Bitcoin Privacy Upgrade Without Forking Proposed

A new proposal called "Shielded Bitcoin" aims to enhance privacy on the Bitcoin network by obscuring transaction amounts and counterparties, mirroring features found in privacy-focused cryptocurrencies like Zcash. Crucially, the proposed metaprotocol is designed to operate on Bitcoin's existing base layer without requiring a contentious soft fork or relying on trusted intermediaries.

The system utilizes encrypted "notes" to hold Bitcoin value, with each note containing the amount and ownership details. When a transaction occurs, such as Alice paying Bob, Alice's wallet would publish these encrypted notes along with a zero-knowledge proof to the Bitcoin ledger. This proof mathematically confirms the transaction's validity and that Alice is authorized to spend the funds, all without revealing sensitive details publicly.

Independent indexers would verify these proofs and track unique "nullifiers" to prevent double-spending, ensuring the integrity of the system without a central authority. The design also introduces distinct keys for spending, viewing, and recovering transaction history, allowing users greater control over what information they share. This approach aims to add a robust layer of privacy to Bitcoin transactions, distinct from existing solutions like CoinJoin that offer partial anonymity but leave transaction amounts visible.