Zero-Knowledge Proofs in Everyday Apps: Beyond zk-SNARKs

Zero-knowledge proofs, or ZKPs, are one of the most exciting developments in modern cryptography. They allow you to prove that something is true without giving away any unnecessary details. Imagine you’re trying to convince a friend that you know the password to a secret club, but you don’t want to tell them the password itself. With a zero-knowledge proof, you can demonstrate your knowledge through a clever interaction or a mathematical trick, leaving your friend certain but clueless about the actual secret.

In today’s world of 2025, where data breaches and privacy invasions are everyday news, ZKPs offer a powerful tool for protecting personal information. They’re not just theoretical anymore; they’re starting to appear in apps we use daily, from secure logins to anonymous voting. While zk-SNARKs have been the star of the show, other systems like Bulletproofs and Halo2 are gaining ground for their unique strengths. Let’s break this down step by step, keeping things simple and accessible for everyone.

A Gentle Introduction to Zero-Knowledge Proofs

The concept of zero-knowledge proofs dates back to the early 1980s, when computer scientists Shafi Goldwasser, Silvio Micali, and Charles Rackoff first described them in their groundbreaking paper “The Knowledge Complexity of Interactive Proof Systems.” They were inspired by everyday scenarios, like proving you know how to solve a puzzle without revealing the solution.

At its heart, a ZKP involves two parties: the prover, who knows a secret and wants to prove something about it, and the verifier, who wants to be convinced without learning the secret. The proof must satisfy three key properties:

1. Completeness: If the prover’s claim is true and both parties follow the rules, the verifier will always be convinced.

2. Soundness: If the claim is false, no cheating prover can fool an honest verifier, except with a tiny chance of luck.

3. Zero-knowledge: The verifier gains no information about the secret beyond whether the claim is true or false.

These properties ensure that ZKPs are both reliable and privacy-preserving. As Goldwasser and Micali explained in their work, “The verifier learns nothing from the proof except the validity of the assertion.”

ZKPs come in two main types: interactive and non-interactive. Interactive proofs require back-and-forth communication between prover and verifier, like a game of questions and answers. Non-interactive proofs, on the other hand, can be sent in a single message and verified later. This makes them more practical for digital applications.

Moving Beyond zk-SNARKs: Exploring Bulletproofs and Halo2

zk-SNARKs (Succinct Non-Interactive Arguments of Knowledge) have been popular because they create very short proofs that can be verified quickly. However, they often require a “trusted setup” - a one-time ceremony where special parameters are created securely. If this setup is compromised, the entire system’s security could be at risk. This limitation has led researchers to develop alternatives that avoid trusted setups while maintaining strong privacy guarantees.

Bulletproofs: Efficient and Trustless Range Proofs

Bulletproofs were introduced in 2017 by a team of researchers including Benedikt Bunz, Jonathan Bootle, Dan Boneh, Andrew Poelstra, Pieter Wuille, and Greg Maxwell. They were designed to solve a common problem in cryptography: proving that a number falls within a certain range without revealing the number itself.

For example, imagine you want to prove that your bank balance is between $100 and$10,000 without showing the exact amount. Bulletproofs make this possible with proofs that are much shorter than previous methods. The key innovation is their use of inner-product arguments and a technique called the “logarithmic derivative,” which allows them to compress proofs efficiently.

One of the biggest advantages of Bulletproofs is that they don’t require any trusted setup. This makes them more secure and easier to deploy in real-world systems. They’re particularly useful for “range proofs” - statements like “this value is greater than X but less than Y.”

Bulletproofs have found a home in cryptocurrency, especially in privacy-focused coins like Monero. In Monero’s confidential transactions, Bulletproofs hide the amounts being sent while still proving that the transaction is valid and that no money is being created out of thin air. As Maxwell noted in the original paper, “Bulletproofs provide a new way to construct efficient zero-knowledge proofs of knowledge for general arithmetic circuits.”

Since their introduction, Bulletproofs have inspired improvements like Bulletproofs++, which offer even better performance for certain types of proofs.

Halo2: Recursive Proofs for Complex Computations

Halo2, developed by the Zcash team and released in 2020, takes a different approach. It’s built on the PLONK proving system but adds powerful features like recursion and universality. Recursion means that a Halo2 proof can verify other Halo2 proofs, creating a chain of verification that’s incredibly efficient for complex applications.

The “halo” in Halo2 refers to the way proofs can be linked together, like a halo of light connecting multiple points. This allows for “proof aggregation,” where multiple small proofs can be combined into one larger proof that verifies everything at once.

Halo2 is designed for “arbitrary computations,” meaning it can prove the correctness of almost any mathematical statement or program execution. This makes it versatile for applications beyond simple range checks. For instance, it could prove that a complex financial calculation was done correctly without revealing the input data.

In Zcash’s Orchard network, Halo2 enables “shielded transactions” where users can hide their transaction details while proving compliance with network rules. The system’s efficiency is impressive: proofs are typically 1-2 KB in size, and verification is fast enough for real-time applications.

As Sean Bowe, one of Halo2’s creators, explained, “Halo2 allows us to build recursive proof systems that can verify themselves, opening up new possibilities for scalable privacy-preserving applications.”

Zero-Knowledge Proofs in Everyday Applications

ZKPs are no longer confined to academic papers or niche blockchain projects. They’re starting to appear in mainstream apps, offering privacy-preserving alternatives to traditional authentication methods.

