Researchers at Bitcoin cryptography firm Alloc Init published a 56-page paper on September 24 proposing Shielded Bitcoin, a design for Zcash-style private transfers on the Bitcoin base layer that needs no soft fork or consensus change. The specification, authored by Clara Shikhelman, Mikhail Komarov and Aleksei Moskvin , would hide the sender, recipient and amount of a payment while Bitcoin records and orders the data without validating it.
How the design borrows Zcash’s privacy model
Shielded Bitcoin holds value in encrypted notes, and each transfer carries a zero-knowledge proof that the sender controls the notes being spent and that inputs and outputs balance. A public marker called a nullifier lets software reject double spends without revealing which note was used. Unlike Zcash, which enforces those rules through its own blockchain, the proposal treats Bitcoin as a “neutral publication and ordering layer”: separate software called indexers verifies the proofs and rebuilds the shielded state. “Like Zcash and Monero, Shielded Bitcoin preserves the privacy of who paid whom and how much, not that a shielded transfer happened,” the paper states.
The unfinished parts: deposits, withdrawals and cost
The paper covers only transfers inside the system. How BTC enters and leaves is left to a separate paper built on Bitcoin PIPEs v2, earlier Alloc Init research that encrypts a Bitcoin signing key so it can be recovered only with a valid proof. The current design uses the Groth16 proof system, whose security rests on a trusted setup ceremony, and publishes each transfer in an OP_RETURN output of roughly 625 vbytes for two inputs and two outputs. Komarov estimates a private transfer at about 700 vbytes, roughly four times the miner fees of a standard Bitcoin transaction. The proposal joins a wider effort to add Zcash-style privacy to Bitcoin, including Citrea’s acquisition of privacy builder Crest .
Reaction and the state of privacy coins
The paper lands amid a privacy-coin revival. Zcash’s shielded pools held about 4.9 million ZEC as of Friday, roughly 29 percent of issued coins and worth about $7.8 billion, and 21Shares listed Europe’s first Zcash ETP in late September. Some developers were skeptical: Vadim Zavodil argued that a new shielded pool “starts at zero,” while Zerocash co-author Eli Ben-Sasson welcomed the direction, noting the original goal of Zerocash was to bring privacy to Bitcoin.
