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
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