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+# 0025: Encrypt `course_chunks.chunk_text` — restore the encryption boundary, but the embedding stays plaintext
+
+- Status: accepted
+- Date: 2026-07-31
+- Relates to: #484 (this decision), #483 (blocked on it), #482 (RAG hardening),
+  #231 (storage/RLS lockdown), migration 0030 (`documents.extracted_text`),
+  migration 0039 (the vector store)
+- Supersedes: none
+
+## Context
+
+#484 names a real asymmetry. `documents.extracted_text` is encrypted (0030)
+because it is student-uploaded content; `course_chunks.chunk_text` holds *the
+same text, chunked*, in plaintext. One column is treated as PII and the other
+isn't, for no reason anyone wrote down.
+
+**The issue's stated premise is wrong, and it matters.** It assumes "pgvector
+similarity can't run over ciphertext." `match_course_chunks` (0039) computes
+`1 - (c.embedding <=> query_embedding)`, orders by `c.embedding <=>
+query_embedding`, and filters `WHERE c.embedding IS NOT NULL`. `chunk_text` is
+only ever SELECTed as payload — the ranking never reads it. Nothing in the
+codebase queries it by content either (no `ILIKE` / `LIKE` / FTS; verified
+across `services/`, `routes/`, `scripts/`). So encryption does not block
+retrieval at all, and this decision is far cheaper than the issue implies.
+
+But the correction cuts both ways, and this is the part worth recording: **the
+reason it's cheap is the reason it's partial.** The `embedding` column cannot
+be encrypted — pgvector must compute distance over it — and an embedding is a
+lossy but real representation of its source text; embedding-inversion
+techniques recover substantial content from vectors alone. Encrypting
+`chunk_text` therefore does *not* make the row opaque.
+
+Threat model, for calibration: the backend connects with the service-role key
+and RLS locks out `anon`/`authenticated` (#231), so direct table reads imply a
+Supabase credential compromise or an insider. `ENCRYPTION_KEY` is a separate
+secret held in the app environment, so column encryption genuinely raises the
+bar against a database-only compromise — the same bar every other encrypted
+column is already set at.
+
+## Decision
+
+Encrypt `chunk_text` for **every** row in `course_chunks` — document *and*
+catalog — through the standard `encrypt_if_present` / `decrypt_if_present`
+helpers, and compute content-addressed ids on the **plaintext, before
+encryption**.
+
+Uniform rather than scoped to document chunks, even though catalog chunks are
+public BU course-catalog text with nothing to protect:
+
+- One invariant — "`chunk_text` is always ciphertext" — is assertable by the
+  existing `ciphertext` e2e oracle. A per-category rule is not.
+- `decrypt_if_present` returns the **raw value** when it cannot decrypt. In a
+  mixed table that makes a genuine decrypt failure indistinguishable from a
+  legitimately-plaintext catalog row, which is precisely the kind of silent
+  degradation #482 was about.
+- The cost is ~5 AES-GCM decrypts per retrieval (`k=5`). Immaterial.
+
+Ids stay keyed on plaintext because AES-GCM uses a random nonce per call: the
+same plaintext encrypts to different ciphertext every time, so ciphertext can
+never be a dedup key. `chunk_id(course, text)` remains the merge key, and
+re-uploads of identical content still converge on one row.
+
+**This ADR does not claim chunk confidentiality.** It restores boundary
+consistency and removes trivially-readable plaintext. The embedding remains,
+and it is the residual exposure.
+
+## Consequences
+
+- (+) The encryption boundary is consistent: the same student text is
+  protected in `documents.extracted_text` and in the chunks derived from it.
+- (+) Retrieval is unaffected — ranking never touched `chunk_text`.
+- (+) Unblocks #483. Notes indexing would otherwise write decrypted note
+  bodies (`notes.body` is encrypted) into a plaintext column, deepening the
+  very asymmetry this closes.
+- (+) A uniform invariant the `ciphertext` oracle can assert, so a future
+  regression fails a lane instead of sitting unnoticed.
+- (−) **Residual exposure: the embedding stays plaintext and is partially
+  invertible.** This is defense in depth, not confidentiality. Anyone reading
+  this ADR to answer "is chunk content protected?" must read this line.
+- (−) `scripts/dedupe_course_chunks.py:70` re-derives ids from the **stored**
+  `chunk_text`. Run against encrypted rows it would hash ciphertext and
+  destroy content-addressing. It must decrypt before hashing, or be retired —
+  its own docstring calls it a one-time migration.
+- (−) A backfill is required for existing rows (the
+  `db/backfill_encryption.py` precedent). Until it completes the table is
+  mixed, carried by `decrypt_if_present`'s raw-value fallback.
+- (−) Catalog chunks pay encryption cost for no privacy benefit. Accepted as
+  the price of the uniform invariant.
+
+## Implementation (not done here)
+
+Write sites: `services/rag_service.py::index_document_chunks` and
+`scripts/ingest_catalog.py`. Read site:
+`services/rag_service.py::retrieve_chunks`, decrypting each returned chunk
+before `format_rag_context`. Plus the dedupe-script fix above, a backfill, and
+extending the `ciphertext` oracle to cover the column.