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⚠️ Draft Standards Track: Core

EIP-6475: SSZ Optional

New SSZ type to represent optional values

Authors Etan Kissling (@etan-status), Zahary Karadjov (@zah)
Created 2023-02-09
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This EIP introduces a new Simple Serialize (SSZ) type to represent Optional[T] values.


Optional values are currently only representable in SSZ using workarounds. Adding proper support provides these benefits:

  1. Better readability: SSZ structures with optional values can be represented with idiomatic types of the underlying programming language, e.g., Optional[T] in Python, making them easier to interact with.

  2. Compact serialization: SSZ serialization can rely on the binary nature of optional values; they either exist or they don’t. This allows more compact serialization than using alternative approaches based on workarounds.


The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “NOT RECOMMENDED”, “MAY”, and “OPTIONAL” in this document are to be interpreted as described in RFC 2119 and RFC 8174.

Type definition

Optional[T] is defined as a type that can represent:

  • A value of SSZ type T
  • Absence of a value, indicated by None

Default value

The default value of Optional[T] is None.


if value is None:
    return b""
    return b"\x01" + serialize(value)


The deserialization of an Optional[T] depends on the input length:

  • If the input length is 0, the value is None.
  • Otherwise, the first byte of the deserialization scope must be checked to be 0x01, the remainder of the scope is deserialized same as T.


An Optional[T] is merkleized as a List[T, 1].

  • If the value is None, the list length is 0.
  • Otherwise, the list length is 1, and the first list element contains the underlying value.


Why not Union[None, T]?

Union[None, T] leaves ambiguity about the intention whether the type may be extended in the future, i.e., Union[None, T, U].

Furthermore, SSZ Union types are currently not used in any final Ethereum specification and do not have a finalized design themselves. If the only use case is a workaround for lack of Optional[T], the simpler Optional[T] type is sufficient, and support for general unions could be delayed until really needed. Note that the design of Optional[T] could be used as basis for a more general Union.

Why not List[T, 1]?

The serialization is less compact for variable-length T, due to the extra offset table at the beginning of the list to indicate the list length.

Backwards Compatibility

Union[None, T] and List[T, 1] workarounds are not used at this time to represent Optional[T].

Test Cases

See EIP assets.

Reference Implementation

  • Python: See EIP assets, based on protolambda/remerkleable
  • Nim: status-im/nim-ssz-serialization

Security Considerations


Copyright and related rights waived via CC0.


Please cite this document as:

Etan Kissling (@etan-status), Zahary Karadjov (@zah), "EIP-6475: SSZ Optional [DRAFT]," Ethereum Improvement Proposals, no. 6475, February 2023. [Online serial]. Available: