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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 ideomatic 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.

Illegal types

Optional[T] with T that might serialize to empty data [] are illegal:

  • T MUST NOT be List[type, N]
  • T MUST NOT be a nested Optional[type]


if value is None:
    return b""
    return serialize(value)


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

  • If the input length is 0, the value is None.
  • Otherwise, deserialize the input as if it represents a value of type 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]?

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 Optional[T], the simpler Optional[T] type is sufficient, and support for general unions could be delayed until really needed.

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

The serialization is less compact, due to the extra selector byte.

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: