Tuesday, April 23, 2024

The Trailing Finality Layer: A stepping stone to proof of stake in Zcash

At Electrical Coin Co. (ECC) we’re exploring a transition in Zcash from the present proof-of-work (PoW) consensus to a proof-of-stake (PoS) consensus. We’re proposing a step on this path that we name the Trailing Finality Layer (TFL). If deployed, this may be mixed with Zcash’s current consensus; the ensuing consensus protocol at that time could be a hybrid of PoW and PoS.

The general aim is to allow finality and PoS on Zcash in a minimally disruptive method. Finality is a assure that when a block is finalized, that block and the transactions it accommodates can’t be rolled again. Finality can scale back delays for some use circumstances (resembling centralized alternate deposit wait occasions) and allow new enhancements resembling safer cross-chain bridges.

If the TFL strategy is adopted by the Zcash group, it may allow some new use circumstances, resembling staking ZEC to earn protocol rewards, whereas minimizing disruption to current use circumstances. Mining is an instance of a course of that may be impacted in a hybrid mannequin, as mining rewards could be diminished whereas the remainder of mining infrastructure and processes would stay unchanged.

We additionally intention to attenuate disruption to evaluation of consensus safety, as lots of the current consensus properties stay intact in a hybrid mannequin.

We’ve solely begun to outline the design of this PoW/PoS hybrid protocol. Many key particulars stay open questions, as we increase on beneath. By sharing our strategy early on this course of, we’re aiming to collect and incorporate suggestions as we go, discover potential collaborators, and stimulate dialogue about this strategy.

Get entangled

When you’re serious about offering suggestions or collaborating on this venture, please get in contact! An excellent alternative to study extra and be part of the dialog is to attend (in individual or nearly) the Interactive Design of a Zcash Trailing Finality Layer workshop I’m main at Zcon4. Additionally be happy to e mail me, [email protected], about your curiosity. We’re in search of contributors from a variety of backgrounds, together with technical, product, group, and any Zcash customers who wish to weigh in because the proposal evolves.

PoS transition background

ECC beforehand shared our rationale for why we imagine it’s in the very best curiosity of present and future ZEC customers to transition the protocol to proof of stake within the weblog submit Ought to Zcash transition from Proof of Work to Proof of Stake? and the Zcon3 presentation Motivations of Proof of Stake. In 2022, we revealed a high-level overview of our Proof-of-Stake Analysis strategy, a extra detailed Method, Focus, and Subsequent Steps companion submit, and gave a Zcon3 presentation about high-level design challenges in Proof of Stake.

A proof-of-stake transition path

Our imaginative and prescient for a transition to proof of stake contains not less than two main milestones:

  1. Shifting Zcash from its present proof-of-work mannequin to a hybrid PoW/PoS system.
  2. Shifting Zcash from a hybrid PoW/PoS system to pure PoS.

Our major motivation for proposing (not less than) two steps is to attenuate disruption of usability, security, safety, and the ecosystem throughout every step.

This strategy of transitioning from PoW to hybrid to PoS was executed by Ethereum with the deployment of the Beacon Chain (hybrid) then The Merge (pure PoS). 

Design objectives for a hybrid PoW/PoS system

ECC is refining the design of TFL with a number of objectives in thoughts, and we’ll doubtlessly be including extra as we proceed to develop this proposal. At the moment:

  • We wish to reduce disruption to current pockets use circumstances and UX. For instance, nothing ought to change within the consumer flows for storing or transferring funds, the format of addresses, and many others.
  • We wish to reduce complexity of safety analyses by preserving current evaluation outcomes the place potential.
  • We wish to allow new PoS use circumstances that enable cell shielded pockets customers to earn a return on delegated ZEC.
  • We wish to allow trust-minimized bridges and different advantages by offering a protocol with finality.
  • We wish to enhance the modularity of the consensus protocol. Modularity has a number of loosely outlined and associated meanings, e.g., it’s potential to know some consensus properties solely given data of a element of the protocol, and it’s potential to implement consensus guidelines in modular code parts with clear interfaces.

