necessary 0.1.1

Device trust for Flutter apps: a device-bound key the plugin never exports, signed measurements of the phone and the app, request proofs. Your backend decides.

necessary

Device trust for Flutter apps on Android and iOS. The plugin gives your app a key it never exports — made in the phone's keystore, in secure hardware where the phone has it and in software where it does not — signed measurements of the phone and of the app running on it, and a proof on each request your app sends — and, if you turn them on, a second key the person must unlock to approve a request, and a PIN your backend checks. It never scores and never decides: your backend does.

Android 7.0 (API 24) and later; iOS 15 and later; Flutter 3.44 (Dart 3.12) and later — the example app needs Flutter 3.47.

Documentation

• The integration guide (doc/integration_guide.md) — the protocol: what the plugin sends, and how your backend checks it.
• The measurements (doc/measurements.md) — everything the plugin measures on the phone, field by field: what each measurement is, how it is taken, and what a complete answer holds.
• The measurement contract (contract/report-1/android-8-ios-5/) — the report's schema, its rules and sample reports. The folder's name is two versions: report-1 is the report's format, and android-8-ios-5 the lowest versions of the measuring code, the collector, that the contract accepts — 8 on Android and 5 on iOS (collector_version; this plugin's are 8 and 6). They are not the Android or iOS versions the plugin runs on, which are above.
• The example app (example/) puts every piece below in a working app, against the sample backends (in the repository, not the package). Try the example app, below, installs it and tests it step by step, measurement by measurement.

Install

dependencies:
  necessary: ^0.1.1

The plugin declares what its own features need. Your app adds only what you choose to use:

• Location measurements: on Android ACCESS_COARSE_LOCATION (and ACCESS_FINE_LOCATION for a precise location) in your manifest; on iOS NSLocationWhenInUseUsageDescription in Info.plist. The plugin never asks by itself: call trust.requestLocationPermission() from your own screen.
• Biometric step-up on iOS: NSFaceIDUsageDescription in Info.plist.

Start

import 'package:necessary/necessary.dart';

final trust = DeviceTrust(
  baseUrl: 'https://api.example.com',
  // The personal measurements your app allows; your backend asks for what it needs of them.
  personalMeasurements: {PersonalMeasurement.screenCapture},
);

await trust.start(); // loads or makes the key, registers it, proves it holds it

start() makes the guide's first three calls under baseUrl: register the installation, ask for a key challenge, answer it. Your backend answers each as the guide says. A new key is a new installation: trust.identity!.createdThisLaunch tells your app to forget what it kept for the old one.

Scan

final ScanResult scan = await trust.scan();
final answer = scan.answer; // your backend's answer, whole

The plugin measures what your backend's challenge asks for — and, of the personal measurements it asks for, the ones your app enabled — signs the report with the installation key and sends it. It acts on the guide's protocol errors once (a challenge expired, the installation session expired, the backend lost the installation, a short wait); any other refusal reaches your app as an ApiException.

Personal measurements

Seven measurements identify a person's phone, network or habits. Each is collected only when your app enables it and your backend's challenge asks for it (optional_probes); otherwise the report says not_enabled. None is on unless your app turns it on:

trust.personalMeasurements = {
  PersonalMeasurement.screenCapture, // the screen recorded, mirrored or cast
  PersonalMeasurement.location, // the last location the phone already knows
};

Set them in the constructor, or change them whenever the person does — in a privacy setting: the next scan follows, and the installation and its step-up key stay as they are.

PersonalMeasurement	What it reports	Platforms
localAddresses	the phone's own IP addresses, link-local ones left out	both
accessibility	the Accessibility services that are on, and the keyboard in use	Android
notificationAccess	the apps allowed to read notifications	Android
otherAccess	device-administrator apps, and known remote-control apps installed	Android
screenCapture	the screen being recorded, mirrored or cast	both
location	the last location the phone already knows — never a fresh fix, never a prompt	both
mockLocation	whether that location was faked	both

The location needs the person's permission as well: declare it (Install, above) and ask from your own screen with await trust.requestLocationPermission(), which answers precise, approximate or none. On iOS the three Android-only measurements answer unsupported.

Prove your own requests

Every request your app sends to your own API can carry a proof that this installation's key signed it, so a copied session token is useless on another phone:

final body = jsonEncode({'amount': 25, 'to': 'payee-7'});
final proof = await trust.signRequest(
  method: 'POST',
  path: '/v1/transfer',
  bearer: accountSession, // your backend's session for this installation
  body: body, // sent exactly as signed
);
await http.post(
  Uri.parse('https://api.example.com/v1/transfer'),
  headers: {
    'Authorization': 'Bearer $accountSession',
    'Content-Type': 'application/json',
    ...proof.headers, // X-Access-Proof, X-Access-Signature
  },
  body: body,
);

Each proof serves its request once. trust.send(proof) sends it through the plugin's transport instead.

