goose d7d09482da docs: ADR for zero-knowledge encryption Phase 1

Records the decision: client-side AES-256-GCM, double-PBKDF2 auth/enc-key
split, server-as-blind-store, in-memory key lifecycle, and Phase 1 limitations
(forgotten password = data loss; Phase 2 recovery wrapping deferred).

2026-06-28 21:51:47 -03:00

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ADR: Zero-Knowledge Encryption (Phase 1)

Status: Implemented (Phase 1) Date: 2026-06-28

Context

The original design (docs/product/encryption.md, docs/adr/mongodb-schema-decision.md) specified client-side zero-knowledge encryption: the server should never hold plaintext user data or encryption keys. Until this change, the EncryptedField type and recovery-phrase fields were aspirational scaffolding — data was always stored and processed as plaintext JSON.

Decision

Implement client-side zero-knowledge encryption using AES-256-GCM via the browser's Web Crypto API. The server becomes a blind store: it holds opaque ciphertext and can never decrypt user data.

Key derivation: double PBKDF2

From the user's password, the client derives two independent values:

Auth secret: PBKDF2(password, "normogen-auth-v1", 150k iters, SHA-256) → base64, sent to the server as the "password". The server PBKDF2-hashes it (as before).
Encryption key: PBKDF2(password, "normogen-enc-v1", 150k iters, SHA-256) → AES-GCM CryptoKey, kept in memory only, never transmitted.

The server cannot derive the encryption key because it never sees the raw password or the enc-domain PBKDF2 output.

What's encrypted

Medication data blobs (name, dosage, frequency, route, etc.)
Appointment data blobs (title, provider, date/time, etc.)
Profile display name

What stays plaintext (queryable)

IDs (medicationId, appointmentId, userId, profileId)
Medication active flag (top-level, filterable)
Appointment status (top-level, filterable)
Timestamps

In-memory key lifecycle

The encryption key lives only in browser memory for the authenticated session. It is NOT persisted. A page reload requires re-entering the password to re-derive it. This is the core ZK trade-off.

Consequences

Phase 1 limitations (documented)

Forgotten password = data loss. No recovery key-wrapping yet (Phase 2).
Page reload requires re-authentication to re-derive the enc key.
No data migration was needed (no real user data existed).

Phase 2 (future)

Recovery key-wrapping (wrap the enc key under a recovery-derived key so a forgotten password doesn't lose data).
Password-change re-wrapping (re-wrap the enc key under the new password).
Wrapped-DEK model (separate data-encryption key + key-encryption key).

Relationship to the original design doc

docs/product/encryption.md describes a fuller zero-knowledge architecture (per-user keys, deterministic encryption for search, key rotation). This Phase 1 implements the core property (server can't read data) with the simplest viable design. The fuller features remain future work.

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