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Eth 2.0 - Simple Serialize

Simple Serialize (SSZ) is a standard for the encoding and merkleization of structured data, designed for ETH 2.0.



Because of resource constraints (both space and speed) and a niche use-case for many interactions with merkleized data, existing encodings were abandoned in favor of SSZ on the ETH 2.0 stack.

The design choices for SSZ originate from the desire for:

  • Efficiency and Elegance in proof structures with binary trees, and a design that separates opinionated sparse structures from merkleization, learning from issues in ETH 1.0.
  • Consistency in a wide range of use-cases for minimal and efficient encoding and proofs in the core of ETH 2.0, as well as the layers being built on top.
  • Flexibility and Transparency for tracing proofs through history, building shallow variants of types, or proofs to linked data such as between ETH 2.0 shards.
  • Stability of proof data for stateless light clients and smart contracts. These can count on deterministic and stable locations of merkle tree leaves of interest.
  • Fast data reads by making full deserialization optional, data can be retrieved with a very minimal amount of operations, largely pre-computable at compile time.

Minimalism and advanced use cases are almost orthogonal. To work around this, this SSZ specification maintains a strong separation of the different stack elements, and a feature introduction model with stages.

Feature stages

Features are introduced by progressing through stages, alike to the TC39 Process implemented by EcmaScript.

In summary, this translates to:

  1. Strawperson Allow input into the specification
  2. Proposal
    • Make the case for the addition
    • Describe the shape of a solution
    • Identify potential challenges
  3. Draft Precisely describe the syntax and semantics.
  4. Candidate Indicate that further refinement will require feedback from implementations and users.
  5. Finished Indicate that the addition is ready for inclusion.

Implementations can build on the stable stage, and offer support for unfinished feature stages separately.

The proposals, drafts, candidates and finished documents are discussed through issues and PRs on GitHub, along with the relevant ETH 2.0 communication channels.

The Strawperson stage is only applicable to open PRs, and will need to become a rounded Proposal before being merged.

One difference with the regular process is that since SSZ is not widely running in production yet, and features have been modeled already, but not formally verified yet, the spec is more welcoming to some types of modifications:

  • Language, cosmetics, examples and references in any spec document may still change
  • Spec documents may be merged or split, to aid in the organization process of the SSZ spec as a whole.

Once finished however, the effective functionality does not change without an upgrade path for implementers.


Language Project Maintainer License Status Features / Notes Implementation
Python Eth 2.0 Ethereum Foundation CC0 Spec Spec
Go ZRNT Diederik Loerakker (@protolambda) MIT Active Unsafe Go, fast, streaming protolambda/zssz
Go ZRNT Diederik Loerakker (@protolambda) MIT Active Generic, caching, datasharing protolambda/ztyp
Go Prysm Ferran Borreguero (@ferranbt) MIT Active Code-gen, fast ferranbt/fastssz
Java Teku Consensys/PegaSys Eng. Apache-2.0 Active Caching, datasharing PegaSysEng/teku/ssz
Nim Nimbus Status MIT and Apache-2.0 Active In-place decode, Caching status-im/nim-beacon-chain/ssz.nim
Python Trinity Ethereum Foundation MIT Active Pyrsistent, partial caching ethereum/py-ssz
Python Pyspec / Diederik Loerakker (@protolambda) MIT Active Datasharing, caching, streaming protolambda/remerkleable
Rust Lighthouse Sigma Prime Apache-2.0 Active Partial caching, fast sigp/lighthouse/ssz
Typescript Lodestar Chainsafe Systems LGPL-v3.0 Active Both Tree & structural, caching ChainSafe/lodestar/ssz
C# Cortex Sly Gryphon (@sgryphon) LGPL-v3.0 Under construction Experimental sgryphon/cortex-ssz
Go Prysm Prysmatic Labs MIT Deprecated Generic, reflection, known bugs prysmaticlabs/go-ssz
Java Harmony Harmony Apache-2.0 Deprecated Partially merged into Teku harmony-dev/beacon-chain-java/ssz
TypeScript Lodestar ChainSafe Systems Apache-2.0 Archived See new Lodestar SSZ ChainSafe/ssz-js
Rust Shasper ParityTech GPL-3.0 Discontinued No updates paritytech/shasper/ssz
Swift Yeeth Dean Eigenmann (@decanus, @yeeth) MIT Discontinued Incomplete yeeth/SimpleSerialize.swift
Java Cava ConsenSys Apache-2.0 Discontinued Outdated, incomplete ConsenSys/cava/ssz


CC0 1.0 Universal. See License file.


Draft of SSZ specs repo







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