Implementing Local Data Encryption in a Mobile Application

Implementing Local Data Encryption in a Mobile Application

We have been integrating local data encryption on iOS and Android for over 5 years. Clients often come with a problem: after a device is compromised, user data ends up in plain sight. The reason is simple — encryption is either not implemented or implemented with errors. An unencrypted SQLite database on Android is just a file: adb pull /data/data/com.yourapp/databases/app.db on a rooted device — and all data is readable with any SQLite browser. On iOS, the situation is slightly better due to the Data Protection API, but only if the developer hasn't forgotten to set the correct NSFileProtectionKey — and this is often overlooked. We guarantee that after our work, your data remains protected even with physical access to the device.

What we encrypt and how

The task breaks down into three independent layers: database, files, secrets.

Database. The standard is SQLCipher. A fork of SQLite with transparent AES-256 encryption at the page level. On Android it is integrated via net.zetetic:android-database-sqlcipher, on iOS via SQLCipher.xcframework. Room on Android works with SQLCipher through SupportOpenHelperFactory — switching involves simply replacing the factory in Room.databaseBuilder() and adding the key. The key is generated once, stored in Keystore/Keychain, never stored in SharedPreferences or UserDefaults in plaintext.

On first launch on Android:

val key = generateAes256Key() // via KeyGenerator with KeyStore provider
val encryptedKey = encryptWithKeystore(key) // RSA/AES through AndroidKeyStore
prefs.putString("db_key_enc", Base64.encode(encryptedKey))

Then every time the database is opened, we decrypt the key and pass it to SQLCipher. Without PRAGMA key, the database simply won't open.

Files. For images, PDFs, cache — AES-256-GCM via javax.crypto.Cipher on Android or CryptoKit.AES.GCM on iOS (Swift 5.5+). GCM is important: it provides both confidentiality and integrity authentication. CBC without a MAC is a poor choice, vulnerable to padding oracle attacks.

On Flutter, the flutter_secure_storage package is convenient for secrets and encrypt for files, but under the hood both use the same native APIs — wrappers, not replacements.

Secrets (API keys, tokens). Only Keychain (iOS) and Android Keystore. Not UserDefaults, not SharedPreferences, not AsyncStorage in React Native. Keychain on iOS is encrypted with keys bound to the Secure Enclave; Keystore on Android with API 23+ binds keys to TEE or SE — they cannot be exported even with root.

Why encrypting the database specifically is important

The database is the most vulnerable point. It typically stores user profiles, shopping lists, operation history. If an attacker gains physical access to the device (loss, theft) and bypasses the lock screen, an unencrypted database means full data access. SQLCipher makes the database unreadable without the key. Even when analyzing a RAM dump, the key does not surface because it is stored in hardware-backed Keystore.

Typical mistakes that break the entire scheme

First — incorrect protection class on iOS. FileProtectionType.complete means the file is inaccessible while the device is locked. But if the app receives a push notification in the background and tries to read the database — crash. Developers panic and switch to completeUnlessOpen or remove protection altogether. The correct solution is to separate data: critical under .complete, background operations under .completeUnlessOpen.

Second — storing the key alongside the data. We encountered a case where the database encryption key was in the same directory as the encrypted database, simply in a file key.bin. That's not encryption, that's renaming.

Third — using the user's password directly as the key. AES requires 128 or 256 bits. A password like "qwerty123" is not a key. A KDF is needed: PBKDF2 with a minimum of 100,000 iterations or Argon2id. On iOS — CommonCrypto.CCKeyDerivationPBKDF, on Android — SecretKeyFactory with PBKDF2WithHmacSHA256.

Integration with biometrics

An advanced option — the database encryption key is protected by biometrics via Keystore/Keychain. On Android: KeyGenParameterSpec.Builder with .setUserAuthenticationRequired(true) and .setUserAuthenticationParameters(0, KeyProperties.AUTH_BIOMETRIC_STRONG). The key is created once on first biometric login; then each time the app opens, the user authenticates, the key is unlocked from Keystore, and the database opens.

