[bitcoin-dev] Graftroot: Private and efficient surrogate scripts under the taproot assumption

In my post on taproot I showed a simple commitment scheme for scripts that is very efficient that there exists some collection of pubkeys (like an M-of-N or even N-of-N) whos authorization is an acceptable alternative to whatever other conditions we might want to impose on a coin. If this holds then when spends happen via the plain signature path the existence of the alternative is never revealed, providing privacy with improved efficiency compared to not being private at all. Taproot suffers from a limitation that it only natively provides for one alternative. Trees or cascades of taproots can be done, but they have less privacy and efficiency than just a single level. E.g. a tree commitment has overhead that grows with the log of the number of alternatives. However, under the taproot assumption-- that there exists some monotone function on plain public keys and nothing else that is sufficient to authorize a transaction-- we can do even better. With graftroot, the participants establish a threshold key, optionally with a taproot alternative, just as they do with taproot. At any time, they can delegate their ability to sign to a surrogate script by signing that script (and just the script) with their taproot key, and sharing that delegation with whomever they choose. Later, when it comes time to spend the coin, if the signers aren't available and the script must be used, the redeeming party does whatever is required to satisfy the script (e.g. provides their own signature and a timelock, or whatnot) and presents that information along with the signer's signature of the script. The result is that instead of allowing only one alternative an unlimited number of alternatives can be provided. All are executed with equal efficiency to a single alternative, and the number of them is hidden without overhead. Alternatives can be provided for existing coins too, without requiring they get moved-- movement is only required to destroy the ability to use alternatives by changing keys. Allowing this kind of delegation makes sense because the same signers could have just signed the transaction outright. The new script simply stands in for them, if they're not available or cooperating. No special conditions are needed outside of the surrogate script on when the surrogate is allowed, because they can be written inside the surrogate. We've discussed delegation in script back to at least 2012-- with speculation that enabling it may have been an original motivation behind codeseperator. ... but these design discussions have gotten mired in how to express and connect the levels of delegation. But the case where delegation is accomplished with a simple unconditional signature is an especially simple case, and under the taproot assumption the only case that is ever needed. A naive implementation of this idea requires a complete signature every time a surrogate is used, which means 64 bytes of data (assuming 128 bit ECC). This is higher overhead than taproot. However, the non-interactive schnorr aggregation trick[1] can be applied to merge the S values of all graftroots and signatures in a transaction into a single aggregate. With this approach only a single R value for each graftroot need be published, lowering the overhead to ~32 bytes-- the same as taproot. This has a side benefit of binding the published grafts to a particular transaction, which might help avoid some screwups. In cases where the taproot assumption doesn't hold, taproot can still be used by setting the public key to a NUMS point, which preserves privacy (e.g. you can't distinguish txn where the key could never have been used.) A similar thing can be done for graftroot if the signature is not a proof of knowledge (commits to the public key): you select the signature in a NUMS manner, and then recover the applicable public key. Though this can't be done if the signature is a PoK, and it's probably a pretty good idea to make it a PoK. The primary limitation of this approach compared to taproot alternatives and trees is that it requires that anyone who wants to make use of a particular surrogate to interact with the participants and store the resulting signature because a single party couldn't compute it again on their own from public data. For trees and taproot alternatives, the alternatives can be setup without any interaction with the participants. The primary advantage is that it scales to any number of alternatives with small constant overhead, can be delegated after the fact, and can still be spent by the participants without overhead. Summarizing: A coin's authorizing contract is decomposed into a top level OR between a monotone function of pubkeys (such as N of N) and any number of arbitrary surrogate scripts which are acceptable authorizations. A key aggregate (see [2]) is formed, and is used to sign each of the the surrogates. Participants save these signatures. Later, when it comes time to spend the coin, if the pubkey holders are unwilling or unavailable, the spender presents and satisfies the relevant surrogate along with it's signature R-value and non-interactively aggregates the S-value into the transaction's overall aggregate signature. The result is 0-overhead if the signers cooperate, or ~32-byte overhead (plus the script) if they don't. This avoids the log() overhead of tree based schemes, and allows delegation to take place before or after the fact but requires storage. The potential for unexpected surrogate replay if keys are reused in foolish ways also needs to be kept in mind, though it may be somewhat mitigated by aggregation. The existence of unused surrogates is completely hidden. I believe this general design is simple and powerful enough that it avoids the rathole that earlier delegation discussions have suffered. [1] https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2017-May/014272.html And the secure construction at: https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2017-May/014308.html [2] https://eprint.iacr.org/2018/068