Maxwell Eyes Bitcoin Smart Contracts After Blockstream

One of bitcoin's most respected developers, Gregory Maxwell, is returning to his cypherpunk roots with a series of new projects.

After nearly four years as CTO of high-profile bitcoin technology startup Blockstream, he's departed that position to focus exclusively on code. Mainly because, as Maxwell explained in his departure letter, he accomplished what he set out to do at the startup, addressing the "significant under-investment" in bitcoin's technology at the time he joined.

But with a "much larger and more active" developer community around bitcoin today, Maxwell is going into 2018 an untethered man set on improving bitcoin smart contracts.

In this pursuit, Maxwell published a paper on something called "Taproot" in mid-January, an idea that improves upon the privacy of MAST, an idea, long in the making, that could beef up bitcoin's smart contract abilities. Days later, Maxwell released another proposal called "Graftroot," improving on MAST further.

So, why is this focus so attractive for Maxwell?

Maxwell told CoinDesk:

"I expect every transaction to eventually use these tools, at least in limited ways. They are an incremental improvement, making things that were already more or less possible more private and efficient. They replace or make much better things like MAST."

And so far, many developers have praised Maxwell's new work.

"Taproot is annoyingly clever," Lightning Network creator Tadge Dryja quipped on Twitter, adding that while the idea sounds simple in hindsight, no one had thought about it before Maxwell.

Like a dandelion?

Maxwell's interest seems to be aligning with greater attention to MAST now that SegWit (a code change MAST depends on) has been activated on bitcoin.

To understand MAST, it's helpful to start by looking one of the common use cases of bitcoin today – M-and-N multi-signatures, which require that coins can only be spent if a certain number of users (such as two-of-two, three-of-five) approve the transaction. One problem that can arise in these types of transactions is that one party loses their private key to sign with or just decides altogether not to comply, and at that point the money is unspendable.

MAST allows users to add additional conditions for when a transaction can be spent in a more efficient way, helping to solve the above issue.

For example, a transaction can be set to lose the need for multiple signatures, if the multi-signature funds aren't spent after, say, 10 years. The magic of MAST is that it can cram all of this logic into one transaction efficiently.

In short, with Taproot and Graftroot, Maxwell has found a way to further improve privacy for these advanced transactions.

In Maxwell's eyes, the problem with MAST as it stands is that each MAST transaction looks different than a normal transaction, which can be harmful for privacy, since people viewing bitcoin's public ledger could theoretically glean which transactions are using MAST and in turn, more about financial transactions they have no business knowing anything about.

Taproot improves privacy in MAST instances where multi-signatures is used, by making those transactions, once settled on the blockchain, look the same as other transactions.

While Maxwell admits the use case is narrow, he told CoinDesk:

"There has been a lot of hype about smart contracts, but real and meaningful useage of them hasn't caught up with that hype yet."

But taking baby steps backed by real uses cases could help expand bitcoin's value proposition as programmable money.

Both proposals, according to Maxwell, make smart contracts "easier to implement, more fee efficient and more private," he said. "Taproot and Graftroot improve the backend technology for these advanced applications and by doing so will contribute to making them more accessible to people."

And this ability to accomplish complex transactions without exposing that complexity is where Taproot specifically got its name.

"Taproot is most efficient to use for smart contract usage that resembles the root system of plants like a dandelion – a thick central path and small alternatives," Maxwell said.

Simple but useful

While Maxwell is sold on the ideas, Taproot has attracted minor debate.

One of Maxwell's former co-workers at Blockstream, Mark Friendenbach, argued that Taproot shows that MAST, if implemented a certain way, could cause problems in the future. His contention isn't that Taproot itself is a bad proposal (in fact he argues the exact opposite), but that many of the MAST implementations on the table today aren't built with future iterations in mind.

While Chaincode developer Matt Corallo said Taproot's additional privacy is "absolutely massive to the ecosystem" and "should not be handwaved away for vague possibly-advantages."

As long as Taproot and Graftroot get approval from developers and the community, though, Maxwell said it is possible to roll out the technologies alongside "future signature system upgrades," such as aggregate signatures, another project Maxwell has contributed to.

But there could be some barriers still. According to Maxwell, bitcoin smart contracts are still a long way off.

"For real smart contracts like these to gain wide use a lot of additional work is required especially in the area of providing good user interfaces to use them," he said.

But still, if disagreements are overcome, implementing and rolling out MAST with Taproot and Graftroot will be relatively painless.

Maxwell concluded:

"Taproot is one of these ideas which are very simple to implement but very useful."

Disclosure: CoinDesk is a subsidiary of Digital Currency Group, which has an ownership stake in Blockstream.

Image via Consensus