This MIT Researcher Has a Plan to Put Privacy in Every Bitcoin Wallet

— PayJoin DevKit project aims to make privacy tooling easy for wallet developers to integrate, with two production wallets (CakeWallet, BullBitcoin Mobile) and 8–12 integrations in progress.

— Wallet fingerprints can decompose PayJoin transactions and recover payment amounts by identifying artifacts of collaboration between sender and recipient; Armin demonstrated this attack on three known PayJoins.

— Common input heuristic used by chain analysis clusters transaction inputs to assumed common owners; PayJoin introduces false positives but lacks counterparty privacy since recipients see sender inputs.

— Standardization efforts like BIP 69 backfired by creating fingerprints when only a few wallets adopted them; randomization of transaction fields may offer better privacy resilience.

— Intersection attacks occur when co-mixing parties dox themselves, compounding de-anonymization; privacy metrics and theoretical frameworks are underdeveloped in the community.

— Roadmap includes integrating PayJoin into a dozen more wallets and developing multi-party coinjoins (NS1R design) while researching transaction graph structures.

— Privacy is not a one-time action but requires active maintenance over time. Monitor your counterparties' wallet fingerprints and transaction behavior, as updates or deviations signal collaboration to chain analysis.

— Wallet developers integrating PayJoin must ensure transaction fields (sequence, locktime, input/output structure) match counterparties' fingerprints consistently across the entire transaction graph, not just single transactions.

— Evaluate which wallets you pair with for privacy transactions; mismatched fingerprints between wallets undermine PayJoin effectiveness even before future transactions compound the risk.