Watch this: Alice sends Zcash. Bob receives USDC on Aztec. Nobody, not even the system facilitating it, knows who Alice or Bob are.

And Bob can now do something with that money. Privately.

This is the connection between private money and a private economy where that money can actually be used.

Zcash has already achieved something monumental: truly private money. It’s the store of value that Bitcoin promised (but made transparent). Like, digital gold that actually stays hidden.

But here's the thing about gold - you don't buy coffee with gold bars. You need an economy where that value can flow, work, and grow. Privately.

Money Under the Mattress

While other projects are trying to bolt privacy onto existing chains as an afterthought, Zcash is one of the oldest privacy projects in Web3. It's achieved what dozens of projects are still chasing: a truly private store of value.

This is critical infrastructure for freedom. The ability to store value privately is a fundamental right, a hedge against surveillance, and a given when using cash. We need a system that provides the same level of privacy guarantees as cash. Right now, there's over $1.1 billion sitting in Zcash's shielded pool, private wealth that's perfectly secure but essentially frozen.

Why frozen? Because the moment that shielded $ZEC tries to do anything beyond basic transfers: earn yield, get swapped for stablecoins, enter a liquidity pool, it must expose itself. The privacy in this format is destroyed.

This isn't Zcash's failure. They built exactly what they set out to build: the world's best private store of value. The failure is that the rest of crypto hasn't built where that value can actually work.

The Privacy Landscape Has an Imbalance

What happens when you want to do more than just send money? What happens when you want privacy after you transfer your money?

Private Digital Money (i.e., “Transfer Privacy,” largely solved by Zcash):

• Zcash: est. 2016
• Everyone else: building variants of digital money at the transaction or identity level
  
  • Monero
  • Ethereum privacy pools
  • 0xbow
  • Payy
  • Every privacy stablecoin project
  • Every confidential L2
  • Every privacy project you've ever heard of

Private World Computer (i.e., After-the-Transfer Privacy):

• Aztec

Everyone else is competing to build better ways to hide money. Zcash has already built the private store of value, and Aztec has built the only way to use hidden money.

The Locked Liquidity Problem

Here's the trillion-dollar question: What good is private money if you can't use it?

Right now, Zcash's shielded pool contains billions in value. This is money in high-security vaults. But unlike gold in vaults that can be collateralized, borrowed against, or deployed, this private value just sits there.

Every $ZEC holder faces two impossible choices:

1. Keep it shielded and forfeit all utility
2. Unshield it to use it and forfeit all privacy

Our demo breaks this false sense of choice. For the first time, shielded value can move to a place where it remains private AND becomes useful.

The Private World Computer

Here's how you can identify whether you’re dealing with a private world computer, or just private digital money:

Without a private world computer (every other privacy solution):

• Receive salary privately → Can't invest it
• Store savings privately → Can't earn yield
• Send money privately → Recipient can't use it privately

With a private world computer (only Aztec):

• Receive salary privately → Invest it privately
• Store savings privately → Earn APY privately
• Send payment privately → Recipient spends it privately

This is basic financial common sense. Your money should grow. It should work. It should be useful.

The technical reality is that this requires private smart contracts. Aztec is building the only way to interact privately with smart contracts. These smart contracts themselves can remain completely hidden. Your private money can finally do what money is supposed to do: work for you.

What We Actually Built

Our demo proves these two worlds can connect:

1. The Vault: Zcash
2. The Engine: Aztec (where private money becomes useful)

We built the bridge between storing privately and doing privately.

The technical innovation - "partial notes" - are like temporary lockboxes that self-destruct after one use. Money can be put privately into these lockboxes, and a key can be privately handed to someone to unlock it. No one knows who put the money in, where the key came from, or who uses the key. You can read more about how they work here. But what matters isn't the mechanism. 

What matters is that Alice's Zcash can become Bob's working capital on Aztec without anyone knowing about either of them.