Age Verification Without Revealing Personal Details

One of the most promising applications is age verification for online services. Traditionally, websites might ask users to upload government ID or provide birthdates, creating privacy risks. With ZKPs, you can prove you’re over a certain age without sharing your exact birthdate or any identifying information.

For example, imagine accessing an adult website or purchasing age-restricted content. Instead of submitting your ID, you could generate a ZKP that mathematically proves your age meets the requirement. The website verifies the proof and grants access, but learns nothing else about you.

Projects like Sismo and Polygon ID are already experimenting with this technology for decentralized identity. As Vitalik Buterin, Ethereum’s co-founder, has said, “Zero-knowledge proofs will be crucial for privacy-preserving identity systems in Web3.”

Passwordless and Privacy-First Authentication

Traditional password-based logins are vulnerable to interception, phishing, and data breaches. ZKPs offer a way to authenticate without sending sensitive information over the network.

In a ZKP-based login system, you prove knowledge of your credentials (like a password hash or private key) without actually transmitting them. The server verifies the proof and logs you in, but if the communication is intercepted, the attacker learns nothing useful.

This approach could also enable “multi-device authentication,” where you prove you have the same secret across different devices without syncing the secret itself. It’s like proving you have the same key for multiple locks without showing the key to anyone.

Other Everyday Uses: Voting, Credit Scoring, and More

ZKPs have potential in many areas:

• Anonymous Voting: Prove you’ve voted without revealing who you voted for, ensuring election integrity while protecting voter privacy.

• Privacy-Preserving Credit Scoring: Banks could verify creditworthiness without accessing full financial histories.

• Secure Messaging: Prove message authenticity without decrypting content.

• Supply Chain Verification: Companies could prove product origins without revealing proprietary processes.

These applications are still emerging, but they show how ZKPs can enhance privacy in our digital lives.

Scalability Challenges and the Road Ahead in 2025

While ZKPs offer incredible privacy benefits, scaling them for billions of users presents significant challenges. As adoption grows in 2025, these issues are becoming more pressing.

Proof Size and Verification Speed

Even the most efficient ZKP systems produce proofs that are larger than traditional digital signatures. A Halo2 proof might be 1-10 KB, which is small by data standards but can be cumbersome for mobile apps or IoT devices. Verification times, while fast on powerful computers, can take seconds on smartphones, making real-time applications difficult.

This is particularly problematic for global services. As one researcher noted, “The bottleneck isn’t computation anymore; it’s the bandwidth and energy required to transmit and verify proofs.”

Computational Overhead and Energy Consumption

Generating ZKPs requires significant computational power, especially for complex proofs. This could strain servers and increase energy consumption at a time when sustainability is a growing concern. In AI-heavy 2025, where data centers are already pushing limits, adding ZKP workloads might be challenging.

Integration with Existing Systems

Most apps today weren’t designed with ZKPs in mind. Retrofitting privacy-preserving authentication means major changes to user interfaces, backend systems, and development workflows. There’s also the challenge of user education - explaining ZKPs to non-technical users without scaring them off.

Security and Quantum Threats

While many ZKP systems are believed to be quantum-resistant, not all are. As quantum computers advance, we may need to upgrade or replace certain implementations. Additionally, the complexity of ZKP systems can introduce subtle bugs or vulnerabilities if not implemented carefully.

Despite these hurdles, progress is rapid. Hardware accelerators specifically for ZKP verification are emerging, and techniques like proof aggregation are reducing overhead. Recursive systems like Halo2 could make on-chain verification more efficient for blockchain applications.

Conclusion: The Future of Privacy-Preserving Technology

Zero-knowledge proofs represent a fundamental shift in how we approach privacy and trust in digital systems. By allowing verification without revelation, they open doors to applications we once thought impossible. Beyond the well-known zk-SNARKs, systems like Bulletproofs and Halo2 offer powerful alternatives with different strengths, from trustless range proofs to recursive verification.

As we navigate 2025’s privacy landscape, ZKPs will play an increasingly important role in protecting user data while enabling new forms of interaction. The key to widespread adoption will be balancing their privacy benefits with usability and performance.

For those interested in diving deeper, I recommend starting with the original Bulletproofs paper at https://eprint.iacr.org/2017/1066.pdf and the Halo2 documentation at https://zcash.github.io/halo2/. Resources like the Zcash blog (https://electriccoin.co/blog/) and Vitalik Buterin’s writings on Ethereum research provide excellent overviews.

What aspect of zero-knowledge proofs intrigues you most? Whether it’s their mathematical elegance or their potential to revolutionize privacy, there’s no doubt they’re shaping the future of secure computing.

References and Further Reading

• Goldwasser, S., Micali, S., & Rackoff, C. (1989). The knowledge complexity of interactive proof systems. SIAM Journal on Computing, 18(1), 186-208.
• Bunz, B., et al. (2018). Bulletproofs: Short proofs for confidential transactions and more. In 2018 IEEE Symposium on Security and Privacy (SP) (pp. 315-334). IEEE.
• Bowe, S., et al. (2020). Halo: Recursive proof composition without a trusted setup. Cryptology ePrint Archive, Report 2020/499.
• Buterin, V. (2021). ZK-SNARKs: Under the hood. Ethereum Foundation. https://vitalik.ca/general/2021/01/26/snarks.html
• Zcash Foundation. (2023). Halo 2: A flexible proving system. https://zcash.github.io/halo2/
• Monero Project. (2023). Bulletproofs in Monero. https://www.getmonero.org/resources/moneropedia/bulletproofs.html