On iOS, similarly via kSecAttrAccessControl with SecAccessControlCreateWithFlags and the .biometryAny or .biometryCurrentSet flag. Difference: .biometryCurrentSet invalidates the key when new fingerprints are added — this is important for banking apps (according to official Apple documentation).

What is included in the work

Full-cycle encryption implementation turnkey:

• Inventory of stored data and classification by criticality.
• Key scheme selection and integration with Keystore/Keychain.
• Implementation of encryption layer for database, files, and secrets.
• Migration of existing data (if any).
• Testing all scenarios: app update, restore from backup, biometric changes.
• Documentation and training for your team.

Process

We start with an inventory: what is stored where, whether encryption already exists, which data falls under requirements (PCI DSS, GDPR, local regulations). Then — key scheme selection, encryption layer implementation, integration with the existing storage. Separately — testing scenarios: app update, restore from backup, user biometric changes.

The timeline depends on data volume and the existence of a current storage scheme. If a database already exists and needs migration to SQLCipher — 3–5 days including testing. File cache encryption and Keychain integration — an additional 1–2 days. A full scheme from scratch for a new project is faster.

We will assess your project for free. Contact us — we will analyze your current storage scheme and offer the optimal solution with a security guarantee.

Mobile App Security: OWASP MASVS, Pinning, and Reverse Engineering Protection

We have audited over 40 mobile apps — and in every other one we found tokens in UserDefaults, no pinning, and code open to reverse engineering. Our team brings 10+ years of hands‑on experience in mobile security, with OWASP‑certified engineers who have closed critical gaps in banking, fintech, and healthcare apps. Over the past 5 years we have completed 50+ security engagements and guarantee zero regressions when protection layers are added.

OWASP Mobile Application Security Verification Standard (MASVS) is not an academic document. It's a pentester's checklist. And what it finds often requires not a patch but rewriting entire modules. Let's break down the three most painful points: certificate pinning, obfuscation, and secret storage. And show how to fix them without production downtime.

Why does certificate pinning break production?

Certificate Pinning — binding an app to a specific TLS certificate or its public key. Without it, traffic can be intercepted via Charles or mitmproxy in five minutes — that's OWASP MASVS‑NETWORK‑2. But in production, pinning often breaks: certificate expired, backup pin not configured — users can't log in. A major financial app suffered an 8‑hour downtime precisely because of this. In our practice, 80% of pinning failures come from missing backup pins.

On iOS, it is implemented via URLSessionDelegate.urlSession(_:didReceive:completionHandler:) with a SecTrust check. Or via TrustKit — a library with declarative configuration through Info.plist. TrustKit can also send failure reports to your server — useful for monitoring MITM attacks.

On Android — network_security_config.xml:

<network-security-config>
  <domain-config>
    <domain includeSubdomains="true">api.example.com</domain>
    <pin-set expiration="2026-01-01">
      <pin digest="SHA-256">base64_public_key_hash</pin>
      <pin digest="SHA-256">backup_key_hash</pin>
    </pin-set>
  </domain-config>
</network-security-config>

Critical rule: always two pins — primary and backup. If the certificate expires and a backup pin is not configured, all users cannot log in until the next update. That's how production builds break.

Another point of failure: CDN and third‑party SDK. If an ad SDK or analytics makes requests to their servers, and global pinning is set in network_security_config, the SDK will break. Configuration must be subdomain‑specific.

<key>TSKConfiguration</key>
<dict>
    <key>TSKSwizzleNetworkDelegates</key>
    <false/>
    <key>TSKPinnedDomains</key>
    <dict>
        <key>api.example.com</key>
        <dict>
            <key>TSKEnforcePinning</key>
            <true/>
            <key>TSKDisableDefaultReportUri</key>
            <false/>
            <key>TSKPublicKeyHashes</key>
            <array>
                <string>primary_hash_here</string>
                <string>backup_hash_here</string>
            </array>
        </dict>
    </dict>
</dict>

How to protect data in Keychain and Keystore?

