What is CISA?

  • “Cross-input signature aggregation”

What is that?

  • Combining together signatures on different inputs
  • Not the same thing as key aggregation
  • Can be either of:
    • Half Aggregation (s values combined)
    • Full Aggregation (r and s values combined)
    • Hybrid?
  • Can be either of:
    • Transaction-wide Aggregation
      • Efficient coinjoins
    • Block-wide Aggregation

What are the benefits of this?

  • Space and fee savings
  • Privacy
    • common-input ownership heuristic
    • anonymity set
    • collaborative transactions become the default
      • reduced stigma to coinjoins
  • Computational efficiency (full-agg only)
  • Bandwidth
  • Lightning gossip (no soft fork required)
  • Incentivizes reducing utxoset

Drawbacks

  • Needs a soft fork
  • Full-agg
    • needs an interactive coordination protocol before it makes sense to recommend a soft fork
      • as it would make the common-input ownership heuristic even worse (obvious that full-agg inputs belong to the same user)
  • Block-agg
    • makes reorgs a mess
  • Groot conflicts

Use cases

  • Coinjoins and other interactive protocols
  • Consolidation (dust would be easier to rescue)
  • New business models for collaborative txs

Status

  • Half-agg BIP
  • Need BIPs for:
    • New segwit version

Questions

  • What are the relative benefits of half vs full aggregation?
  • What are the relative benefits of transaction-wide vs block-wide aggregation?