Biometric Authentication (Fingerprint) in Android – BiometricPrompt with CryptoObject

You integrated biometrics into an Android app, but it doesn't work on Xiaomi? Or using the deprecated FingerprintManager that will crash on Android 14? We help implement reliable fingerprint authentication with BiometricPrompt and CryptoObject. Turnkey solution with compatibility guarantee across all brands. We have 5+ years of mobile development experience and have implemented biometrics in 30+ projects. Integration cost is determined after analysis—contact us for a quote.

Why BiometricPrompt is better than FingerprintManager?

FingerprintManager (deprecated since API 28) supports only fingerprint, doesn't work with CryptoObject, and is unstable on custom ROMs. BiometricPrompt from the androidx.biometric library (works from API 23+) supports fingerprint, face, iris—and most importantly, allows binding authentication to a cryptographic key via CryptoObject. This raises security to Class 3 (Strong)—the highest biometric category in Android.

Common mistakes

Mistake 1: Calling from ViewModel

BiometricPrompt requires a FragmentActivity or Fragment. Developers sometimes try to call it from ViewModel or Repository and get an IllegalStateException at runtime. The prompt lives in the UI layer, period. If you need to initiate authentication from business logic, use callbacks or an event-based approach.

Mistake 2: Missing CryptoObject

Many implementations call BiometricPrompt.authenticate() without CryptoObject, meaning they only check biometric presence but don't bind it to a cryptographic operation. This is "weak" biometrics: an attacker with root access could theoretically inject SUCCESS into the AuthenticationCallback. The correct approach is Class 3 (Strong) biometrics with CryptoObject. As per Android documentation, only CryptoObject provides cryptographic binding of authentication to the encryption operation.

Mistake 3: Android fragmentation

On MIUI 12–13, BiometricManager.canAuthenticate(BIOMETRIC_STRONG) returns BIOMETRIC_ERROR_NONE_ENROLLED even with registered fingerprints due to Xiaomi's customization. You need to add a fallback check using FingerprintManagerCompat for such cases. We guarantee correct operation on Samsung, Xiaomi, Pixel, and others—tested on 20+ models.

Correct implementation with CryptoObject

Key generation in Android Keystore

The essence: generate a key in Android Keystore bound to biometrics. Upon authentication, a Cipher is initialized with this key and passed to CryptoObject. If biometrics succeed, the cipher is unlocked and can encrypt/decrypt data.

val keyGenerator = KeyGenerator.getInstance(KeyProperties.KEY_ALGORITHM_AES, "AndroidKeyStore")
keyGenerator.init(
    KeyGenParameterSpec.Builder(KEY_NAME, KeyProperties.PURPOSE_ENCRYPT or KeyProperties.PURPOSE_DECRYPT)
        .setBlockModes(KeyProperties.BLOCK_MODE_CBC)
        .setEncryptionPaddings(KeyProperties.ENCRYPTION_PADDING_PKCS7)
        .setUserAuthenticationRequired(true)
        .setInvalidatedByBiometricEnrollment(true)
        .build()
)
keyGenerator.generateKey()

setInvalidatedByBiometricEnrollment(true)—the key is invalidated when a new fingerprint is enrolled. Without this flag, the old key remains valid after the user changes biometrics, reducing security.

Initializing Cipher and CryptoObject

val cipher = Cipher.getInstance("${KeyProperties.KEY_ALGORITHM_AES}/${KeyProperties.BLOCK_MODE_CBC}/${KeyProperties.ENCRYPTION_PADDING_PKCS7}")
val keyStore = KeyStore.getInstance("AndroidKeyStore").apply { load(null) }
val secretKey = keyStore.getKey(KEY_NAME, null) as SecretKey
cipher.init(Cipher.ENCRYPT_MODE, secretKey)

val cryptoObject = BiometricPrompt.CryptoObject(cipher)

Then pass cryptoObject to biometricPrompt.authenticate(promptInfo, cryptoObject).

Handle callback completely

object : BiometricPrompt.AuthenticationCallback() {
    override fun onAuthenticationSucceeded(result: BiometricPrompt.AuthenticationResult) {
        val cipher = result.cryptoObject?.cipher ?: return
        // decrypt token from EncryptedSharedPreferences
    }
    override fun onAuthenticationError(errorCode: Int, errString: CharSequence) {
        when (errorCode) {
            BiometricPrompt.ERROR_LOCKOUT -> showFallback()
            BiometricPrompt.ERROR_LOCKOUT_PERMANENT -> showPermanentLockout()
            BiometricPrompt.ERROR_NEGATIVE_BUTTON -> showPinAuth()
            BiometricPrompt.ERROR_USER_CANCELED -> { /* do nothing */ }
        }
    }
    override fun onAuthenticationFailed() {
        // attempt failed but limit not exhausted—BiometricPrompt itself shows error
    }
}

onAuthenticationFailed is not a final error. The system updates the prompt UI itself. Do not hide the prompt or show your own errors in this callback.

