# PCH-02-005 — Document the legacy PBKDF2 import KDF + recommend re-export

**Severity:** Informational
**Target files:** `src/shared/backup.ts:146` (`derivePeachExportKey`),
`src/popup/views/Import.tsx` (re-export notice)
**Report:** peach-code-review.md, Finding PCH-02-005

## Correction to the fix brief

The brief's comment text says "100,000 iterations". The code (and the report)
say **250,000** (`backup.ts:166`). The comment below uses the value the code
actually enforces — a comment that misstates its own parameters is worse than
no comment.

## Edit 1 — KDF comment at backup.ts:146

```typescript
// BEFORE (backup.ts:146-150)
export async function derivePeachExportKey(
  password: string,
  salt: Uint8Array,
  opts: { raw?: boolean } = {}
): Promise<Uint8Array> {

// AFTER
/**
 * LEGACY IMPORT KDF (PCH-02-005):
 * This path derives with PBKDF2-SHA256 (250,000 iterations) for backwards
 * compatibility with `.json` exports sealed before the Argon2id migration —
 * the encrypted-JSON export UI was removed in v2.5.0 (see header), so this
 * KDF is import-only (plus the test fixture generator above).
 *
 * PBKDF2 has no memory-hardness and is materially weaker than the current
 * Argon2id envelope (m=64 MiB, t=3, p=4 — peachPayload.ts) against GPU/ASIC
 * attackers at equal wall-clock cost. Do NOT use this helper for any new
 * format. Users importing an old `.json` backup are prompted to export a
 * fresh backup afterwards (Import.tsx legacy-KDF notice).
 */
export async function derivePeachExportKey(
  password: string,
  salt: Uint8Array,
  opts: { raw?: boolean } = {}
): Promise<Uint8Array> {
```

(The existing LV-1 NFC comment inside the function body stays as is.)

## Edit 2 — post-import re-export notice in Import.tsx

Verified: there is currently **no** user-facing recommendation after a
successful PBKDF2 (`peach-json`) import. The import success path keeps the
view mounted and already surfaces a post-import `duplicateWarning` notice
(Import.tsx:790) — the legacy-KDF notice mirrors that exact mechanism.

2a. Add a signal next to `duplicateWarning` (Import.tsx:~425):

```typescript
// AFTER the existing line:
  const [duplicateWarning, setDuplicateWarning] = createSignal<number>(0);
// ADD:
  // PCH-02-005: set when a legacy PBKDF2-encrypted `.json` backup imports
  // successfully — prompts the user to re-export under the Argon2id format.
  const [legacyKdfNotice, setLegacyKdfNotice] = createSignal(false);
```

2b. In `handleImport`, alongside `setDuplicateWarning(dupes)` (the
success tail, Import.tsx:~604):

```typescript
// BEFORE
      setDuplicateWarning(dupes);
      await props.onImport();

// AFTER
      setDuplicateWarning(dupes);
      // PCH-02-005: "peach-json" is the legacy PBKDF2-SHA256 envelope; every
      // other recognised format is plaintext-source (no KDF) or current-format.
      setLegacyKdfNotice(currentPreview.format === "peach-json");
      await props.onImport();
```

Also reset it where the other notices reset (`handleFileSelect`, alongside
`setError(null)` / `setAwaitingReplaceConfirm(false)`):

```typescript
      setLegacyKdfNotice(false);
```

2c. Render next to the duplicate-warning block (Import.tsx:~790, same
`<Show>` pattern and styling class as the sibling notice):

```tsx
        <Show when={legacyKdfNotice() && !importing()}>
          <div class="peach-settings-group">
            <div class="peach-restore-warning">
              This backup was created with Peach's older encryption format
              (PBKDF2). It imported fine — but we recommend exporting a fresh
              backup now so your paper/file backups use the current, stronger
              format.
            </div>
          </div>
        </Show>
```

(Class choice: `peach-restore-warning` is the existing soft-warning surface
used three blocks above; no new CSS.)

## Tests

UI-notice coverage belongs in `src/popup/views/contracts.test.tsx` style; the
notice is a pure render-state change. Suggested single contract case (optional,
not counted as a security regression test): mock `parseImportSource` to return
`format: "peach-json"`, drive `handleImport`, assert the notice text renders.
The KDF comment itself needs no test.

## Save message (integration)

`fix: PCH-02-005 document legacy PBKDF2 import path + recommend re-export after legacy import`