Privacy and programmability have traditionally been at odds in public blockchains. The transparent execution environment of Ethereum provides strong smart contract programmability, but at the cost of making all transactions publicly visible by default. While efforts to add privacy have tended to constrain the kinds of applications that can be built, programmable blockchains have traditionally had to forgo privacy. Noir & The Streaming EVM: How Aztec Solved the “Privacy vs. Programmability” Paradox for Ethereum reflects an important technological shift that tries to solve this problem. Aztec’s vision is for a world where private computation and highly programmable smart contracts can coexist on Ethereum.
This article will explore the technical underpinnings of Aztec’s vision, centered on the Noir programming language and the Streaming EVM, in the context of larger trends such as zero-knowledge rollups and the privacy layer wars. It is an educational and informative piece, written to explain how the technology works, why it matters, and what it might mean for the future of Ethereum.
The Privacy vs. Programmability Problem in Ethereum
Ethereum’s design prioritizes openness. Every transaction, contract state change, and function call is visible to the network. While this transparency supports trust minimization, it introduces several limitations:
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Sensitive financial data is publicly accessible
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Business logic can be reverse-engineered
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User behavior is easily traceable
Privacy-focused systems have attempted to address these issues using cryptographic techniques such as zero-knowledge proofs (ZKPs). However, early implementations often supported only limited transaction types, making them unsuitable for complex decentralized applications.
In short, Ethereum faced a paradox:
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Programmability requires flexible, expressive execution environments.
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Privacy demands restricted visibility and cryptographic abstraction.
Aztec’s architecture is designed specifically to bridge this divide rather than choosing one side.
Aztec’s Architectural Overview
Aztec is structured as a zero-knowledge rollup that settles on Ethereum while executing transactions privately off-chain. Its design centers on two core components:
1. Noir, a domain-specific programming language for private computation
2. The Streaming EVM, an execution model that enables composable private smart contracts
Together, these elements form what Aztec describes as a privacy-first layer compatible with Ethereum’s security guarantees.
Noir: A Language for Private Smart Contracts
Noir is a high-level programming language designed to compile into zero-knowledge circuits. Instead of writing low-level cryptographic logic, developers express constraints and computations in a syntax closer to conventional smart contract languages.
Key characteristics of Noir
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Abstracts zero-knowledge proof construction
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Enables private state and private function inputs
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Compiles into circuits verifiable on Ethereum
From a developer perspective, Noir reduces the cognitive overhead traditionally associated with ZK systems. Rather than designing circuits manually, developers focus on application logic while the compiler handles proof generation.
Aztec has also upgraded its proving architecture to a system known as Honk, an evolution of the earlier Plonk proving system widely used in zero-knowledge rollups. Honk improves proving efficiency, verification speed, and modularity, allowing more complex private smart contracts to execute with reduced computational overhead. This transition reflects Aztec’s broader effort to scale private computation without compromising programmability or performance.
Why Noir matters
Noir addresses a critical adoption barrier: usability. Without accessible tooling, privacy-preserving systems remain niche. Noir’s role is to make private computation as approachable as writing a standard smart contract, without exposing sensitive data on-chain.
The Streaming EVM Explained
The Streaming EVM is Aztec’s execution environment for private contracts. Unlike Ethereum’s conventional EVM, which operates on globally visible state, the Streaming EVM processes data as encrypted streams.
Core ideas behind the Streaming EVM
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Execution occurs over encrypted inputs
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State transitions are validated via zero-knowledge proofs
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Only proofs and commitments are published to Ethereum
This model allows contracts to remain programmable while keeping execution details private. Importantly, it preserves composability—contracts can interact with one another without revealing internal logic or data.
How Noir and the Streaming EVM Work Together
The integration of Noir and the Streaming EVM is central to resolving the privacy-programmability paradox.
High-level execution flow:
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A developer writes a private contract in Noir
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The contract compiles into a zero-knowledge circuit
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Users submit encrypted inputs
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The Streaming EVM executes the logic privately
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A proof of correct execution is posted to Ethereum
Ethereum validators do not see the underlying data, but they can verify that execution followed the rules.
Benefits of Aztec’s Approach
Advantages
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Strong privacy guarantees without sacrificing expressiveness
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Ethereum security through rollup settlement
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Developer accessibility via high-level abstractions
Trade-offs
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Higher computational complexity
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New tooling and mental models required
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Performance constraints compared to fully transparent execution