Token storage

Use EncryptedSharedPreferences from androidx.security:security-crypto. Encrypt the token using the cipher from a successful CryptoObject, store encrypted bytes + IV in EncryptedSharedPreferences. On subsequent authentication: initialize biometrics in DECRYPT_MODE with the saved IV → get plaintext token.

Work process

What’s included

• Integration documentation
• Source code with comments
• Build and signing instructions
• Support for 30 days after delivery
• Compatibility guarantee with Android 6.0+ and brands (Xiaomi, Samsung, Huawei)

We will assess your project within 1 day. Contact us for a consultation—we'll help you choose the right approach. Get a free timeline and cost estimate.

Additionally, if you want to save on development, consider our standard implementation—it covers 90% of scenarios and reduces integration costs. Order biometric implementation right now—we'll prepare a commercial proposal within a day.

What breaks authentication in mobile

We've seen a banking app where a PIN login issued a JWT, and the token was stored in SharedPreferences as plaintext. Not hypothetical — real fintech projects that later had to rewrite the authentication module from scratch. SharedPreferences on Android can be read by any app with root access without additional permissions. On iOS, the equivalent is UserDefaults instead of Keychain. The mistake is costly: the average damage from such a leak exceeds $50,000 including fines and reputational losses.

Authentication in mobile is fundamentally more complex than the web: no HttpOnly cookies, no browser session mechanism, but there are platform storage and biometrics. We have developed authorization modules for 30+ projects (fintech, marketplaces, social networks) and guarantee compliance with App Store and Google Play rules.

How to protect tokens during OAuth 2.0 authentication?

iOS Keychain — OS-level encrypted storage. Data is protected by Secure Enclave on devices with Face ID/Touch ID. Correct scenario: JWT refresh token is stored with attribute kSecAttrAccessibleWhenUnlockedThisDeviceOnly — token is accessible only when device is unlocked and not transferred during iCloud backup.

// Saving to Keychain via Security framework
let query: [String: Any] = [
    kSecClass as String: kSecClassGenericPassword,
    kSecAttrService as String: "com.yourapp.auth",
    kSecAttrAccount as String: "refresh_token",
    kSecValueData as String: tokenData,
    kSecAttrAccessible as String: kSecAttrAccessibleWhenUnlockedThisDeviceOnly
]
SecItemAdd(query as CFDictionary, nil)

Android Keystore System — hardware (or software on older devices) cryptographics key storage. Keys cannot be exported — encryption/decryption operations inside Keystore. Pattern: generate a key in Keystore, encrypt refresh token with it, store encrypted blob in EncryptedSharedPreferences (Jetpack Security).

EncryptedSharedPreferences — wrapper around SharedPreferences with encryption via Keystore. Adds in 5 minutes and eliminates a class of vulnerabilities present in half of Android apps.

Biometric authentication

iOS LocalAuthentication. LAContext.evaluatePolicy(.deviceOwnerAuthenticationWithBiometrics) — standard call for Face ID/Touch ID. Integrates with Keychain via kSecAccessControl with flag .biometryCurrentSet: key becomes inaccessible after biometric data changes.

Typical scenario: on first login — password login, refresh token → Keychain with biometric protection. On subsequent launches — biometrics unlock access to token, token is exchanged for a new access token. Using biometrics with Keychain reduces token compromise risk by 99% compared to storage in UserDefaults.

Android BiometricPrompt. Unified API for fingerprint, face, and iris. BiometricManager.canAuthenticate(BIOMETRIC_STRONG) checks availability of Class 3 biometrics (required for financial apps). BIOMETRIC_STRONG + Keystore key with setUserAuthenticationRequired(true) — key used only after successful biometrics in current session.

Why is OAuth 2.0 authentication with PKCE the standard?

OAuth 2.0 Authorization Code Flow with PKCE (Proof Key for Code Exchange) is the mandatory pattern for mobile apps. Implicit Flow is officially deprecated in RFC 8252. PKCE introduces code_verifier (random string) and code_challenge (SHA-256 of verifier). The authorization server verifies the match when exchanging code for token. This protects against interception of authorization code via custom URL scheme. Comparison: PKCE increases OAuth security over 1000 times compared to Implicit Flow, because without proof key the code can be stolen before exchange.

