A brand new Bitcoin Enchancment Proposal titled “Chain Code Delegation for Personal Collaborative Custody” has been opened on the Bitcoin BIPs repository, concentrating on a long-standing privateness leak in multisig collaborations that depend on shared prolonged public keys.
The method, authored by Bitkey engineers and collaborators, with a public explainer from Bitkey, proposes withholding BIP32 chain codes from non-privileged individuals in order that cosigners may also 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 customary.
How The BIP Enhances Bitcoin Privateness
The crux of the privateness drawback 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 method, they argue, “goals to take away that tradeoff” by withholding chain codes solely and revealing solely what’s minimally needed at spend time.
The proposed BIP’s summary is crisp concerning the change in belief boundaries: “We suggest a brand new BIP for Chain Code Delegation, a collaborative custody method that entails privileged individuals (delegatee) withholding BIP32 chain codes at key setup time from a delegator, and sharing solely sufficient data for non-privileged individuals to offer their signature.”
Within the non-blinded circulation, “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 circulation layers Schnorr blind signing on high in order that the cosigner stays oblivious to the ultimate message whereas nonetheless implementing the per-spend tweak, leveraging the linearity of Schnorr for correctness.
Functionally, the method narrows what a cosigner can study and when. Somewhat 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 total balances.”
If extensively 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 enticing to mainstream and enterprise customers.
The design has been incubating within the open. A technical dialogue thread on Delving Bitcoin over the summer season summarized two key advantages that reach past privateness: limiting the “safety blast radius” as a result of, with out the chain code or undisclosed tweaks, a custodian’s secret’s successfully unspendable for UTXOs they haven’t been explicitly delegated for; and tightly scoping what will get revealed in the meanwhile of spend, typically simply earlier than these outputs are consumed. That thread foreshadowed the BIP now filed and affords helpful context for reviewers tracing how the proposal hardened by way of 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 non-public collaborative pockets—one thing that hasn’t been doable till now.” The express intention is for the method to be an “open, community-vetted customary that any pockets or custody supplier can undertake,” not a vendor-locked characteristic.
Distinguished trade accounts amplified the announcement on X. Principal govt officer and chairman of Block, Inc Jack Dorsey highlighted Bitkey’s deal with pushing privateness enhancements from product to protocol.
At press time, Bitcoin traded at $111,398.
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