Trailing Finality Layer in a nutshell

The hybrid PoW/PoS protocol we envision at ECC is structured like at present’s Zcash NU5 protocol with a brand new Trailing Finality Layer. We describe it as a layer, as a result of the present nodes and most of their logic will proceed to function largely as-is with minimal adjustments, whereas a lot of the brand new performance will probably be supplied by new, supplementary parts and community protocols.

The left diagram reveals the present Zcash community, with a element that illustrates how two nodes are linked to one another throughout the context of your entire community. The correct reveals the addition of the TFL after deployment: Every node continues to have its authentic PoW element, however now has a further TFL element. The PoW parts nonetheless join to one another, as earlier than, and TFL parts use distinct connections to different TFL parts.

This new layer gives the blockchain with a trailing finality assure: after blocks are mined they are often finalized that means they might not be rolled again. This assure extends to any of the transactions throughout the blocks. It’s trailing as a result of this finality property follows the PoW mining system “trailing behind it.”

As a result of this hybrid design depends totally on PoW for producing new blocks, this protocol is immune to halting in the identical means Bitcoin or present Zcash is — though the finality assure could stall, as we describe subsequent.This design paradigm has each a theoretical and sensible monitor report: It’s analyzed in a analysis paper, Ebb-and-Movement Protocols, and it’s the identical paradigm utilized by Ethereum each within the pre-Merge hybrid design of the Beacon chain in addition to in present day Ethereum.

Why finality issues

Nakamoto PoW consensus, launched with Bitcoin and inherited by Zcash, gives probabilistic finality. This implies the possibility {that a} block could be rolled again falls as extra blocks are mined.

In our view, the first problem with this type of finality is that totally different individuals independently react to rollbacks. For instance, in all probability most individuals anticipate 1-block rollbacks (that are comparatively frequent), however as the scale of rollbacks develop bigger, three challenges emerge:

  1. Bigger rollbacks turn out to be extra uncommon, so some individuals could not have a course of or coverage for dealing with that scenario.
  2. Completely different individuals could have totally different insurance policies, so within the occasion of a giant rollback, the ecosystem could fracture as totally different individuals disagree on how you can get well.
  3. When counterparties require a sufficiently low tolerance for rollbacks, their interplay should incur a considerable delay.

Instance: trust-minimized bridge

To drive this level house, think about the precious use case of a trust-minimized bridge: ZEC despatched right into a bridge have to be locked up whereas an equal variety of proxy tokens are issued on one other community.

If a rollback reverts a bridge deposit after the proxy tokens are issued elsewhere, these ZEC are now not locked within the bridge, and the proxy tokens are actually unbacked. This breaks the peg of the bridge, and lots of bridge customers will concurrently lose funds. If the bridge designers resolve to require sufficient PoW blocks to make the chance of this occasion astronomically small, then issuing the proxy tokens on the opposite community may have a particularly massive delay.

Instance: alternate deposits

If a consumer deposits ZEC on a centralized alternate, their account on the alternate is credited the suitable quantity. If a rollback of this residue happens, the alternate accounting now has extra ZEC liabilities than precise ZEC held.

Exchanges search to deal with this by requiring extra PoW blocks to succeed in a sufficiently low chance of this occasion. Nevertheless, it is a balancing act: If an alternate imposes a delay of n hours, the chance continues to be not “astronomically small,” so customers are inconvenienced by n hours and the alternate nonetheless carries a sensible threat of a legal responsibility overhang occasion.

Moreover, due to problem quantity 2 above, totally different exchanges require totally different deposit delays, which doubtlessly confuses customers and places exchanges into competitors to tackle extra threat by accepting fewer block confirmations.


In distinction to probabilistic finality, a consensus protocol could present a finality assure. Protocols that do that be sure that all individuals agree on which set of blocks and transactions are last. The trade-off is that finality could fail to make progress within the occasion of community disruptions. As soon as the community recovers, the trailing finality can “catch up” to the PoW blocks which had been produced within the interim.1

Virtually, this implies if individuals are ready for a transaction to turn out to be last, generally they could want to attend an arbitrarily lengthy period of time. 