According to the OAuth 2.0 Security Best Current Practice, using PKCE is mandatory for public clients, including mobile apps.

iOS: ASWebAuthenticationSession — system browser for OAuth. Session cookies are not accessible to the app, no phishing risk via embedded WebView. Apple rejects apps using WKWebView for OAuth (Guideline 5.1.1).

Android: AppAuth-Android — standard library for OAuth/OIDC with PKCE support. Custom Tabs (Chrome) instead of WebView — the same security principle.

Steps to implement OAuth 2.0 authentication with PKCE on iOS

1. Generate code_verifier (minimum 43 characters from unreserved set).
2. Compute code_challenge = SHA256(code_verifier), encode base64url.
3. Open ASWebAuthenticationSession with authorization URL including code_challenge and code_challenge_method=S256.
4. After redirect, obtain authorization code.
5. Send POST request to server with code, code_verifier, client_id.
6. Server verifies code_challenge matches code_verifier, issues token.

Sign in with Apple and Google Sign-In

Sign in with Apple is mandatory if the app offers any other third-party login (Google, Facebook). Apple has required it for years, violation leads to rejection under Guideline 4.8.

Peculiarity: Apple can hide the real user email, providing a relay address ([email protected]). The backend must handle this correctly — not use email as primary identifier.

ASAuthorizationAppleIDProvider on iOS, SignInWithAppleButton in SwiftUI. JWT identity token from Apple contains sub — stable user identifier, unchanged when email is hidden.

Google Sign-In. On Android — via Credential Manager API (replaced former GoogleSignIn API). On iOS — GoogleSignIn SDK, opening Safari or Google App for authorization.

2FA and one-time passwords

TOTP (Time-based One-Time Password, RFC 6238) — standard for 2FA. base32-encoded secret generated on server, user scans QR in Google Authenticator or Authy. Adding TOTP reduces account takeover risk by 99.9% compared to password-only.

On mobile, built-in Authenticator via Password AutoFill (iOS 15+) works from Keychain: one-time code filled automatically without separate app. For this, OTP field must have textContentType = .oneTimeCode.

SMS OTP — least secure option (SIM-swapping), but most conversion-friendly. If used — only via SMS Retriever API on Android (code read automatically without permissions) and ASAuthorizationController with oneTimeCode on iOS.

JWT: access and refresh tokens

Pattern: short-lived access token (15 minutes – 1 hour) + long-lived refresh token (30–90 days). Access token in memory (in-memory — not in Keychain), refresh token in Keychain/EncryptedSharedPreferences. Silent refresh: on receiving 401 — automatic request for new access token with refresh token. If refresh token expired — forced login.

Rotation refresh tokens: each exchange of refresh token for access token issues a new refresh token. Old one invalidated. If old refresh token is attempted — compromise, all user tokens revoked.

What's included in the work

When ordering an authentication module, we provide:

• Source code of the authorization module (Swift/Kotlin) with integration of chosen methods.
• Architecture and token scheme documentation.
• Configured PKCE flow for OAuth 2.0.
• Integration of Sign in with Apple and Google Sign-In using your client IDs.
• Biometric configuration with correct protection flags.
• Deployment and testing instructions (TestFlight, Firebase App Distribution).
• Checklist for App Store and Google Play review.

Timeline and cost

Implementation of basic authentication (email + password + JWT) takes 1 to 2 weeks. Adding OAuth, biometrics, and 2FA adds another 1–3 weeks. The final cost is calculated after auditing your project. Get a consultation — we'll assess complexity and propose the optimal stack.

Common mistakes (and how to avoid them)

• Storing tokens in UserDefaults / SharedPreferences — readable on rooted devices without root. Solution: Keychain / Keystore.
• Lack of certificate pinning in high-security apps — MITM via corporate proxy. Solution: add pinning in URLSession or OkHttp.
• Storing secrets in Info.plist or BuildConfig — trivially decompiled. Solution: use Keychain or server configuration.
• OAuth via WKWebView / WebView instead of system browser — App Store rejection + security risk. Solution: ASWebAuthenticationSession / Custom Tabs.
• Incorrect kSecAttrAccessible — token with kSecAttrAccessibleAlways does not require device unlock. Solution: WhenUnlockedThisDeviceOnly.

We have implemented secure authentication for 30+ projects over 5 years. We guarantee compliance with platform requirements and best practices (OAuth 2.0 + PKCE, Keychain, Keystore). Order development of an authentication module — we'll analyze vulnerabilities and propose a solution within your budget. Get a consultation via the form on the website.