A scan for one action

For a request that matters — a payment, a new payee — have the scan name it, under the account session that will send it, and send the scan's report with the request:

final operation = describeOperation('POST', '/v1/transfer', body);
final scan = await trust.scan(session: accountSession, operation: operation);
final proof = await trust.signRequest(
  method: 'POST',
  path: '/v1/transfer',
  bearer: accountSession,
  body: body,
  reportId: scan.reportId, // X-Integrity-Report
);

Step-up: the person approves one request

Turn it on, and choose how the key is unlocked:

final trust = DeviceTrust(
  baseUrl: 'https://api.example.com',
  stepUp: const StepUpOptions(factor: 'passcode', mode: 'per_use'),
);
await trust.start(); // registers the step-up key with the installation

if (trust.stepUpUnavailable != null) {
  // This phone cannot make the key -- no screen lock, say. Step-up is off here.
}

final proof = await trust.signRequest(
  method: 'POST',
  path: '/v1/transfer',
  bearer: accountSession,
  body: body,
  stepUp: true,
  reason: 'Approve the transfer of 25', // what the phone shows the person
);

Option	Values
factor	passcode (the phone's PIN, pattern or password) or biometric (a fingerprint or face enrolled now)
mode	per_use (unlocked for every signature) or windowed with windowSeconds (one unlock for a while; Android only)

Android 9 and 10 cannot ask for the passcode at every signature: there a passcode key is windowed, and says so to your backend. iOS makes per-use keys only. An app whose backend sets the policy assigns trust.stepUp before start().

Removing the screen lock, or enrolling another biometric for a biometric key, makes the key unusable: an approval then fails with NativeKeyException STEPUP_KEY_INVALIDATED. The person re-enrols a new key with their own re-authentication:

await trust.reenrolStepUp(
  session: accountSession,
  reauthentication: {'password': password}, // as your backend requires
);

A new key is made, the person unlocks it to sign its proof, and your backend takes it for that account alone; approvals use it from then on. The call goes to /v1/installations/stepup-key, or to your API's own path: StepUpOptions(reenrolPath: ...). Until your backend has taken the new key — the person cancelled the prompt, the password was wrong, the answer was lost — approvals fail with STEPUP_REENROL_PENDING, across restarts too: call reenrolStepUp again. The same holds for a key the phone made after the installation was registered — the screen lock was off at the first start() and set later, say — since your backend takes a step-up key only at an installation's first registration: start() reports stepUpUnavailable STEPUP_REENROL_PENDING, and re-enrolment registers the key.

The app PIN: the person's PIN for one request

Turn it on; the two paths default to the guide's and can be your API's own:

final trust = DeviceTrust(
  baseUrl: 'https://api.example.com',
  appPin: const AppPinOptions(), // path: '/v1/account/app-pin', proofPath: '/v1/account/app-pin/proof'
);

// Setting it: with the person's own re-authentication, as your backend requires.
await trust.setAppPin(
  session: accountSession,
  pin: pin,
  reauthentication: {'password': password},
);

// Using it: your backend checks the PIN for this one request, and the request carries its proof.
final proof = await trust.signRequest(
  method: 'POST',
  path: '/v1/transfer',
  bearer: accountSession,
  body: body,
  appPin: pin,
);

The PIN travels once, in a signed body, never in a header, and the plugin never stores it. Protect the screen while it is typed:

await SecureScreen.protect(true); // Android: no screenshots or recording; iOS: covered while captured
// ... the PIN is typed ...
await SecureScreen.protect(false);

Calls nest — each protect(true) needs its protect(false) — and when the last one ends the window gets back what it had: a window your app keeps secure itself stays secure. On iOS the cover is over your app's window, not the system keyboard; on the iPhone it was tested on (iOS 15), the keyboard's key presses did not show in a recording. Opening Control Center to start a recording closes the keyboard, so protect whenever the field has focus, as the example does.

Your backend's refusals reach your app as ApiException with the guide's codes: app_pin_not_set, app_pin_invalid (it counts), app_pin_locked (a new PIN, set with the re-authentication, unlocks it). Step-up and the app PIN combine: your backend decides which requests need which.

