# Sensitive Data Masking

## Objective

Show enough of a sensitive value to recognise it, and no more.

Server-side masking of sensitive fields so the full value never leaves the server, with a gated path to reveal it.

## Before You Begin

This feature is being added to an application that already exists and already
works. Do not scaffold a new project, and do not assume a blank slate.

Inspect the codebase first and establish:

- The existing application structure and where code of this kind already lives.
- The framework and version in use.
- The existing design system — colours, spacing, typography, and component conventions.
- Existing UI components you can reuse instead of writing new ones.
- The existing database structure, if this feature needs to persist anything.
- The existing authentication and authorization system, if this feature is user-scoped.
- Dependencies already installed, so you don't add a library that duplicates one.
- The existing test setup and conventions.

Only start writing code once you understand the above. If the application
already implements part of this feature, extend it rather than replacing it.

## Implementation Instructions

1. Inventory the sensitive fields first — emails, phone numbers, card numbers, API tokens, national identifiers, bank details — and classify each one.
2. Mask at the point the value is serialised on the server. The full value must never be sent to the client and hidden with CSS; that is not masking, it is a suggestion.
3. Write per-type rules: cards keep the last four, emails keep the first character and the domain, tokens keep a short prefix. A single generic rule makes some fields useless and others still identifying.
4. Apply the same rules everywhere the value can escape: UI, API responses, CSV and PDF exports, application logs, error reports, and internal support tooling.
5. Gate any full reveal behind an explicit permission plus recent re-authentication, and record who revealed what and when.
6. Do NOT build a client-side show/hide control here — that is owned by Sensitive Field Reveal, which deliberately refuses to toggle server-masked values. This feature is what produces those masked values; the two must not overlap.

## UI and UX Requirements

Match the application's existing design system exactly. Reuse its components,
spacing, and typography. This feature should look like it was always there.

## Responsive Requirements

Works on mobile, tablet, and desktop. Touch targets are large enough to hit on a
phone, and nothing overflows horizontally at 320px.

## Accessibility Requirements

- Fully keyboard navigable.
- Correct semantic elements and ARIA roles.
- Visible focus states.
- Meets WCAG AA contrast.
- Dynamic changes are announced to screen readers.
- Respects prefers-reduced-motion.

## Edge Cases

- Secrets shown once at creation (API keys) are a separate flow — masked afterwards, never retrievable.
- Search and filtering must work against the real value server-side while still returning masked results, or the feature breaks the app.
- Masking must be consistent across an entire response — one endpoint returning the full value undoes every other.
- Log redaction must cover request bodies, query strings, and exception messages, which are where full values usually leak.
- A reveal must be time-boxed. Do not leave the full value in client memory or in a cached response after the user moves on.
- Some masks still identify: a partial phone number plus a name is often enough. Classify by risk, not by field name.
- Support staff need a different mask level than customers. Make it a permission, not a separate hardcoded view.

## Testing

Exercise the feature end to end in the running application. Cover every edge case
above, then run the existing test suite and confirm nothing regressed.

## Acceptance Criteria

- [ ] No API response, export, or log contains a full sensitive value without an authorized reveal.
- [ ] Each field type has a documented mask rule appropriate to it.
- [ ] Masking is applied identically in UI, exports, logs, and support tooling.
- [ ] Full reveal requires permission plus recent authentication and is recorded in an audit log.
- [ ] Server-side search against real values still functions.
- [ ] The client-side toggle remains owned by Sensitive Field Reveal.
- [ ] The feature matches the existing design system.
- [ ] No existing functionality is broken.

## Adaptation Rules

- Match the existing design system. Do not introduce a new colour palette,
  spacing scale, or component library.
- Reuse existing components and utilities wherever they fit.
- Follow the naming, file layout, and code style already present.
- Do not upgrade, replace, or remove existing dependencies to make this
  feature fit. Adapt the feature to the app, not the app to the feature.
- Do not break existing functionality. If a change is genuinely required in
  existing code, make the smallest one that works and say so.
- If something in these instructions conflicts with how the application is
  built, follow the application and explain the deviation.

## Final Verification

Before you report the work as done:

1. Re-read the acceptance criteria above and check each one against what you
   actually built.
2. Run the application and exercise the feature end to end.
3. Run the existing test suite and confirm you have broken nothing.
4. Check the feature on mobile, tablet, and desktop widths.
5. Check keyboard navigation and focus handling.
6. Summarize what changed: files added, files modified, and anything you
   deliberately did differently because of how this application is built.

If any acceptance criterion is unmet, fix it before reporting completion.