.

Note: Each return will provide:

  • deserialized_object
  • new_index

At each step, the following checks should be made:

Check to perform Check
Ensure sufficient length len(rawbytes) >= current_index + deserialize_length

At the final step, the following checks should be made:

Check to perform Check
Ensure no extra length new_index == len(rawbytes)

uint

Convert directly from bytes into integer utilising the number of bytes the same size as the integer length. (e.g. uint16 == 2 bytes)

All integers are interpreted as little endian.

byte_length = int_size / 8
new_index = current_index + byte_length
assert(len(rawbytes) >= new_index)
return int.from_bytes(rawbytes[current_index:current_index+byte_length], 'little'), new_index

Bool

Return True if 0x01, False if 0x00.

assert rawbytes in (b'\x00', b'\x01')
return True if rawbytes == b'\x01' else False

Bytes

bytesN

Return the N bytes.

assert(len(rawbytes) >= current_index + N)
new_index = current_index + N
return rawbytes[current_index:current_index+N], new_index
bytes

Get the length of the bytes, return the bytes.

Check to perform code
rawbytes has enough left for length len(rawbytes) > current_index + LENGTH_BYTES
bytes to return not greater than serialized bytes len(rawbytes) > bytes_end 
assert(len(rawbytes) > current_index + LENGTH_BYTES)
bytes_length = int.from_bytes(rawbytes[current_index:current_index + LENGTH_BYTES], 'little')

bytes_start = current_index + LENGTH_BYTES
bytes_end = bytes_start + bytes_length
new_index = bytes_end

assert(len(rawbytes) >= bytes_end)

return rawbytes[bytes_start:bytes_end], new_index

List/Vectors

Deserialize each element in the list.

  1. Get the length of the serialized list.
  2. Loop through deserializing each item in the list until you reach the entire length of the list.
Check to perform code
rawbytes has enough left for length len(rawbytes) > current_index + LENGTH_BYTES
list is not greater than serialized bytes len(rawbytes) > current_index + LENGTH_BYTES + total_length 
assert(len(rawbytes) > current_index + LENGTH_BYTES)
total_length = int.from_bytes(rawbytes[current_index:current_index + LENGTH_BYTES], 'little')
new_index = current_index + LENGTH_BYTES + total_length
assert(len(rawbytes) >= new_index)
item_index = current_index + LENGTH_BYTES
deserialized_list = []

while item_index < new_index:
   object, item_index = deserialize(rawbytes, item_index, item_type)
   deserialized_list.append(object)

return deserialized_list, new_index

Container

Refer to the section on container encoding for some definitions.

To deserialize a container, loop over each field in the container and use the type of that field to know what kind of deserialization to perform. Consume successive elements of the data stream for each successful deserialization.

Instantiate a container with the full set of deserialized data, matching each member with the corresponding field.

Check to perform code
rawbytes has enough left for length len(rawbytes) > current_index + LENGTH_BYTES
list is not greater than serialized bytes len(rawbytes) > current_index + LENGTH_BYTES + total_length

To deserialize:

  1. Get the list of the container's fields.
  2. For each name in the list, attempt to deserialize a value for that type. Collect these values as they will be used to construct an instance of the container.
  3. Construct a container instance after successfully consuming the entire subset of the stream for the serialized container.

Example in Python

def get_field_names(typ):
    return typ.fields.keys()

def get_value_for_field_name(value, field_name):
    return getattr(value, field_name)

def get_type_for_field_name(typ, field_name):
    return typ.fields[field_name]

class Container:
    # this is the container; here we will define an empty class for demonstration
    pass

# get a reference to the type in some way...
container = Container()
typ = type(container)

assert(len(rawbytes) > current_index + LENGTH_BYTES)
total_length = int.from_bytes(rawbytes[current_index:current_index + LENGTH_BYTES], 'little')
new_index = current_index + LENGTH_BYTES + total_length
assert(len(rawbytes) >= new_index)
item_index = current_index + LENGTH_BYTES

values = {}
for field_name in get_field_names(typ):
    field_name_type = get_type_for_field_name(typ, field_name)
    values[field_name], item_index = deserialize(data, item_index, field_name_type)
assert item_index == new_index
return typ(**values), item_index

Tree Hash

The below hash_tree_root algorithm is defined recursively in the case of lists and containers, and it outputs a value equal to or less than 32 bytes in size. For the final output only (ie. not intermediate outputs), if the output is less than 32 bytes, right-zero-pad it to 32 bytes. The goal is collision resistance within each type, not between types.

Refer to the helper function hash of Phase 0 of the Eth2.0 specs for a definition of the hash function used below, hash(x).

uint8..uint256, bool, bytes1..bytes32

Return the serialization of the value.

uint264..uintN, bytes, bytes33..bytesN

Return the hash of the serialization of the value.

List/Vectors

First, we define some helpers and then the Merkle tree function.

# Merkle tree hash of a list of homogenous, non-empty items
def merkle_hash(lst):
    # Store length of list (to compensate for non-bijectiveness of padding)
    datalen = len(lst).to_bytes(32, 'little')

    if len(lst) == 0:
        # Handle empty list case
        chunkz = [b'\x00' * SSZ_CHUNK_SIZE]
    elif len(lst[0]) < SSZ_CHUNK_SIZE:
        # See how many items fit in a chunk
        items_per_chunk = SSZ_CHUNK_SIZE // len(lst[0])

        # Build a list of chunks based on the number of items in the chunk
        chunkz = [b''.join(lst[i:i+items_per_chunk]) for i in range(0, len(lst), items_per_chunk)]
    else:
        # Leave large items alone
        chunkz = lst

    # Tree-hash
    while len(chunkz) > 1:
        if len(chunkz) % 2 == 1:
            chunkz.append(b'\x00' * SSZ_CHUNK_SIZE)
        chunkz = [hash(chunkz[i] + chunkz[i+1]) for i in range(0, len(chunkz), 2)]

    # Return hash of root and length data
    return hash(chunkz[0] + datalen)

To hash_tree_root a list, we simply do:

return merkle_hash([hash_tree_root(item) for item in value])

Where the inner hash_tree_root is a recursive application of the tree-hashing function (returning less than 32 bytes for short single values).

Container

Recursively tree hash the values in the container in the same order as the fields, and return the hash of the concatenation of the results.

return hash(b''.join([hash_tree_root(getattr(x, field)) for field in value.fields]))

Implementations

Language Implementation Description
Python https://github.com/ethereum/py-ssz Python implementation of SSZ
Rust 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 Nim Implementation maintained SSZ.
Rust 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 Javascript Implementation maintained SSZ
Java 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 Go implementation of SSZ mantained by Prysmatic Labs
Swift https://github.com/yeeth/SimpleSerialize.swift Swift implementation maintained SSZ

Copyright

Copyright and related rights waived via CC0.