As a result, Bob receives USDC that he can:

• Earn yield on
• Trade with
• Pay suppliers with
• Build a business on
• All privately

Why This Required Starting from Scratch (and 8 years of building)

You can't bolt privacy onto existing systems. You can't take Ethereum and make it private. You can't take a transparent smart contract platform and add privacy as a feature.

Aztec had to be built from the ground up as a private world computer because after-the-transfer privacy requires rethinking everything:

• How state is managed
• How contracts execute
• How proofs are generated
• How transactions are ordered

This is why there's only one name building fully private smart contracts. From the beginning, Aztec has been inspired by the work Zcash has done to create a private store of value. That’s what led to the vision for a private world computer.

Everyone else is iterating on the same transfer privacy problem. Aztec solves a fundamentally different problem.

The Obvious Future

Once you see it, you can't unsee it: Privacy without utility is only the first step.

Every privacy project will eventually need what Aztec built. Because their users will eventually ask: "Okay, my money is private... now what?"

• Zcash users will want their $ZEC to earn yield
• Privacy pool users will want to do more than just mix
• Private stablecoin users will want to actually… use their stablecoins

This demo that connects Zcash to Aztec is the first connection between the old world (private transfers) and the new world (private everything else).

What This Means

For Zcash Holders: Your shielded $ZEC can finally do something without being exposed.

For Developers: Stop trying to build better mattresses to hide money under. Start building useful applications on the only platform that keeps them private. 

For the Industry: The privacy wars are over. There's transfer privacy (solved by Zcash) and after-the-transfer privacy (just Aztec).

What’s Next? 

This demo is live. The code is open source. The bridge between private money and useful private money exists.

But this is just the beginning. Every privacy project needs this bridge. Every private payment network needs somewhere for those payments to actually be used.

We're not competing with transfer privacy. We're continuing it.

Your private money yearns for the private economy.

Welcome to after-the-transfer privacy. Welcome to Aztec.

Privacy has emerged as a major driver for the crypto industry in 2025. We’ve seen the explosion of Zcash, the Ethereum Foundation’s refocusing of PSE, and the launch of Aztec’s testnet with over 24,000 validators powering the network. Many apps have also emerged to bring private transactions to Ethereum and Solana in various ways, and exciting technologies like ZKPassport that privately bring identity on-chain using Noir have become some of the most talked about developments for ushering in the next big movements to the space. 

Underpinning all of these developments is the emerging consensus that without privacy, blockchains will struggle to gain real-world adoption. 

Without privacy, institutions can’t bring assets on-chain in a compliant way or conduct complex swaps and trades without revealing their strategies. Without privacy, DeFi remains dominated and controlled by advanced traders who can see all upcoming transactions and manipulate the market. Without privacy, regular people will not want to move their lives on-chain for the entire world to see every detail about their every move. 

While there's been lots of talk about privacy, few can define it. In this piece we’ll outline the three pillars of privacy and gives you a framework for evaluating the privacy claims of any project. 

The Three Pillars of Privacy 

True privacy rests on three essential pillars: transaction privacy, identity privacy, and computational privacy. It is only when we have all three pillars that we see the emergence of a private world computer. 

Transaction: What is being sent?

Transaction privacy means that both inputs and outputs are not viewable by anyone other than the intended participants. Inputs include any asset, value, message, or function calldata that is being sent. Outputs include any state changes or transaction effects, or any transaction metadata caused by the transaction. Transaction privacy is often primarily achieved using a UTXO model (like Zcash or Aztec’s private state tree). If a project has only the option for this pillar, it can be said to be confidential, but not private. 

Identity: Who is involved?

Identity privacy means that the identities of those involved are not viewable by anyone other than the intended participants. This includes addresses or accounts and any information about the identity of the participants, such as tx.origin, msg.sender, or linking one’s private account to public accounts. Identity privacy can be achieved in several ways, including client-side proof generation that keeps all user info on the users’ devices. If a project has only the option for this pillar, it can be said to be anonymous, but not private. 

Computation: What happened? 

