Proposal: Use SSZ hash_tree_root instead of keccak(payload) for SequencerCommitment signatures

[simon-jentzsch]

Is your feature request related to a problem? Please describe.

Currently, sequencer commitments (preconfirmations) are signed over

keccak256(
domain ++ chain\_id ++ keccak256(payload)
)

This ties the signature to the raw byte serialization of the full payload.
As a result, verification always requires transmitting the entire payload. This makes proofs unnecessarily large and prevents the use of Merkle proofs for lightweight verification. For light clients and zk-based systems (e.g. Colibri stateless clients), this is a major efficiency bottleneck.

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Describe the solution you'd like

Define the signature message using an SSZ container and its hash_tree_root, similar to Ethereum consensus:

class SequencerCommitment(Container):
    parent_beacon_block_root: Bytes32
    execution_payload_header: ExecutionPayloadHeader

Then the signing root becomes:

keccak256(
    domain ++ chain_id ++ hash_tree_root(SequencerCommitment)
)

This enables:

• Merkle-friendly proofs: clients can verify only the necessary fields.
• Smaller proofs: no need to transmit the full payload, only Merkle branches.
• Consistency with Ethereum consensus: aligns with how signing roots are already defined.
• Forward-compatibility: SSZ evolution is safer than raw byte hashing.

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Describe alternatives you’ve considered

• Status quo: keep keccak(payload) – works but bloats proofs and prevents efficient verification.
• Application-layer Merkleization: replicate hashing on top of the keccak payload, but this duplicates work and doesn’t integrate cleanly with existing SSZ tooling.

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Additional context

• Current spec reference: [Rollup Node P2P – Block signatures](https://specs.optimism.io/protocol/rollup-node-p2p.html)
• Ethereum SSZ reference: SSZ hash_tree_root
• Backwards compatibility path: introduce a new topic/domain (e.g. v5), sequencers can dual-sign during migration, clients accept both formats.

This change would drastically reduce proof sizes and make OP Stack more compatible with stateless light clients and zk-provers.