comparisons
Published by Floriva · Updated 2026-04-16 · How Floriva checks its guides
Stardust vs Clue: Privacy, Features, Architecture (2026)
Stardust markets zero-knowledge but stores data server-side. Clue is GDPR-compliant with no FTC actions. Neither uses on-device-only storage.
Stardust markets itself as a privacy-first, zero-knowledge period tracker. Researchers have contested those encryption claims, and data is still stored on Stardust servers. Clue has a cleaner verified track record, GDPR jurisdiction, and academic partnerships, but it uses cloud storage on EU servers. Neither app keeps cycle data only on your device.
Two apps, different privacy claims
Stardust and Clue both position themselves as safer choices for period tracking. Stardust uses zero-knowledge marketing language and an astrology-tied differentiator. Clue uses GDPR jurisdiction, a subscription business model, and academic research partnerships.
We compared the two because users considering them are usually trying to move off Flo or another data-selling app. They want a tracker with a credible privacy story. The question is whether either app delivers privacy at the architecture level, or only at the marketing level.
How Stardust stores data
Stardust is a US-based app marketed as privacy-first. Its public-facing copy invokes zero-knowledge encryption. When researchers examined the actual storage model, they found that user data sits on Stardust servers in a form the company can decrypt. That is not what zero-knowledge means.
True zero-knowledge means the encryption keys never leave the user's device, so even the service provider cannot read the stored data. Stardust's architecture does not match that standard, based on the reviews referenced in our Stardust privacy claims guide. The marketing implies one thing. The architecture does another.
Stardust is also account-based. You sign up, your cycle data is linked to that account, and it lives on company infrastructure. Pricing runs about $29.99/year.
How Clue stores data
Clue is operated by Biowink GmbH out of Berlin. It has been on the market longer than Stardust. Its business model is subscription-based with no advertising. The company has research partnerships with Oxford and Stanford that feed into cycle science and product predictions.
Clue stores cycle data on cloud servers in the EU. That keeps the data under GDPR, which limits how Clue can use, share, or transfer user data commercially. There is no documented FTC enforcement history and no significant privacy settlement on record. Clue Plus is about $39.99/year, with a free tier that has no ads.
GDPR is a stronger commercial framework than what US-based apps operate under by default. It is still a policy framework, not an architectural one. The data is still on Clue's servers.
Why neither is subpoena-proof
The structural issue for both apps is the same. If a company holds a copy of your cycle data on its servers, a court with valid jurisdiction can subpoena that data. In states with abortion restrictions, prosecutors have sought digital evidence including period tracking records. GDPR does not block compelled disclosure under valid legal process, especially when the request crosses jurisdictions.
Stardust's marketing framing does not change this. If the company can decrypt the data, the data can be produced. Clue's stronger commercial privacy posture does not change this either. If the data is on a server, it is reachable through legal process.
The only architecture that removes this exposure is on-device-only storage, where the company never creates a server copy in the first place. Floriva, Drip, and Euki use this model. Stardust and Clue do not.
Pricing snapshot
Stardust: $29.99/year. Clue Plus: $39.99/year (free tier with no ads). Floriva: paid app with no free tier, with on-device storage for every user and no account required.
If annual cost is the deciding factor, Stardust is cheaper. If a verifiable privacy track record is the deciding factor, Clue is cleaner. If your cycle data should never exist on a company server, neither Stardust nor Clue is the answer.