Computation privacy means that any activity that happens is not viewable by anyone other than the intended participants. This includes the contract code itself, function execution, contract address, and full callstack privacy. Additionally, any metadata generated by the transaction is able to be appropriately obfuscated (such as transaction effects, events are appropriately padded, inclusion block number are in appropriate sets). Callstack privacy includes which contracts you call, what functions in those contracts you’ve called, what the results of those functions were, any subsequent functions that will be called after, and what the inputs to the function were. A project must have the option for this pillar to do anything privately other than basic transactions. 

From private money to a private world computer 

Bitcoin ushered in a new paradigm of digital money. As a permissionless, peer-to-peer currency and store of value, it changed the way value could be sent around the world and who could participate. Ethereum expanded this vision to bring us the world computer, a decentralized, general-purpose blockchain with programmable smart contracts. 

Given the limitations of running a transparent blockchain that exposes all user activity, accounts, and assets, it was clear that adding the option to preserve privacy would unlock many benefits (and more closely resemble real cash). But this was a very challenging problem. Zcash was one of the first to extend Bitcoin’s functionality with optional privacy, unlocking a new privacy-preserving UTXO model for transacting privately. As we’ll see below, many of the current privacy-focused projects are working on similar kinds of private digital money for Ethereum or other chains. 

Now, Aztec is bringing us the final missing piece: a private world computer.

A private world computer is fully decentralized, programmable, and permissionless like Ethereum and has optional privacy at every level. In other words, Aztec is extending all the functionality of Ethereum with optional transaction, identity, and computational privacy. This is the only approach that enables fully compliant, decentralized applications to be built that preserve user privacy, a new design space that we see as ushering in the next Renaissance for the space. 

Where are we now? 

Private digital money

Private digital money emerges when you have the first two privacy pillars covered - transactions and identity - but you don’t have the third - computation. Almost all projects today that claim some level of privacy are working on private digital money. This includes everything from privacy pools on Ethereum and L2s to newly emerging payment L1s like Tempo and Arc that are developing various degrees of transaction privacy 

When it comes to digital money, privacy exists on a spectrum. If your identity is hidden but your transactions are visible, that's what we call anonymous. If your transactions are hidden but your identity is known, that's confidential. And when both your identity and transactions are protected, that's true privacy. Projects are working on many different approaches to implement this, from PSE to Payy using Noir, the zkDSL built to make it intuitive to build zk applications using familiar Rust-like syntax. 

The Private World Computer 

Private digital money is designed to make payments private, but any interaction with more complex smart contracts than a straightforward payment transaction is fully exposed. 

What if we also want to build decentralized private apps using smart contracts (usually multiple that talk to each other)? For this, you need all three privacy pillars: transaction, identity, and compute. 

If you have these three pillars covered and you have decentralization, you have built a private world computer. Without decentralization, you are vulnerable to censorship, privileged backdoors and inevitable centralized control that can compromise privacy guarantees. 

‍

Aztec: the Private World Computer 

What exactly is a private world computer? A private world computer extends all the functionality of Ethereum with optional privacy at every level, so developers can easily control which aspects they want public or private and users can selectively disclose information. With Aztec, developers can build apps with optional transaction, identity, and compute privacy on a fully decentralized network. Below, we’ll break down the main components of a private world computer.

Private Smart Contracts 

A private world computer is powered by private smart contracts. Private smart contracts have fully optional privacy and also enable seamless public and private function interaction. 

Private smart contracts simply extend the functionality of regular smart contracts with added privacy. 

As a developer, you can easily designate which functions you want to keep private and which you want to make public. For example, a voting app might allow users to privately cast votes and publicly display the result. Private smart contracts can also interact privately with other smart contracts, without needing to make it public which contracts have interacted. 

Aztec’s Three Pillars of Privacy

Transaction: Aztec supports the optionality for fully private inputs, including messages, state, and function calldata. Private state is updated via a private UTXO state tree.

