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A reusable credential is convenient — and dangerous. If the same credential produced the same on-chain marker everywhere, it would become a tracking beacon: anyone watching the chain could link a user’s activity across every app that accepts it. Nullis keeps credentials reusable without making them trackable, by domain-separating the nullifier.

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The threat, concretely

If the nullifier were just a function of the secret, the same value would surface in every app — a permanent, cross-app identifier. That’s the exact tracking harm private credentials are supposed to avoid.

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The mechanism

The nullifier folds in an app_domain term: 
nullifier = Poseidon(credential_secret, policy_id, app_domain, action_id)
Because Poseidon is a cryptographic hash, changing any input scrambles the output unpredictably. Change the app, change the app_domain, and the nullifier is a completely different, uncorrelatable field element — even for the identical credential. Given only two nullifiers, an observer cannot tell whether they came from one credential or two.
The app_domain also does double duty for replay protection: because it’s folded into the nullifier alongside action_id, each app maintains its own independent spent-set. A nullifier spent in App A has no bearing on App B.

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Proven, not asserted

This is a real, reproducible result — the same credential used in the remittance app and the RWA-access app yields two different nullifiers: 
npm run build -w @nullis/core -w @nullis/sdk -w @nullis/issuer
node examples/unlinkability.mjs
AppPolicyapp_domainNullifier
Remittance777442400x09e01e5e…436ba08c
RWA access888488790x0532e4a7…357f2911
Same credentialCommitment, two unrelated nullifiers. Two apps, one engine, cryptographically unlinkable at the proof and nullifier layer.

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The scope of the claim

Unlinkability is scoped precisely. The same credential yields different nullifiers per app, unlinkable at the proof and nullifier layer. It does not hide:
  • Wallet addresses — the submitting account is public on both apps.
  • Funding sources — shared funding can correlate accounts.
  • IP and timing — network-level metadata lives outside the artifact layer.
Nullis never overclaims beyond the artifact layer.
This precision is itself a feature: Nullis tells you exactly where its privacy guarantee ends, so you can reason about the rest of your stack — wallets, RPC, funding — honestly, instead of assuming a blanket “anonymous.”

Next: revocation

How access is revoked without touching anyone’s identity.