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0027-max-witness-size-policy.md

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Maximum Witness Size Policy

Summary

Given concern over Bitcoin "ordinal inscriptions", this RFC proposes restricting the size of each input's witness data for standard transactions to 100KB. Nodes that use the new policy will not relay, mint or mine transactions with large inscriptions.

Conventions

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

Motivation

The ordinal protocol may be ported to Peercoin which could cause large amounts of superfluous, non-transactional data to be included in blocks. Other protocols may also utilise Taproot witness elements to add large amounts of data.

Limiting the witness data size for each input through a policy change mitigates against protocols that use excessive amounts of data within transactions. In particular, this change deters large inscriptions of the ordinal protocol which is a particular concern.

Detailed design

The standard transaction policy of the Peercoin client shall be changed. It is recommended that alternative software abide by the same policy unless there is a specific reason to deviate from the policy.

A transaction shall be non-standard if any input has total witness data that exceeds 100KB. To simplify calculation of the total witness data for each input, it shall be calculated by summing the size of each witness element in the stack without including varints.

The IsWitnessStandard function in src/policy/policy.cpp shall return false under the new non-standard condition. The rule shall apply regardless of the witness program version. Any changes shall only apply to standard transaction policy and must not affect consensus.

Drawbacks

It remains possible for transactions with large witness data over 100KB per input to be included in blocks by non-standard nodes. It is also possible to include 100KB of data in multiple inputs such that the total transaction witness data is much larger.

Restricting the size of data in transaction inputs restricts the possible use cases of Peercoin. This includes ordinal inscriptions but may impact other protocols that rely on large amounts of input data.

Alternatives

A consensus change may limit witness data or provide a fee-based mitigation but this requires a new fork which would interfere with the ongoing v0.12 fork. A consensus change is less straight-forward and imposes stricter limits that are not as simple to change.

100KB is a compromise. A larger limit will allow for larger ordinal inscriptions and other possible data uses but encourages more superfluous data usage on the chain. A much smaller limit of ~5KB should cover multisig inputs of 40 keys or more, but places a heavier limit on applications that depend upon data availability, including ordinal inscriptions.