Identity: Using client-side proofs and function execution, Aztec can optionally keep all user info private, including tx.origin and msg.sender for transactions. 

Computation: The contract code itself, function execution, and call stack can all be kept private. This includes which contracts you call, what functions in those contracts you’ve called, what the results of those functions were, and what the inputs to the function were. 

Decentralization

A decentralized network must be made up of a permissionless network of operators who run the network and decide on upgrades. Aztec is run by a decentralized network of node operators who propose and attest to transactions. Rollup proofs on Aztec are also run by a decentralized prover network that can permissionlessly submit proofs and participate in block rewards. Finally, the Aztec network is governed by the sequencers, who propose, signal, vote, and execute network upgrades.

What Can You Build with a Private World Computer?

Private DeFi

A private world computer enables the creation of DeFi applications where accounts, transactions, order books, and swaps remain private. Users can protect their trading strategies and positions from public view, preventing front-running and maintaining competitive advantages. Additionally, users can bridge privately into cross-chain DeFi applications, allowing them to participate in DeFi across multiple blockchains while keeping their identity private despite being on an existing transparent blockchain.

Private Dark Pools

This technology makes it possible to bring institutional trading activity on-chain while maintaining the privacy that traditional finance requires. Institutions can privately trade with other institutions globally, without having to touch public markets, enjoying the benefits of blockchain technology such as fast settlement and reduced counterparty risk, without exposing their trading intentions or volumes to the broader market.

Private RWAs & Stablecoins

Organizations can bring client accounts and assets on-chain while maintaining full compliance. This infrastructure protects on-chain asset trading and settlement strategies, ensuring that sophisticated financial operations remain private. A private world computer also supports private stablecoin issuance and redemption, allowing financial institutions to manage digital currency operations without revealing sensitive business information.

Compliant Apps

Users have granular control over their privacy settings, allowing them to fine-tune privacy levels for their on-chain identity according to their specific needs. The system enables selective disclosure of on-chain activity, meaning users can choose to reveal certain transactions or holdings to regulators, auditors, or business partners while keeping other information private, meeting compliance requirements.

Let’s build

The shift from transparent blockchains to privacy-preserving infrastructure is the foundation for bringing the next billion users on-chain. Whether you're a developer building the future of private DeFi, an institution exploring compliant on-chain solutions, or simply someone who believes privacy is a fundamental right, now is the time to get involved.

Follow Aztec on X to stay updated on the latest developments in private smart contracts and decentralized privacy technology. Ready to contribute to the network? Run a node and help power the private world computer. 

The next Renaissance is here, and it’s being powered by the private world computer.

Special thanks to Santiago Palladino, Phil Windle, Alex Gherghisan, and Mitch Tracy for technical updates and review.

On September 17th, 2025, a new network upgrade was deployed, making Aztec more secure and flexible for home stakers. This upgrade, shipped with all the features needed for a fully decentralized network launch, includes a completely redesigned slashing system that allows inactive or malicious operators to be removed, and does not penalize home stakers for short outages. 

With over 23,000 operators running validators across 6 continents (in a variety of conditions), it is critical not to penalize nodes that temporarily drop due to internet connectivity issues. This is because users of the network are also found across the globe, some of whom might have older phones. A significant effort was put into shipping a low-memory proving mode that allows older mobile devices to send transactions and use privacy-preserving apps. 

The network was successfully deployed, and all active validators on the old testnet were added to the queue of the new testnet. This manual migration was only necessary because major upgrades to the governance contracts had gone in since the last testnet was deployed. The new testnet started producing blocks after the queue started to be “flushed,” moving validators into the rollup. Because the network is fully decentralized, the initial flush could have been called by anyone. The network produced ~2k blocks before an invalid block made it to the chain and temporarily stalled block production. Block production is now restored and the network is healthy. This post explains what caused the issue and provides an update on the current status of the network. 

Note: if you are a network operator, you must upgrade to version 2.0.3 and restart your node to participate in the latest testnet. If you want to run a node, it’s easy to get started.

