Mobile App Development for Internal Employee Communication

Note: When a company's staff exceeds 5,000 people, standard messengers become a bottleneck: data leaks, message delays, lack of offline sync. Without the right approach, a corporate messenger turns into a source of vulnerabilities and failures. We develop corporate messengers turnkey—from protocol to store publication. In one project for a retailer with 5,000+ employees, we processed 100+ messages per second with 99.9% uptime. Below are the technical solutions that underpin it. Investment in development pays off by reducing communication time by up to 40%.

What infrastructure does a corporate messenger require?

A corporate messenger is not just CRUD with WebSocket. It includes real-time transport, E2E encryption, push notifications, history storage, media sharing, and cross-device synchronization. Each of these components requires a separate approach. For example, for a group of 5,000 users with a peak load of 200 messages per second, an architecture with horizontal scaling and WebSocket connection balancing is necessary.

Which transport protocol to choose for a corporate messenger?

The choice of protocol determines scalability and maintenance complexity. WebSocket paired with a custom protocol gives full control but requires implementing reconnect, heartbeat, acknowledgement, and message queuing on connection loss. XMPP is a mature standard with ready-made servers (Ejabberd, OpenFire, Prosody) and libraries for iOS/Android. It provides presence, MUC, and OMEMO for encryption out of the box. Matrix is a more modern option with E2E by default and federation, but it's tightly coupled to a homeserver (Synapse). For corporate tasks, we most often choose WebSocket + custom backend (NestJS + Redis Pub/Sub + PostgreSQL). XMPP is justified when integrating with an existing XMPP infrastructure; Matrix if federation between organizations is needed. The key decision is made based on your IT landscape.

How to implement E2E encryption in group chats?

Signal Protocol is the de facto standard for E2E. Used in Signal, WhatsApp, Skype. For mobile: libsignal-protocol-java (Android), SignalProtocolKit (iOS). The protocol is based on the Double Ratchet Algorithm: each message is encrypted with a new key; compromising one does not reveal the rest (forward secrecy).

// Encrypting an outgoing message
val sessionCipher = SessionCipher(signalStore, recipientAddress)
val encryptedMessage = sessionCipher.encrypt(plaintext.toByteArray())
val payload = Base64.encodeToString(encryptedMessage.serialize(), Base64.NO_WRAP)

For groups—Sender Key Distribution: one key encrypts for all, more efficient than encrypting individually. Keys are stored only on the device. When the app is deleted, the history is lost—this is the trade-off. Solution: optional key backup via iCloud Keychain / Google Drive with user consent.

Step-by-step plan for E2E implementation:

1. Choose the Signal Protocol library for each platform.
2. Set up a server for storing encrypted messages and key management.
3. Integrate encryption/decryption on clients.
4. Conduct a key compromise test.

History storage and synchronization

Local database—Room (Android) / Core Data (iOS). Messages with fields: id, conversationId, senderId, encryptedContent, timestamp, status (sent/delivered/read). Resync on reinstall: if E2E without backup—history unavailable. With backup—paged loading via cursor-based pagination. Delivery and read statuses—via WebSocket ack. In offline mode, messages accumulate and are sent in batch.

Media and files

Upload via a separate HTTP endpoint. For images: first thumbnail (JPEG, 200px, quality 40), then original. Video—HLS or progressive download. Chunked upload with resume (S3 Multipart Upload). Progress via okhttp3.MultipartBody. Cache via Glide/Kingfisher with LRU eviction of old files.

Push notifications and badges

For a closed app—FCM (Android) / APNs (iOS). On iOS, UNNotificationServiceExtension decrypts E2E before display. On Android 13+, request POST_NOTIFICATIONS permission. Badge on iOS: UNUserNotificationCenter.setBadgeCount() (since iOS 16) or via push payload. On Android—ShortcutBadger (launcher-dependent).

Stages and timelines

MVP without E2E (basic chats, push, history) — 3-4 months. Full version with E2E — 5-7 months. Cost is calculated individually.

What's included in the work

• Technical specification and architectural documentation
• Source code for iOS and Android with comments
• Integration with your IT infrastructure (SSO, API, corporate email)
• CI/CD setup and app store publication
• Administrator and user training
• 3-month warranty support

Checklist for readiness verification

• [ ] WebSocket connection in background on iOS and Android
• [ ] APNs and FCM certificates
• [ ] Penetration test on E2E
• [ ] History synchronization after reinstall
• [ ] Load testing with 5,000+ concurrent users

Our team has 10+ years of experience in mobile development and over 40 corporate projects. We use Signal Protocol for encryption and WebSocket API for real-time. Investment in development pays back on average in 12-18 months, and savings on operational costs can reach 40%. Request a consultation to assess your project—get a preliminary estimate in 2 days. Contact us to discuss the details of your corporate messenger.

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.