Stardust vs Clue vs Floriva: Privacy & Features
| Feature | Floriva | Stardust | Clue |
|---|---|---|---|
| Data storage | On-device only | Server-side (contested zero-knowledge) | Cloud (EU servers) |
| Account required | No | Yes | Yes |
| Subpoenable | No | Yes | Yes |
| FTC enforcement history | No | No | No |
| Jurisdiction | On-device (no jurisdiction) | US | Germany (GDPR) |
| Academic partnerships | N/A | No | Yes (Oxford, Stanford) |
| Partner sharing | No (by design) | No | Yes (Connect) |
| Annual price | Paid app (no free tier) | $29.99 | $39.99 |
Strengths and trade-offs
Floriva
Pros
- On-device storage, no cloud copy
- No account required
- Structurally unsubpoenable because there is nothing to subpoena
- Default privacy for all users, not a paid tier
Cons
- Smaller feature set than Clue
- No automatic cloud backup
Stardust
Pros
- Marketed as privacy-first
- Astrology integration as a differentiator
- Lower annual price than Clue Plus
Cons
- Zero-knowledge claims have been contested by researchers
- Data decryptable on Stardust's side
- Still account-based and server-stored
- Subpoena exposure despite marketing framing
Clue
Pros
- No documented FTC actions or privacy violations
- GDPR jurisdiction out of Berlin
- Oxford and Stanford research partnerships
Cons
- Still cloud-based, EU server storage
- Account required
- GDPR limits commercial use but not legal compulsion
- Full feature access behind Clue Plus
What the experts say
These apps promised users privacy while systematically routing reproductive health data to third-party advertising networks. Policy claims were never backed by architecture.
Quick answers to the obvious questions.
Is Stardust more private than Clue?
Stardust markets itself as more private, but the actual architecture does not back that up. Researchers have questioned Stardust's zero-knowledge claims because the data is still decryptable on the company's servers. Clue does not use zero-knowledge language, but has a cleaner verified record: GDPR-compliant, no FTC actions, and academic research partnerships. On verifiable privacy posture, Clue is the stronger pick.
Does Stardust actually use zero-knowledge encryption?
Independent reviews of Stardust's architecture have raised doubts about the zero-knowledge framing. True zero-knowledge means the provider cannot decrypt user data at all, because the keys never leave the user's device. Stardust stores data server-side in a form the company can access. That does not match the zero-knowledge definition. The marketing language and the architecture do not line up.
Is Clue subpoenable?
Yes. Clue stores user data on cloud servers in the EU. GDPR limits how Clue can use that data commercially, but it does not prevent a court with valid jurisdiction from ordering the data produced. Any server-based tracker, including Clue and Stardust, carries this legal exposure. Only on-device-only trackers remove the subpoena surface entirely.
Questions people ask before they switch.
What is the difference between Stardust and Clue?
Stardust is a US-based period tracker marketed as privacy-first with an astrology-tied angle and $29.99/year pricing. Clue is a Berlin-based tracker (Biowink GmbH) that has been around longer, operates under GDPR, has academic research partnerships with Oxford and Stanford, and costs $39.99/year for Clue Plus. Stardust's privacy claims have been contested. Clue has no comparable enforcement history.
Are Stardust's privacy claims accurate?
Stardust's marketing uses zero-knowledge language, but researchers who examined the architecture found that data is stored in a form Stardust can decrypt on their servers. That does not match a strict zero-knowledge definition. The privacy framing is stronger in marketing than in the underlying technical architecture. See our guide on Stardust privacy claims for a full breakdown.
Does Clue sell user data?
Clue does not have a documented record of selling user health data. Biowink GmbH operates under GDPR out of Berlin, its business model is subscription-based rather than ad-supported, and it has no FTC enforcement history. GDPR still allows compelled disclosure under valid legal process, so the commercial picture is cleaner than the legal-process picture.
Which is cheaper, Stardust or Clue?
Stardust costs about $29.99/year. Clue Plus costs about $39.99/year. Stardust is cheaper on annual price. Price is not the same as privacy value: the architecture difference between them is small because both use server-side storage. Pay for whichever feature set you actually use.
Is there a period tracker with no cloud storage?
Yes. Floriva keeps core cycle records on your device using encrypted local storage, with no account required for core tracking. Optional sync is encrypted so Floriva cannot read synced records. Drip (Android) and Euki use local-only storage by default.