What’s included in the upgrade? 

This upgrade was a team-wide effort that optimized performance and implemented all the mechanisms needed to launch Aztec as a fully decentralized network from day 1. 

Feature highlights include: 

• Improved node stability: The Aztec node software is now far more stable. Users will see far fewer crashes and increased performance in terms of attestations and blocks produced. This translates into a far better experience using testnet, as transactions get included much faster.

• Boneh–Lynn–Shacham (BLS) keys: When a validator registers on the rollup, they also provide keys that allow BLS signature aggregation. This unlocks future optimizations where signatures can be combined via p2p communication, then verified on Ethereum, while proving that the signatures come from block proposers.

• Low-memory proving mode: The client-side proving requirements have dropped dramatically from 3.7GB to 1.3GB through a new low-memory proving mode, enabling older mobile devices to send Aztec transactions and use apps like zkPassport. 

• AVM performance: The Aztec Virtual Machine (AVM) performance has seen major improvements with constraint coverage jumping from 0% to approximately 90-95%, providing far more secure AVM proving and more realistic proving performance numbers from provers. 

• Flexible key management: The system now supports flexible key management through keystores, multi-EOA support, and remote signers, eliminating the need to pass private keys through environment variables and representing a significant step toward institutional readiness. 
• Redesigned slashing: Slashing has been redesigned to provide much better consensus guarantees. Further, the new configuration allows nodes not to penalize home stakers for short outages, such as 20-minute interruptions. 

• Slashing Vetoer: The Slasher contract now has an explicit vetoer: an address that can prevent slashing. At Mainnet, the initial vetoer will be operated by an independent group of security researchers who will also provide security assessments on upgrades. This acts as a failsafe in the event that nodes are erroneously trying to slash other nodes due to a bug.

With these updates in place, we’re ready to test a feature-complete network. 

What happened after deployment? 

As mentioned above, block production started when someone called the flush function and a minimum number of operators from the queue were let into the validator set. 

Shortly thereafter, while testing the network, a member of the Aztec Labs team spun up a “bad” sequencer that produced an invalid block proposal. Specifically, one of the state trees in the proposal was tampered with. 

The expectation was that this would be detected immediately and the block rejected. Instead, a bug was discovered in the validator code where the invalid block proposal wasn't checked thoroughly enough. In effect, the proposal got enough attestations, so it was posted to the rollup. Due to extra checks in the nodes, when the nodes pulled the invalid block from Ethereum, they detected the tampered tree and refused to sync it. This is a good outcome as it prevented the attack. Additionally, prover nodes refused to prove the epoch containing the invalid block. This allowed the rollup to prune the entire bad epoch away. After the prune, the invalid state was reset to the last known good block.

The prune revealed another, smaller bug, where, after a failed block sync, a prune does not get processed correctly, requiring a node restart to clear up. This led to a 90-minute outage from the moment the block proposal was posted until the testnet recovered. The time was equally split between waiting for pruning to happen and for the nodes to restart in order to process the prune.

The Fix

Validators were correctly re-executing all transactions in the block proposals and verifying that the world state root matched the one in the block proposal, but they failed to check that intermediate tree roots, which are included in the proposal and posted to the rollup contract on L1, were also correct. The attack tweaked one of these intermediate roots while proposing a correct world state root, so it went unnoticed by the attestors. 

As mentioned above, even though the block made it through the initial attestation and was posted to L1, the invalid block was caught by the validators, and the entire epoch was never proven as provers refused to generate a proof for the inconsistent state. 

A fix was pushed that resolved this issue and ensured that invalid block proposals would be caught and rejected. A second fix was pushed that ensures inconsistent state is removed from the uncommitted cache of the world state.

What’s Next

Block production is currently running smoothly, and the network health has been restored. 

Operators who had previously upgraded to version 2.0.3 will need to restart their nodes. Any operator who has not upgraded to 2.0.3 should do so immediately. 

