Bitcoin Journal
Not ECDSA. Not Schnorr. Meet DahLIAS.
Mixture signatures aren’t new. They’ve been round because the early 2000s. However constructing one that truly works in Bitcoin’s safety mannequin, with Bitcoin’s elliptic curve, has by no means been confirmed. Builders speculated it is perhaps doable. They shared hand-wavy sketches and stated, “possibly it’d work like MuSig2, however throughout transaction inputs.” The thought lingered for years as developer folklore, shut, by no means provably confirmed.
That modified lately, when Jonas Nick and Tim Ruffing of Blockstream Analysis, along with Yannick Seurin of Ledger, revealed a paper that turned this cryptographic ghost story right into a concrete, provable consequence. DahLIAS is the primary formal, safe building of a full constant-size combination signature (CISA) scheme that works on Bitcoin’s native curve!
However that’s numerous phrases, so let’s break that down:
- Full aggregation: A number of signatures throughout totally different inputs are mixed into one — and the result’s a 64 byte signature whose measurement stays fixed, regardless of what number of signers or inputs.
- Cross-input: Every signer can authorize totally different inputs, and all mix into one signature.
It provides no vital new assumptions past these already relied on by Bitcoin. DahLIAS builds a brand new cryptographic primitive utilizing the identical math Bitcoin already depends on, unlocking a wholly new form of signature.
Let’s Speak About Curves and Signatures
Digital signatures are how Bitcoin proves {that a} person has licensed a transaction. Whenever you go to spend bitcoin, your pockets makes use of a personal key to signal a message, and the community verifies that signature utilizing the matching public key.
Bitcoin makes use of the secp256k1 curve. It’s quick, environment friendly, and has been battle-tested over time. It helps signature schemes like ECDSA (Bitcoin’s authentic signature algorithm) and Schnorr (added by Taproot in 2021), that are presently the one signature schemes permitted by Bitcoin consensus.
Historically, full signature aggregation relied on mathematical operations not supported by Bitcoin’s curve, secp256k1, which made it appear out of attain. These options have usually relied on different forms of elliptic curves. For instance, BLS (Boneh–Lynn–Shacham) signatures use a particular form of curve known as a pairing-friendly curve, which permits superior operations like combining many signatures, even on totally different messages, into one.
The issue is that BLS signatures don’t work on secp256k1. Whereas Schnorr was a pure improve from ECDSA, since each depend on the identical form of elliptic curve, including BLS could be a a lot larger leap and a departure from Bitcoin’s present safety mannequin. Although technically doable, it might introduce new cryptographic assumptions and add vital complexity to the protocol. Supporting a curve that’s pairing-friendly, like BLS12-381, could be a serious change for Bitcoin.
That is a part of why full signature aggregation has by no means been carried out on secp256k1.
Till now.
What Mixture Signatures Really Do
Most Bitcoin customers are aware of multisignatures. In a multisig pockets, a number of individuals collectively authorize the spending of a single UTXO or some particular “coin”. Everybody indicators the identical enter information. This setup is beneficial for issues like shared custody wallets.
Mixture signatures work in another way. As a substitute of a number of individuals signing the identical enter or coin, every signer authorizes a unique UTXO in a transaction. These separate signatures are then compressed into one compact proof. With DahLIAS, meaning a single 64-byte signature on Bitcoin’s secp256k1 curve that verifies all inputs without delay.
Meaning in case you have 5 inputs from 5 totally different individuals, the transaction wants 5 totally different signatures. With an combination signature, all of these might be bundled into one. Even when every signer is spending a unique enter and signing a unique a part of the transaction, the result’s one signature that proves the whole transaction was correctly licensed.
It’s like zipping a complete checklist of approvals into one file. The signature is compact, however nonetheless verifiably proves that every signer licensed their particular UTXO.
As a substitute of verifying 10 separate signatures, you confirm one.
This helps realign incentives for privateness. By lowering the signature overhead to a single 64-byte proof, DahLIAS lowers the price of combining inputs in CoinJoins, making it financially smarter to decide on privateness than to go with out it.
Why Half-Aggregation Bought Shut
Shortly after Schnorr signatures have been launched on Bitcoin, builders explored half-aggregation, as a strategy to compress a number of signatures however they weren’t mounted measurement. Every enter contributes to the scale of the signature, so the transaction nonetheless grows with each participant. DahLIAS fixes this by enabling full-aggregation throughout inputs and signers. Irrespective of how many individuals are concerned or what they’re signing, all their signatures compress into one constant-size, 64-byte proof.
What DahLIAS Really Unlocks
The primary profit right here is that DahLIAS are lowering the scale of advanced transactions.
DahLIAS makes use of a two-round interactive signing course of. It’s much like MuSig2 in that regard, nevertheless it isn’t a multisignature protocol as a result of it doesn’t require all contributors to co-sign the identical message. As a substitute, it aggregates totally different signatures on totally different messages throughout the transaction.
DahLIAS can be sooner to confirm than checking every signature individually, as much as twice as quick in some circumstances. Decrease verification prices make it simpler for extra individuals to run full nodes, which helps protect Bitcoin’s decentralization over time.
Importantly, DahLIAS comes with sturdy cryptographic ensures. The scheme contains formal safety proofs. Earlier ‘folklore’ approaches to full signature aggregation lacked this, and a few have been even later proven to be insecure. Thankfully they weren’t adopted prematurely.
It’s value repeating: DahLIAS just isn’t a multisig protocol. It isn’t corresponding to MuSig2 or FROST from a practical standpoint, even when it shares comparable cryptographic constructing blocks. It serves a unique goal. It gives a brand new strategy to encode many impartial approvals into one clear, verifiable bundle.
Future Instructions
You would possibly suppose: if DahLIAS is so highly effective, why isn’t it a BIP? Why not suggest it for Bitcoin consensus?
DahLIAS signatures don’t seem like Schnorr or ECDSA signatures. The verification algorithm is totally different. As a substitute of taking a single public key, message, and signature, a DahLIAS verifier takes lists of public keys and messages, and a single 64-byte proof.
This makes DahLIAS incompatible with Bitcoin’s present consensus guidelines. Supporting it on the base layer would require a consensus change. This paper doesn’t suggest that change, nevertheless it does one thing equally vital.
This paper exhibits {that a} full signature aggregation scheme for Bitcoin’s native curve is feasible.
That alone is a serious step ahead.
To make DahLIAS a part of Bitcoin, somebody would wish to put in writing a Bitcoin Enchancment Proposal (BIP), possibly even utilizing secp256k1lab. Meaning specifying the scheme intimately, contemplating its implications for consensus and implementation, and constructing group assist. This paper lays the cryptographic basis for that dialog.
The true worth of the DahLIAS paper is what it proves. Full signature aggregation on secp256k1 is not only a thought experiment. It’s concrete. It’s environment friendly. It’s safe. For years, the concept lived in developer folklore. Now, it’s written down, analyzed, and confirmed. All that’s left is to convey it to Bitcoin—if we wish it.
This can be a visitor put up by Kiara Bickers. Opinions expressed are fully their very own and don’t essentially mirror these of BTC Inc or Bitcoin Journal.
This put up Not ECDSA. Not Schnorr. Meet DahLIAS. first appeared on Bitcoin Journal and is written by Kiara Bickers.