Finality addresses all three challenges to a point:

  1. Members now now not have to anticipate rollbacks of various possibilities when designing their procedures and insurance policies. As an alternative, they need to anticipate the danger that generally finality fails to progress in a well timed vogue.
  2. All individuals agree precisely which blocks and transactions are last, although they could disagree on how you can react if finality is stalled for lengthy durations of time.
  3. Members could now depend on the finality assure to make sure they solely react to some transactions when there’s zero probability of the transaction being reverted.

Within the examples above:

  • A trust-minimized bridge can depend on finality for issuing proxy tokens. This ensures the bridge won’t ever be under-collateralized. The trade-off is that so long as finality stalls, cross-bridge transfers can even be stalled.
  • All exchanges can use the identical finality assure, so customers can anticipate the identical deposit delay in all places (and it’s more likely to be notably decrease than the established order). The trade-off is that if finality stalls, new deposits can even be stalled, although all exchanges will behave constantly on this regard.

The top consequence for customers is that some high-value interactions (resembling bridging or alternate deposits) will now be sooner and safer more often than not. Typically finality could stall. When finality resumes, it’s going to “catch up” to the PoW chain, so customers who don’t want the finality assure can proceed utilizing this hybrid protocol, equally to how they use Zcash at present, and be unaffected if finality stalls.

Standing and open questions

This weblog submit introduction covers most of our R&D, to date, on the TFL design. There are nonetheless many open points that have to be resolved with collaboration and enter from the Zcash group earlier than a TFL design is prepared as a proposal for a Zcash improve.

An incomplete record of points that also have to be resolved embrace:

  1. Is that this common strategy acceptable to the Zcash group?
  2. How will new ZEC issuance be distributed between PoW, PoS, and any potential Dev Fund successor? This can be a key concern for each miners and potential stakers or delegators.
    1. How can we combine some other adjustments to issuance mechanics, such because the Zcash Sustainability Fund?
  3. The entire PoS accounting mechanics, resembling how bonding works, how delegation works, what sorts of slashing could happen, withdrawal delays and mechanics, and many others.
  4. How would PoS operations work together with all different Zcash ledger actions, resembling shielded swimming pools, and many others.? This will probably be a key space for understanding how privateness and staking work together.
  5. Ought to the chain tip ever stall with a view to certain the hole between the finalized block and the chain tip?
  6. Detailed safety analyses, together with financial safety, a case evaluation of PoW seize, PoS seize, community safety (esp. given two separate community protocols/layers).
  7. How can we guarantee cell shielded wallets are first-class individuals? For instance, can we guarantee they’ll safely and effectively delegate stake and handle stake-delegation positions? This contains each UX points, like delegation UI flows, in addition to implementation, resembling lightwalletd protocol adjustments.
  8. The best way to combine any TFL protocol adjustments with different proposed protocol adjustments in a protected and well timed method. Examples: Zcash Sustainability Fund, Zcash Shielded Property, bridging efforts, Namada integrations, and many others.
  9. Number of a particular finalizing PoS protocol. We’re at the moment specializing in utilizing Tendermint and ABCI as a strategy to quickly prototype and validate the design.
  10. Prototypes, testnets, code structure, and many others.

As we deal with the open points above, particularly the interplay with different protocol options and coordination with these groups on timing, we will start to refine a deployment timeline.

Subsequent steps

Our subsequent steps for this TFL R&D initiative are to get group suggestions on this weblog, host a workshop at Zcon4, and produce R&D updates on the open points above.

As we collaborate with different protocol growth groups, we’d wish to create a tentative deployment timeline that comes with all the different protocol characteristic efforts such because the Zcash Sustainability Fund, Zcash Shielded Property, and potential cross-chain bridge efforts.

Lastly, we’re looking for out different groups or people who’re serious about collaborating on this TFL venture, so if you happen to’re , please see the Get entangled part above.

1 In a pure finalizing PoS protocol, the equal “stalls” are literally network-wide halts. One energy of this hybrid design is that PoW needn’t halt, and if it doesn’t then customers who don’t rely on finality can proceed utilizing the community.

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