Slashing has also been functioning as expected. Below you can see the slashing signals for each round. A single signal can contain votes for multiple validators, but a validator's attester needs to receive 65 votes to be slashed.

Join us this Thursday, September 25, 2025, at 4 PM CET on the Discord Town Hall to hear more about the 2.0.3 upgrade. To stay up to date with the latest updates for network operators, join the Aztec Discord and follow Aztec on X.

The TL;DR:

Payy, a privacy-focused payment network, just rewrote its entire ZK architecture from Halo2 to Noir while keeping its network live, funds safe, and users happy. 

Code that took months to write now takes weeks (with MVPs built in as little as 30 minutes). Payy’s codebase shrank from thousands of lines to 250, and now their entire engineering team can actually work on its privacy infra. 

This is the story of how they transformed their ZK ecosystem from one bottlenecked by a single developer to a system their entire team can modify and maintain.

Starting with Halo2

Eighteen months ago, Payy faced a deceptively simple requirement: build a privacy-preserving payment network that actually works on phones. That requires client-side proving.

"Anyone who tells you they can give you privacy without the proof being on the phone is lying to you," Calum Moore - Payy's Technical Lead - states bluntly.

To make a private, mobile network work, they needed:

• Mobile proof generation with sub-second performance
• Minimal proof sizes for transmission over weak mobile signals
• Low memory footprint for on-device proving
• Ethereum verifier for on-chain settlement

To start, the team evaluated available ZK stacks through their zkbench framework:

STARKs (e.g., RISC Zero): Memory requirements made them a non-starter on mobile. Large proof sizes are unsuitable for mobile data transmission.

Circom with Groth16: Required trusted setup ceremonies for each circuit update. It had “abstracted a bit too early” and, as a result, is not high-level enough to develop comfortably, but not low-level enough for controls and optimizations, said Calum.

Halo2: Selected based on existing production deployments (ZCash, Scroll), small proof sizes, and an existing Ethereum verifier. As Calum admitted with the wisdom of hindsight: “Back a year and a half ago, there weren’t any other real options.”

Bus factor = 1 😳

Halo2 delivered on its promises: Payy successfully launched its network. But cracks started showing almost immediately.

First, they had to write their own chips from scratch. Then came the real fun: if statements.

"With Halo2, I'm building a chip, I'm passing this chip in... It's basically a container chip, so you'd set the value to zero or one depending on which way you want it to go. And, you'd zero out the previous value if you didn't want it to make a difference to the calculation," Calum explained, “when I’m writing in Noir, I just write ‘if’. "

With Halo2, writing an if statement (programming 101) required building custom chip infra. 

Binary decomposition, another fundamental operation for rollups, meant more custom chips. The Halo2 implementation quickly grew to thousands of lines of incomprehensible code.

And only Calum could touch any of it.

The Bottleneck

"It became this black box that no one could touch, no one could reason about, no one could verify," he recalls. "Obviously, we had it audited, and we were confident in that. But any changes could only be done by me, could only be verified by me or an auditor."

In engineering terms, this is called a bus factor of one: if Calum got hit by a bus (or took a vacation to Argentina), Payy's entire proving system would be frozen. "Those circuits are open source," Calum notes wryly, "but who's gonna be able to read the Halo2 circuits? Nobody."

Evaluating Noir: One day, in Argentina…

During a launch event in Argentina, "I was like, oh, I'll check out Noir again. See how it's going," Calum remembers. He'd been tracking Noir's progress for months, occasionally testing it out, waiting for it to be reliable.

"I wrote basically our entire client-side proof in about half an hour in Noir. And it probably took me - I don't know, three weeks to write that proof originally in Halo2."

Calum recreated Payy's client-side proof in Noir in 30 minutes. And when he tested the proving speed, without any optimization, they were seeing 2x speed improvements.

"I kind of internally… didn't want to tell my cofounder Sid that I'd already made my decision to move to Noir," Calum admits. "I hadn't broken it to him yet because it's hard to justify rewriting your proof system when you have a deployed network with a bunch of money already on the network and a bunch of users."

