PrivyCycle and the unfinished business of health app privacy on Ethereum

Inspiration rarely strikes where people expect. The origin of PrivyCycle did not stem from the predictable bomb of laptops on the Hackathon floor of ETHCC, but rather from the electrical voltage of a Berlin summer phase in June 2025.

There, among the spotlights of D/ACC Berlin and for a crowd of Ethereum’s sharpest spirits, Migle Rakitaite – Visionary was behind WXMen Web3 Privacy Now – Face to Eathereum’s own Vitalik Buterin. The sky crackled as Rakitaite, accompanied by fellow privacy advocaten, Buterin challenged directly: why had the Ethereum Foundation failed to defend and finance crypto-native solutions for protecting sensitive health data, especially for following apps of the blockchain period? In that charged moment, while questions about privacy and responsibility were echoing by the auditorium, the seeds of PrivyCycle were sown.

It is not theoretical problem. In the United States, the criminalization of abortion in various states has made historical tracking apps dangerous. Data from services such as Flo and Clue can be – and has been – summons in lawsuits. And although regular apps promise privacy, most user data earn through opaque ADTech and analysis deals. The gap between privacy rhetoric and practice is striking.

So at Ethglobal Cannesccs 2025 Hackathon, Rakitaite collaborated with developers from Taco.build (FKA Nucypher), Vialabs.io” 0G.AIAnd WAKU/LOGOS (A team fully composed of men, with the remarkable exception of Rakitaite), in an attempt to implement crypto-native alternative to the problems surrounding the existing period tracking apps, that is, the lack of privacy and flagrant sales of user data. The result, Privycycle, was one of the top 10 of the ETHCC projects from 334 entries and winner of three separate sponsor prices. It is a proof-of-concept, not a product but it comes to the key questions about whether Ethereum can support privacy-first applications outside of Defi and Token Swaps.

Zero knowledge cryptography (ZK) has long been the proposed answer. ZK-proofs enable one party to prove the possession of certain data without revealing the data itself. In a fertility tracking context, this means that a doctor proves that the cycle has been regularly for six months without disclosing the dates or symptoms. In theory.

But in practice, ZK Tooling continues to open on Ethereum. “Defi ZK”-Privé swaps, ZK rollups for scale or ZK-NFT certificates is far ahead of privacy tooling for irregular, longitudinal, non-gigantic data sets such as medical records or menstrual logs. ZK-SNAUK circuits remain expensive to implement, Layer-2 Solutions Fragment Data Access and Off-Chain storage (such as IPFs) introduces its own security assumptions. From mid-2025 there is no production of production quality, open-source ZK protocol for health data.

Privycycle has combined existing infrastructure that focuses on the use of technology offered by the sponsors of Ethglobal Cannes Hackathon: Privy.io for identity abstraction, zurcuit for ZKs and 0G.AI for lightweight, AI-driven suggestions. During the Demo, Rakitaite explained how the app allows users to log symptoms locally, cod data, upload it to IPFs and selectively share it with partners or doctors using threshold keys.

But even these AI functions increase privacy flags. “During this period, perhaps stay away – or bring her chocolate and flowers,” Rakitaite joked on stage and explained how the AI of the app would offer anonymous relationship – tips based on aggregated data. But every AI system that offers personalized recommendations, even ‘anonymous’, implies a certain degree of inference from user data. Without open-source models, audits or reproducible training processes, these claims of anonymity will not continue to be overgoverted.

The presentation also contained an option to export encrypted PDFS or CSV files for doctors, and a dashboard for doctor to interpret health insights generated by AI. But here too the infrastructure is hypothetical. Nowadays, most healthcare providers cannot decode, verify or store ZK-protected medical files. The idea that users “can take their data wherever they want” only works if there is willing to receive it somewhere.

The financial pitch was shaky in the same way. On stage, Ryan Caruso or Threshold claimed that the menstrual tracking app market would reach $ 10 billion in 2034. But that projection comes from a Market Research Future Report that the wider Femtech market treasures – include wearables, diagnostics, fertility services and telhealth platforms – could reach that figure. The actual historical biting income is unknown. Major players such as Flo Health and Clue have been detained privately and historically opaque about data income and financial data.

What is clear is the risk. In 2021, Flo was caught sharing sensitive health data with third parties and satisfied with the FTC. In the legal environment after Dobbs, where reproductive health data can be armed, this is not just a privacy problem-it is a policy.

That is why projects such as PrivyCycle Matter, even in their imperfections. Not because they deliver finished products, but because they are uncovering what Ethereum is still missing: exposure to credible, scalable, decentralized privacy tooling for sensitive, personal information. During the Mainstage presentation it was noted that the route map of PrivyCycle includes the implementation of better tooling for access control and data exchange that is coding end-to-end and “more important, end-to-end” Taco (Threshold access control). Taco was not a sponsor of the hackathon.

That line landed differently in Paris than a month earlier in Berlin, when Rakitaite immediately called on the foundation. The truth is that the Ethereum ecosystem has postponed privacy tooling for years. While Tornado-Contant was punished and zero knowledge of L2’s prolified for Defi-scaling, its applications for medical, identity and social data are left behind.

Privycycle is not a finished solution. It is a provocation. A reminder that, as Ethereum, must evolve from a financial settlement layer to a social infrastructure protocol, the and end-to-end coding tools and zero-Knisprivacy for non-finance data, should not take only in Hackathon demos, but at the level of financing and the level of protocol.

As Rakitaite said the ETHCC:

“On the first day of my daughter’s period I want to give this as a gift. So that she can calculate her fertility much easier and much better.”