What happens when

When	What the plugin does
The key is lost — an Android reinstall, a cleared keystore	start() makes a new key and a new installation ID; trust.identity!.createdThisLaunch is true, so forget what you kept for the old installation. Your backend links the two only provisionally, by the reinstall hint
iOS keeps the key across a reinstall	with its ID, the same installation comes back; without it, the plugin proposes a new ID, your backend answers with the one it holds for the key, and the plugin keeps that one
The app's process restarts	nothing is kept in memory: start() again finds the same key and installation, and proves it for a new installation session
The activity, a scene or a window is replaced	screen protection follows: Android sets it on each new window; on iOS the cover is placed again, on the window the person sees in each scene, whenever a window becomes key, visible or hidden or a scene comes to the foreground — and protection asked for before any window exists covers the first one. A step-up prompt the recreation cancelled answers STEPUP_CANCELLED: ask again
The Flutter engine is torn down	iOS drops the protection, its covers and its observers with the engine
Two calls at once	start(), prove(), loadIdentity() and the same scan() run once, and the second caller gets the first's result; scans for different operations, or asked after the personal measurements changed, run apart
A step-up prompt is open	on iOS the plugin's other calls — a scan, a request's signature — wait until the person answers it; Android runs them meanwhile
reset() or close() while calls run	the calls end with InterruptedCallException and keep nothing — no session, no installation ID, no report sent; after a reset, new calls wait for the deletion and never join one made before
A call abandoned, or its answer lost	a scan is simply asked for again — a challenge serves once; the plugin never sends a signed request twice: sign it again, and let your backend decide whether the first one reached it
A report over 65,536 bytes	it is never sent: the app lists the contract lets a collector shorten are shortened, and say truncated; anything else answers an ApiException report_too_large

Your own transport

final trust = DeviceTrust.over(MyTransport()); // implements DeviceTrustTransport

A transport sends each message the plugin builds and answers your backend's JSON object, or throws an ApiException with its code, message and details. To carry the messages over an API of your own shape instead, use the pieces directly — trust.client, collectReport, signRequest — and make the guide's checks in your backend.

What the plugin never does

It never scores, decides, or tells your app a score; never stores your account session; never sends the PIN in a header; and uses no platform attestation (Play Integrity, SafetyNet, App Attest, DeviceCheck). It never exports a private key, but it cannot prove to your backend where a key is kept: the registration's key_security is the phone's own word, and where the secure hardware declines — the iOS Simulator, some configurations — the key is a software key, which a compromised phone can copy. A fully compromised phone can falsify what it measures and can use a genuine key to sign what it likes: the measurements raise the cost of fraud, and your backend weighs them with its own business logic.

Try the example app

The example app (example/) puts every piece above in a working app, against a sample backend. Its own README carries the same steps.

Install it

The sample backends live in the repository, not in the package, so start from a clone.

1. Clone the repository:

   git clone https://github.com/merajhasan88/necessary_flutter.git
   cd necessary_flutter
   

2. Start a sample backend on your computer. It answers on 127.0.0.1 only. In Python 3.10 or later:

   pip install --require-hashes -r backend/python/requirements.txt
   python3 backend/python/server.py --port 8787 --schema contract/report-1/android-8-ios-5/report.schema.json
   

   The TypeScript and C# samples run the same way; their commands are in backend/README.md.

3. Run the example app, which needs Flutter 3.47, from example/:

   • On an Android emulator, a debug build reaches your computer at 10.0.2.2:

     flutter run --dart-define=NECESSARY_BACKEND_URL=http://10.0.2.2:8787
     

   • On an Android phone connected by USB with USB debugging on, forward the port first:

     adb reverse tcp:8787 tcp:8787
     flutter run --dart-define=NECESSARY_BACKEND_URL=http://127.0.0.1:8787
     

   • A release build, on Android or iOS, talks only HTTPS: the plugin refuses an http:// address in a release build. Put the sample behind an HTTPS proxy on a name the phone can resolve, with a certificate the phone trusts, and give that address:

     flutter run --release --dart-define=NECESSARY_BACKEND_URL=https://backend.example.com
     

   • On an iPhone, open example/ios/Runner.xcworkspace in Xcode on a Mac, choose your team under Signing & Capabilities, and run as above with the iPhone connected. An iPhone reaches your computer over your network, so give the address of the HTTPS proxy.

   To offer several backends, name each: --dart-define='NECESSARY_BACKENDS=Python=https://…;TypeScript=https://…'. A debug build measures itself as one — debuggable in app_identity — so judge the measurements on a release build.

Test it on a phone, step by step

With several backends (NECESSARY_BACKENDS), tap one at the top and run the steps once for each. Every result line carries a ✓ when the backend did what it should, a red ✗ when it did not.

