Multisig is a well-known idea for many in Bitcoin: a multisig transaction requires approval from a number of events earlier than it may be executed. We distinguish between “n-of-n” multi-signatures, the place the variety of concerned events is n, they usually all must approve, and “t-of-n” threshold signatures, the place solely a smaller quantity t of individuals must approve. Cryptographic schemes like MuSig, MuSig-DN and MuSig2 for multi-signatures and FROST by Komlo and Goldberg for threshold signatures can scale back transaction value and enhance privateness of multisig wallets.
To date, within the Bitcoin Group FROST has solely been utilized in experimental implementations. On this publish, we clarify why that is the case and the way we intention to advance FROST in a Bitcoin manufacturing setting via our current publication of a BIP draft for the ChillDKG distributed key technology protocol.
First, what are the advantages of FROST?
Privateness and Effectivity Good points with MuSig2 and FROST
With MuSig2 and FROST, despite the fact that a number of individuals contribute to the signing course of, the result is a single signature.
This not solely provides higher privateness to the individuals by making the transaction appear like as unusual singlesig-wallet transaction. It additionally trims down the transaction, decreasing its measurement and due to this fact reducing the transaction price. All nice issues!
MuSig2 and FROST permit Bitcoin customers to function a multisig pockets with the identical transaction value as a daily single-signature pockets. The price advantages are particularly important for techniques with a lot of signers and frequent transactions, similar to federated sidechains like Liquid or Fedimint. In contrast to conventional multisig, which leaves a definite fingerprint that permits blockchain observers to establish transactions of the pockets, FROST-based wallets are indistinguishable from common single-signature wallets on the blockchain. Due to this fact, they supply an enchancment in privateness in comparison with conventional multisig wallets.
Whereas MuSig2 has seen adoption from the Bitcoin trade, the identical can’t be stated for FROST so far as we all know. This can be stunning, contemplating the existence of a number of FROST implementations, similar to in ZF FROST (by the Zcash Basis), secp256kfun (by Lloyd Fournier), and an experimental implementation in libsecp256k1-zkp (by Jesse Posner and Blockstream Analysis). There’s even a IETF specification for FROST, RFC 9591 (although it isn’t suitable with Bitcoin as a consequence of Taproot tweaking and x-only public keys). Probably the most believable explanations is that FROST’s key technology course of is significantly extra advanced in comparison with MuSig2.
The Unresolved Puzzle of FROST in Manufacturing Programs
FROST basically consists of two components: key technology and signing. Whereas the signing course of carefully resembles that of MuSig2, key technology is considerably extra concerned than in MuSig2. Key technology in FROST is both trusted or distributed:
- Trusted key technology includes a “trusted vendor” who generates the important thing and distributes key shares to the signers. The vendor represents a single level of failure: if malicious or hacked, the FROST pockets is susceptible to being emptied.
- Distributed key technology (DKG), whereas eliminating the necessity for a trusted vendor, presents its personal challenges: All individuals have to be concerned in an interactive key technology “ceremony” run earlier than signing can begin.
The Core Problem: Settlement
DKG sometimes requires safe (i.e., authenticated and encrypted) channels between individuals to ship secret shares to particular person signers, and a safe settlement mechanism. The aim of the safe settlement mechanism is to make sure that all individuals finally attain settlement over the outcomes of the DKG, which embrace not solely parameters such because the generated threshold public key, but additionally whether or not no error occurred and the ceremony was not disrupted by a misbehaving participant.
Whereas the IETF specification considers DKG out of scope completely, the FROST implementations talked about above don’t implement safe settlement, leaving this activity to the library consumer. However settlement isn’t trivial to implement: there exist numerous protocols and flavors of settlement, starting from easy echo broadcast schemes to full-fledged Byzantine consensus protocols, and their safety and availability ensures differ considerably, and generally subtly.
Regardless of the confusion which will come up as a consequence of this jungle of settlement protocols, the precise taste of settlement that DKG depends on is commonly not clearly communicated to engineers, leaving them at nighttime.
ChillDKG: a Standalone DKG for FROST
To beat this impediment, we suggest ChillDKG, a brand new “ready-to-use” DKG protocol tailor-made to the use in FROST (draft). We offer an in depth description within the type of a draft of a Bitcoin Enchancment Proposal (BIP), which is meant to function a specification for implementers.
The primary function of ChillDKG is that it’s standalone: The institution of safe communications and safe settlement is completed inside the protocol, whereas all of this underlying complexity is hidden behind a easy and hard-to-misuse API. Consequently, ChillDKG is able to use in follow and doesn’t depend on any setup assumption, besides that every signer has selected the set of co-signers as recognized by particular person public keys. ChillDKG relies on the SimplPedPop protocol, in whose design and formal safety proof Blockstream Analysis has been concerned, see, the CRYPTO 2023 paper “Sensible Schnorr Threshold Signatures With out the Algebraic Group Mannequin” by Chu, Gerhart, Ruffing (Blockstream Analysis), and Schröder
Further objectives for ChillDKG’s design embrace:
- Broad applicability: ChillDKG helps a variety of situations, from these the place the signing units are owned and linked by a single particular person to these the place a number of house owners handle the units from distinct areas.
- Easy backups: As a substitute of getting to again up secrets and techniques acquired from the opposite signers in a safe location, ChillDKG permits restoring the pockets solely from the system seed and public information that’s the identical for all DKG individuals. Consequently, an attacker getting access to the general public backup information doesn’t acquire the key signing key, and if a consumer loses their backup, they’ll request it from one other trustworthy signer.
The ChillDKG BIP is at present in draft stage, and we’re searching for suggestions on design decisions and implementation particulars. Whereas the specification is usually full, it lacks take a look at vectors, and we’re contemplating including some further options (e.g., “identifiable aborts”). As soon as finalized, the ChillDKG BIP can be utilized together with a BIP for FROST signing to instantiate your entire FROST protocol.
It is a visitor publish by Jonas Nick, Kiara Bickers, and Tim Ruffing. Opinions expressed are completely their very own and don’t essentially mirror these of BTC Inc or Bitcoin Journal.
