Secure Storage

Secure Storage on Mobile

Instructions

Follow these guidelines to persist sensitive values without leaking them to disk, backups, or co-resident apps.

1. What Belongs Where

Never place tokens or PII in SharedPreferences, NSUserDefaults, or a plain SQLite file.

2. Android: EncryptedSharedPreferences (Tink-backed)

// build.gradle.kts: androidx.security:security-crypto:1.1.0-alpha06
val masterKey = MasterKey.Builder(context)
    .setKeyScheme(MasterKey.KeyScheme.AES256_GCM)
    .setUserAuthenticationRequired(false) // set true + timeout for biometric gating
    .build()

val prefs = EncryptedSharedPreferences.create(
    context,
    "secure_prefs",
    masterKey,
    EncryptedSharedPreferences.PrefKeyEncryptionScheme.AES256_SIV,
    EncryptedSharedPreferences.PrefValueEncryptionScheme.AES256_GCM,
)
prefs.edit().putString("refresh_token", token).apply()

For new code prefer Tink directly (AeadConfig, KeysetHandle) so you control the keyset lifecycle — EncryptedSharedPreferences is in alpha indefinitely.

3. iOS: Keychain

let query: [String: Any] = [
    kSecClass as String:       kSecClassGenericPassword,
    kSecAttrService as String: "com.example.app.auth",
    kSecAttrAccount as String: "refresh_token",
    kSecValueData as String:   Data(token.utf8),
    kSecAttrAccessible as String:
        kSecAttrAccessibleAfterFirstUnlockThisDeviceOnly,
]
SecItemDelete(query as CFDictionary)
let status = SecItemAdd(query as CFDictionary, nil)
guard status == errSecSuccess else { throw KeychainError.store(status) }

Always use a *ThisDeviceOnly accessibility class so items don't sync via iCloud Keychain.

4. Flutter: flutter_secure_storage

const storage = FlutterSecureStorage(
  aOptions: AndroidOptions(encryptedSharedPreferences: true),
  iOptions: IOSOptions(
    accessibility: KeychainAccessibility.first_unlock_this_device,
  ),
);
await storage.write(key: 'refresh_token', value: token);

On Android < 23 the plugin falls back to RSA + AES in SharedPreferences — document this in the threat model or raise minSdk to 23.

5. React Native: react-native-keychain

import * as Keychain from 'react-native-keychain';

await Keychain.setGenericPassword('user', token, {
  service: 'com.example.app.auth',
  accessible: Keychain.ACCESSIBLE.AFTER_FIRST_UNLOCK_THIS_DEVICE_ONLY,
  accessControl: Keychain.ACCESS_CONTROL.BIOMETRY_CURRENT_SET, // optional
  storage: Keychain.STORAGE_TYPE.AES_GCM, // Android 23+
});

Avoid AsyncStorage for secrets — it is plain JSON on disk.

6. Backups, Exports, and Logs

• Android: set android:allowBackup="false" or provide an explicit backup_rules.xml that excludes secure stores.
• iOS: set NSFileProtectionComplete on anything sensitive written to the filesystem; mark files with isExcludedFromBackupKey = true where appropriate.
• Never log token values. Log only token IDs / hashes for debugging.

7. Migration & Wipe

• On logout, explicitly delete every entry you wrote — do not rely on app uninstall.
• On key rotation (OS upgrade invalidating keystore entries), surface a re-auth flow rather than crashing.

Checklist

• No tokens or PII written to SharedPreferences / NSUserDefaults / AsyncStorage.
• Keys use *ThisDeviceOnly accessibility on iOS.
• Android uses EncryptedSharedPreferences or Tink, backed by Keystore.
• allowBackup="false" or an explicit backup exclusion rule is set.
• Logout path deletes every secure entry.
• No secret values appear in logs, crash reports, or analytics.