1. Installation → Start. Expect: Installation …, proved; step-up on. On a phone without a screen lock, step-up is off: step-up off here (…), and step 7 cannot pass.
2. Measurements: switch on All of them, tap Ask for the location permission and allow it. Once is enough; the switches reset when the app restarts.
3. Scan. Expect: The backend answered next: allow, report … — the sample backends answer scan_again when a measurement could not be read, and review when its figures contradict each other. Under Hooks, gadgets and memory changes every row reads measured, none not asked. Open any row for the fields the phone sent.
4. Account → Sign in. Expect: An account session for this installation.
5. Requests → Protected request. Expect: ✓ {"done":"/v1/demo/echo"}.
6. Sensitive, without approval. Expect: ✓ Refused, as it should be: 403 needs_approval.
7. Sensitive, with step-up, then unlock with the phone's passcode. Expect: ✓ {"done":"/v1/demo/sensitive"}.
8. Type 6 digits in App PIN, then Set the app PIN. Expect: ✓ Set, with the account password.
9. Sensitive, with the app PIN. Expect: ✓ {"done":"/v1/demo/sensitive"}.
10. Requests a backend must refuse: tap each of the four. Expect a ✓ on each: Refused, as it should be: 401 proof_reused for the request sent twice, 401 proof_not_accepted for the other three.

If the phone's screen lock is removed and set again, the step-up key dies: step 7 then shows The phone said STEPUP_KEY_INVALIDATED. Tap Re-enrol the step-up key and unlock with the passcode — expect ✓ {"stepup_key_registered":true} — and step 7 passes again. The new key is that account's alone. If the lock was off at step 1 and set afterwards, Start again says step-up off here (STEPUP_REENROL_PENDING): the backend took the installation without a step-up key, so re-enrol the same way after step 4.

The app PIN belongs to the account, not to the phone. Phones signed in with the same handle share it: a PIN set on one unlocks requests on the other, and before any is set, step 9 answers app_pin_not_set. Sign in with another handle for an account of its own.

A session copied to another phone

A session token copied off one phone must be useless on another. You need two phones running the example against the same backend.

1. Phone A: steps 1 and 4 above, then Requests a backend must refuse → Copy this phone's session. Expect: Copied this phone's session.
2. Get the copied text to phone B, for example in a note or a message to yourself. It holds phone A's session and installation ID; the sample's sessions last ten minutes.
3. Phone B: step 1 above, on the same backend.
4. Phone B: paste the text into A session from another phone, then tap Use it here. Expect: ✓ Refused, as it should be: 401 invalid_installation_signature. Phone B's key cannot sign for phone A's installation, even when the request names it.

If step 4 reads The session is no longer valid: sign in again, phone A's session has expired: sign in again on phone A and copy it again.

Measurement by measurement

What each measurement is and every field it carries: doc/measurements.md. On a phone of your own you can watch these change, one at a time: make the change, tap Scan, open the measurement's row, and change it back afterwards.

Measurement	Change	The row then shows
developer_settings (Android)	Settings → About phone → tap Build number seven times; then Developer options → USB debugging	developer_options_enabled, then adb_enabled, true
network	turn on a VPN app; or set a manual proxy on your Wi-Fi network	vpn_active true, the tunnel's interface in interfaces (Android) or scoped_interfaces (iOS); http_proxy_configured true
local_addresses	switch between Wi-Fi and mobile data	the phone's own addresses, per interface
screen_capture	start the screen recorder (Android 15 and later; on iOS from Control Center); connect an external display	recording true; external_displays 1
location	tap Ask for the location permission and allow it	the permission, the coordinates and their age_seconds
mock_location	Android: Developer options → Select mock location app, and run a mock-location app; iOS: simulate a location from Xcode while the app runs from it	mock true
accessibility (Android)	Settings → Accessibility → turn on a service such as Select to Speak; or choose another keyboard	the service in services; the keyboard in keyboard
notification_access (Android)	Settings → Notification access (under Special app access) → allow an app	the app in listeners
other_access (Android)	install AnyDesk, TeamViewer QuickSupport, RustDesk or AirDroid; or make an app a device administrator	the app in remote_access_apps; in device_admins
app_identity	install a build of the example made with other code	other hashes; a debug build reads debuggable true
debug_state (Android)	in Android Studio, Run → Attach Debugger to Android Process, on a debug build	debugger_connected true
code_signing (iOS)	run a development build from Xcode	get_task_allow true

The rest — root and jailbreak traces, Frida and other hooking tools, code changed in memory, an emulator or the Simulator — needs a device you are allowed to change: never a phone you depend on. On an Android emulator emulator.suspected is true, and a userdebug image shows a debuggable, test-keys build in system_properties and root_files; on the iOS Simulator simulator.is_simulator is true. Frida on a test device shows in runtime_maps, frida_ports, instrumentation_threads, exec_mappings and code_integrity on Android, and in dyld_images and code_integrity on iOS.

Licence

Copyright 2026 Meraj Hasan. Licensed under the Apache License, Version 2.0 (LICENSE, NOTICE).