# Trusted Devices

## Objective

Stop asking for a second factor on the laptop they use every day.

A per-device trust token that skips repeat verification on familiar devices, with expiry, naming, and revocation.

## 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. Issue a signed, revocable token stored on the device when the user opts to trust it after a successful verification, and record the device server-side so it can be listed and revoked.
2. Rotate the token on each use so a stolen copy has a short useful life and reuse of an old token is detectable.
3. Expire trust after a fixed period regardless of activity, and drop all trust on password change, second-factor change, or recovery.
4. Show the user their trusted devices with a name, last-used time, and a revoke control, plus revoke-all.
5. Do NOT trust a browser fingerprint on its own. Fingerprints collide across similar machines and cannot be revoked, which makes them useless as a security boundary.

## 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

- A cleared cookie jar means the device simply is not trusted anymore — verify again rather than trying to recover trust from other signals.
- A copied or cloned browser profile will present a valid token; token rotation plus a reuse alarm is what catches it.
- High-risk actions — changing the password, adding a payment method, exporting data — must require fresh verification even on a trusted device.
- Trust is per user and per device: a shared machine must not skip verification for a second account.
- Cap the number of trusted devices and prompt the user when they hit it, rather than growing the list without bound.
- Signing in from a trusted device in a new country should still trigger verification.

## 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

- [ ] Trust is a revocable server-side record backed by a rotating token, not a fingerprint.
- [ ] Trust expires on a fixed schedule and is cleared on any credential change.
- [ ] Users can name, list, and revoke devices, including all at once.
- [ ] High-risk actions require fresh verification regardless of trust.
- [ ] Token reuse after rotation is detected and revokes the device.
- [ ] Trust does not carry across user accounts on the same machine.
- [ ] 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.