Bitcoin Transcripts

Privacy Metrics for Coin Selection

Date

20 September, 2023

Topics

bitcoin-core privacy-enhancements coin-selection

Speakers

Mark Erhardt

Transcript by

Goal: Get privacy consciousness into coin selection
Configurability
- Privacy vs cost (waste)
- Privacy: weighted on a 0-5 scale
- Cost: weighted on a 0-5 scale
Convert privacy preference (0-5) into satoshis to make it compatible with the waste score
- Combined score = PrivacyScoreWeight x PrivacyScore + CostWeight x WasteMetric
- 20-30 sats per privacy point as a gut feeling
Privacy score example: sending to different script type than inputs of transaction
- We already match the change type to the recipient type, but that can still mean that we have differently typed inputs than outputs
- If we have two input sets, where one has the same type and the other has a different type, the one with the same type inputs should be preferred by the privacy metric
Privacy score example: Preferring spending UTXOs of similar age
- E.g. spending a UTXO received two years ago and one received the same week may leak more information than two UTXOs received two years ago
- The timeframe in which someone spends received coins could be a privacy leak
- Right now, given current 12 input sets and variability of coin selection, might not be a privacy concern
Need to implement privacy heuristics
- Which requires more information about UTXOs
24 potential ideas for metrics so far
- Start with just 3 for first implementation
- Inspired by prior work by Bitcoin Privacy Wiki, Wasabi, LaurentMT
- There are considerations for wallet implementation
- Tx entropy metric (Samourai)
  - Number of interpretations the transaction could have
- At some point get a group together to go through the criteria to get feedback on degree of privacy impact for each
Could use wallet help in terms of pulling out data here and there
Interplay between APS (avoid partial spends) and privacy scoring
- Perhaps turn off APS with a privacy score of 0
  - Potential confusion from users ("I have privacy at 0, but APS still on by default")
- Configuration options that have overlapping areas of concern
Future
- Wallet Health Metric (also 0-5)

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Privacy Metrics for Coin Selection

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