Cheaper Payment Rollups with Ephemeral Calldata

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Top Highlights

• Ephemeral Calldata (ECD) is a new type of transaction calldata that is included in the blockchain but may be removed in the future

• he hash digest of this data can be used by smart-contracts to verify Zero-Knowledge proofs

• Contracts can query parts of the data for onchain arbitration, preventing the data to be removed later

• Removal can be automatic (remove-by-default) or on-request (keep-by-default).

• The huge benefit of ECD is the ability to remove transaction commitments made by rollups after a new state snapshot (consuming much less space) has been committed.

• Cost savings of doing so have been estimated in the 5x-10x range.

• RSKIP281 presents a proposal to add ephemeral data capability to RSK.

• The core idea is that ephemeral calldata (or ECD for short) is priced less than normal calldata

• This can be achieved either by pricing it at 68 gas/byte, and giving a refund of 60 gas/byte (expressed in rBTC) on removal, or by directly pricing it at 8 gas/byte when removal is the default action after some time.

• While the concept is simple, implementing Ephemeral Data is not so straightforward.

• It requires implementing at least 5 changes to different parts of the consensus code of a node

• Creation of a precompiled contract (EphemeralDataManager), which stores references to all ECDs registered and allows ECD querying by contracts.

• Addition of new transaction fields to the transaction and verification of the correctness of such fields before transaction executio

• Execution of the user-defined hasher contract

• Redesign of the blockchain synchronization protocol to detect if a block is complete or not

• Removal of ECD from old transactions when new transactions command it.

• I began exploring the possibility to remove unneeded data from the blockchain in 2016 with the concept of ephemeral data, presented in RSKIP28 and several conference talks.

• There are several interesting topics of research related to ephemeral blockchain data.

• transaction signatures can be posted as ephemeral data, to be later removed and replaced by an aggregated signature or zero-knowledge proof of the signatures and their validity

• Ephemeral signatures combine the benefits of non-interactive transaction signing with the space savings of signature aggregation