Rebuilding (Ship of Theseus-ing) Payy

Convincing a team to rewrite the core of a live financial network takes some evidence. The technical evaluation of Noir revealed improvements across every metric:

Proof Generation Time: Sub-0.5 second proof generation on iPhones. "We're obsessive about performance," Calum notes (they’re confident they can push it even further).

Code Complexity: Their entire ZK implementation compressed from thousands of lines of Halo2 to just 250 lines of Noir code. "With rollups, the logic isn't complex—it's more about the preciseness of the logic," Calum explains.

Composability: In Halo2, proof aggregation required hardwiring specific verifiers for each proof type. Noir offers a general-purpose verifier that accepts any proof of consistent size.

"We can have 100 different proving systems, which are hyper-efficient for the kind of application that we're doing," Calum explains. "Have them all aggregated by the same aggregation proof, and reason about whatever needs to be."

Migration Time

Initially, the goal was to "completely mirror our Halo2 proofs": no new features. This conservative approach meant they could verify correctness while maintaining a live network.

The migration preserved Payy's production architecture:

• Rust core (According to Calum, "Writing a financial application in JavaScript is borderline irresponsible")
• Three-proof system: client-side proof plus two aggregators  
• Sparse Merkle tree with Poseidon hashing for state management

When things are transparent, they’re secure

"If you have your proofs in Noir, any person who understands even a little bit about logic or computers can go in and say, 'okay, I can kinda see what's happening here'," Calum notes.

The audit process completely transformed. With Halo2: "The auditors that are available to audit Halo2 are few and far between."

With Noir: "You could have an auditor that had no Noir experience do at least a 95% job."

Why? Most audit issues are logic errors, not ZK-specific bugs. When auditors can read your code, they find real problems instead of getting lost in implementation details.

Code Comparison

Halo2: Binary decomposition

• Write a custom chip for binary decomposition
• Implement constraint system manually
• Handle grid placement and cell references
• Manage witness generation separately
• Debug at the circuit level when something goes wrong

Payy’s previous 383 line implementation of binary decomposition can be viewed here (pkg/zk-circuits/src/chips/binary_decomposition.rs).

Payy’s previous binary decomposition implementation

Meanwhile, binary decomposition is handled in Noir with the following single line.

pub fn to_le_bits<let N: u32>(self: Self) -> [u1; N]

(Source)

What's Next

With Noir's composable proof system, Payy can now build specialized provers for different operations, each optimized for its specific task.

"If statements are horrendous in SNARKs because you pay the cost of the if statement regardless of its run," Calum explains. But with Noir's approach, "you can split your application logic into separate proofs, and run whichever proof is for the specific application you're looking for."

Instead of one monolithic proof trying to handle every case, you can have specialized proofs, each perfect for its purpose.

The Bottom Line

"I fell a little bit in love with Halo2," Calum admits, "maybe it's Stockholm syndrome where you're like, you know, it's a love-hate relationship, and it's really hard. But at the same time, when you get a breakthrough with it, you're like, yes, I feel really good because I'm basically writing assembly-level ZK proofs."

“But now? I just write ‘if’.”

Technical Note: While "migrating from Halo2 to Noir" is shorthand that works for this article, technically Halo2 is an integrated proving system where circuits must be written directly in Rust using its constraint APIs, while Noir is a high-level language that compiles to an intermediate representation and can use various proving backends. Payy specifically moved from writing circuits in Halo2's low-level constraint system to writing them in Noir's high-level language, with Barretenberg (UltraHonk) as their proving backend.

Both tools ultimately enable developers to write circuits and generate proofs, but Noir's modular architecture separates circuit logic from the proving system - which is what made Payy's circuits so much more accessible to their entire team, and now allows them to swap out their proving system with minimal effort as proving systems improve.

Payy's code is open source and available for developers looking to learn from their implementation.