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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.

About
Terminology
Constants
Overview
- Serialize/Encode
  - uint: 8/16/24/32/64/256
  - Address
  - Hash
    - Hash32
    - Hash96
    - Hash97
  - Bytes
  - List/Vectors
  - Container (TODO)
- Deserialize/Decode
  - uint: 8/16/24/32/64/256
  - Address
  - Hash
    - Hash32
    - Hash96
    - Hash97
  - Bytes
  - List/Vectors
  - Container (TODO)
Implementations

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
`big`	Big 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.

Overview

Serialize/Encode

uint: 8/16/24/32/64/256

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

All integers are serialized as big 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, 'big')

Address

The address should already come as a hash/byte format. Ensure that length is 20.

Check to perform	Code
Length is correct (20)	`len(value) == 20`

assert( len(value) == 20 )
return value

Hash

Hash Type	Usage
`hash32`	Hash size of `keccak` or `blake2b[0.. < 32]`.
`hash96`	BLS Public Key Size.
`hash97`	BLS Public Key Size with recovery bit.

Checks to perform	Code
Length is correct (32) if `hash32`	`len(value) == 32`
Length is correct (96) if `hash96`	`len(value) == 96`
Length is correct (97) if `hash97`	`len(value) == 97`

Example all together

if (type(value) == 'hash32'):
   assert(len(value) == 32)
elif (type(value) == 'hash96'):
   assert(len(value) == 96)
elif (type(value) == 'hash97'):
   assert(len(value) == 97)
else:
   raise TypeError('Invalid hash type supplied')

return value

Hash32

Ensure 32 byte length and return the bytes.

assert(len(value) == 32)
return value

Hash96

Ensure 96 byte length and return the bytes.

assert(len(value) == 96)
return value

Hash97

Ensure 97 byte length and return the bytes.

assert(len(value) == 97)
return value

Bytes

For general byte type:

Get the length/number of bytes; Encode into a 4-byte integer.
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, 'big')
return byte_length + value

List/Vectors

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

Get the number of raw bytes to serialize: it is len(list) * sizeof(element).
- Encode that as a 4-byte big endian uint32.
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, 'big'))

return serialized_len + serialized_list_string

Container

########################################
                 TODO
########################################

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 keeping track of the new index.

At each step, the following checks should be made:

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

uint: 8/16/24/32/64/256

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 big endian.

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

Address

Return the 20 bytes.

assert(len(rawbytes) >= current_index + 20)
new_index = current_index + 20
return rawbytes[current_index:current_index+20], new_index

Hash

Hash32

Return the 32 bytes.

assert(len(rawbytes) >= current_index + 32)
new_index = current_index + 32
return rawbytes[current_index:current_index+32], new_index

Hash96

Return the 96 bytes.

assert(len(rawbytes) >= current_index + 96)
new_index = current_index + 96
return rawbytes[current_index:current_index+96], new_index

Hash97

Return the 97 bytes.

assert(len(rawbytes) >= current_index + 97)
new_index = current_index + 97
return rawbytes[current_index:current_index+97], 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], 'big')

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 object in the list.

Get the length of the serialized list.
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], 'big')
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

########################################
                 TODO
########################################

Implementations

Language	Implementation	Description
Python	https://github.com/ethereum/beacon_chain/blob/master/ssz/ssz.py	Beacon chain reference implementation written in Python.
Rust	https://github.com/sigp/lighthouse/tree/master/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.

12 KiB Raw Blame History

[WIP] SimpleSerialize (SSZ) Spec

ToC

About

Terminology

Constants

Overview

Serialize/Encode

uint: 8/16/24/32/64/256

Address

Hash

Hash32

Hash96

Hash97

Bytes

List/Vectors

Container

Deserialize/Decode

uint: 8/16/24/32/64/256

Address

Hash

Hash32

Hash96

Hash97

Bytes

List/Vectors

Container

Implementations

12 KiB

Raw Blame History