SSZ Specs

Reference implementation of Ethereum's SSZ (Simple Serialize) type system, serialization, and Merkleization, written in Python.

The specification is a faithful port of the SSZ and Merkleization logic used across the Ethereum consensus specifications (ethereum/consensus-specs/ssz).

Overview

SSZ is the serialization and hashing scheme used throughout Ethereum consensus. This repository implements:

• Basic types: Boolean, Uint8/Uint16/Uint32/Uint64.
• Byte arrays: fixed Bytes4 … Bytes64 and variable ByteList.
• Bitfields: Bitvector and Bitlist.
• Composite types: Vector, List, and Container.
• Progressive types (EIP-7916): ProgressiveList and ProgressiveBitlist, which carry no capacity and hash into a tree that grows with the data.
• Progressive containers (EIP-7495): ProgressiveContainer with an ACTIVE_FIELDS layout, whose fields keep their tree positions as later versions add or drop fields.
• (De)serialization: canonical encode/decode with strict, offset-based decoding.
• Merkleization: hash_tree_root, merkleize, merkleize_progressive, mix_in_length, and mix_in_active_fields.

Project Structure

src/ssz/
  __init__.py          # public re-exports of the SSZ types
  base.py              # strict, immutable Pydantic base models
  ssz_base.py          # abstract SSZType / SSZModel bases
  boolean.py           # Boolean
  uint.py              # Uint8/16/32/64
  byte_arrays.py       # fixed byte vectors and byte lists
  bitfields.py         # Bitvector / Bitlist / ProgressiveBitlist
  collections.py       # Vector / List / ProgressiveList
  container.py         # Container / ProgressiveContainer
  exceptions.py        # SSZ error hierarchy
  merkleization.py     # hash_tree_root dispatch and Merkle primitives
tests/                 # pytest unit tests mirroring the source modules
tests/fillers/         # test-vector fillers (generate JSON conformance vectors)
packages/testing/      # ssz-testing package: the vector generation infrastructure

Development

This project uses uv and just.

uv sync                 # Install dependencies
uv tool install just-bin  # Install the just command runner

just            # List all available recipes
just test       # Run the unit tests
just check      # Lint, format, typecheck, spellcheck, mdformat, lock-check
just fix        # Auto-fix lint, formatting, and markdown

• Python 3.12+ is required.
• All types are modeled with Pydantic for strict validation.

Test vectors

Alongside the unit tests, the repository generates language-neutral JSON conformance vectors that other SSZ implementations can replay. The fillers live under tests/fillers/ and the generation infrastructure is the ssz-testing workspace package under packages/testing/.

just fill       # Generate all SSZ conformance vectors into fixtures/

Each vector records the type name, the SSZ serialized bytes, the hash_tree_root, and an _info block with a content hash. Decode-failure vectors carry an empty root and a rejectionReason. The generated tree lands under fixtures/ (git-ignored).

License

MIT