A brand new Bitcoin Enchancment Proposal titled “Chain Code Delegation for Non-public Collaborative Custody” has been opened on the Bitcoin BIPs repository, focusing on a long-standing privateness leak in multisig collaborations that depend on shared prolonged public keys.
The approach, authored by Bitkey engineers and collaborators, with a public explainer from Bitkey, proposes withholding BIP32 chain codes from non-privileged members in order that cosigners may help with restoration and coverage enforcement with out gaining sweeping visibility right into a consumer’s balances and transaction historical past. Bitkey says it plans to implement the scheme first if it turns into an accepted normal.
How The BIP Enhances Bitcoin Privateness
The crux of the privateness downside is well-known to pockets engineers and custodial companions: in typical collaborative or assisted multisig, the cosigner is handed an xpub plus chain code, which lets them deterministically derive the addresses in a consumer’s pockets and, by scanning the blockchain, infer balances and flows.
Bitkey’s submit frames the established order plainly: sharing a key with a 3rd celebration has “historically meant giving that celebration visibility right into a consumer’s pockets stability and transaction historical past.” The brand new strategy, they argue, “goals to take away that tradeoff” by withholding chain codes completely and revealing solely what’s minimally vital at spend time.
The proposed BIP’s summary is crisp in regards to the change in belief boundaries: “We suggest a brand new BIP for Chain Code Delegation, a collaborative custody approach that entails privileged members (delegatee) withholding BIP32 chain codes at key setup time from a delegator, and sharing solely sufficient data for non-privileged members to supply their signature.”
Within the non-blinded movement, “the delegatee derives a per-spend scalar tweak t from the (withheld) chain code, the delegator computes the kid key (x+t, P+tG), and produces a normal signature over the transaction’s sighash.” The blinded movement layers Schnorr blind signing on high in order that the cosigner stays oblivious to the ultimate message whereas nonetheless imposing the per-spend tweak, leveraging the linearity of Schnorr for correctness.
Functionally, the approach narrows what a cosigner can study and when. Reasonably than everlasting, international observability over all derived addresses, the cosigner solely sees per-spend information as wanted. The Bitkey explainer interprets this right into a user-facing promise: cosigners can help with restoration or spend insurance policies “with out studying something about unrelated transactions or general balances.”
If broadly adopted, that shift would make collaborative custody wallets extra similar to DIY multisig on privateness, whereas preserving the operational advantages which have made assisted fashions engaging to mainstream and enterprise customers.
The design has been incubating within the open. A technical dialogue thread on Delving Bitcoin over the summer time summarized two key advantages that stretch past privateness: limiting the “safety blast radius” as a result of, with out the chain code or undisclosed tweaks, a custodian’s secret is successfully unspendable for UTXOs they haven’t been explicitly delegated for; and tightly scoping what will get revealed in the meanwhile of spend, usually simply earlier than these outputs are consumed. That thread foreshadowed the BIP now filed and provides helpful context for reviewers tracing how the proposal hardened by suggestions.
Bitkey positions itself as the primary mover on implementation as soon as the usual is vetted. “Bitkey plans to be the primary to implement Chain Code Delegation in manufacturing,” the corporate wrote, arguing that it’s going to allow “a personal collaborative pockets—one thing that hasn’t been potential till now.” The specific intention is for the approach to be an “open, community-vetted normal that any pockets or custody supplier can undertake,” not a vendor-locked characteristic.
Outstanding trade accounts amplified the announcement on X. Principal government officer and chairman of Block, Inc Jack Dorsey highlighted Bitkey’s concentrate on pushing privateness enhancements from product to protocol.
At press time, Bitcoin traded at $111,398.
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