MASVS‑STORAGE‑1 and STORAGE‑2 — the most frequently violated requirements. A common mistake on iOS: storing auth tokens in UserDefaults. Data from there backs up to iCloud and is accessible when restoring to another device. A token on a new iPhone means a foreign authorized session. Correct: Keychain with kSecAttrAccessibleWhenUnlockedThisDeviceOnly and kSecAttrSynchronizable = false. Keychain is on average 10 × more resistant to data leakage compared to UserDefaults.

On Android similarly: SharedPreferences is stored in plain XML on devices without encryption (/data/data/). Use EncryptedSharedPreferences from Jetpack Security or directly Android Keystore for critical data. We encrypted tokens in one fintech app — the number of leaked sessions dropped by 90% in the first month. Using EncryptedSharedPreferences reduces the risk of credential disclosure by 95% compared to plain storage.

Obfuscation and code protection

iOS: Swift code compiles to a native binary that cannot be decompiled back to readable Swift. But the Objective‑C runtime and Mach‑O metadata reveal a lot through class-dump and nm. Class names, method names, strings in the binary — all visible. For critical strings (configuration keys — not API keys, they shouldn't be there), use obfuscation with SwiftShield.

Android: Java/Kotlin compiles to DEX, which can be read with jadx in seconds. R8 (included by default in release builds) minifies and obfuscates. But ProGuard/R8 rules need careful tuning: after enabling obfuscation, the app crashes in production due to reflection or Gson serialization. Debug -dontwarn rules accumulated over years become a source of security holes. Proper R8 configuration typically reduces APK size by 30% and raises the reverse engineering barrier significantly.

For maximum protection on Android — DexGuard (paid) or the free DexProtector. They add runtime protection, string encryption, and integrity checks. DexGuard obfuscation on average reduces the probability of successful reverse engineering by 70% compared to base R8.

Comparison of obfuscation tools

Detecting jailbreak and root

MASVS‑RESILIENCE‑1 requires detection of compromised devices. Standard checks: presence of /Applications/Cydia.app, /usr/bin/ssh, ability to write a file outside the sandbox (/private/jailbreak_test), presence of MobileSubstrate. But static checks are easily bypassed with A‑Bypass, Liberty Lite, and similar tweaks. Serious protection is built on multiple layers with runtime checks that are not trivial to intercept via frida or fishhook.

Ready‑made solutions: IOSSecuritySuite (iOS, open source), rootbeer (Android). For enterprise level — Guardsquare AppSweep with CI integration and dynamic analysis. Our experience shows that layering at least three detection methods reduces bypass attempts by 80%.

Mobile app security engagement deliverables

How we implement protection: a case study from our practice

One of our clients came with a banking app that failed a security audit. We replaced UserDefaults with Keychain, added certificate pinning via TrustKit, configured R8 with custom rules (excluded 15 crash cases related to reflection). Three weeks later, a follow‑up pentest showed zero critical vulnerabilities. Since implementation — zero incidents in two years. Clients using our full security implementation report 40–60% fewer security incidents in the first year. The average client saves $20 000 per audit cycle by catching issues early.

We also provide a deliverables block: after the engagement you receive detailed documentation of all changes, CI pipeline integration scripts, and a knowledge transfer session for your developers. This ensures your team can maintain security independently.

Timeline and cost

• Security audit per OWASP MASVS L1 — from 1 to 2 weeks.
• Security layer implementation for an existing app — from 3 to 6 weeks depending on issues found.
• Full cycle "audit + implementation + test" — from 4 to 8 weeks.

Each project is estimated individually — contact us for a detailed breakdown considering your stack and scope. We work turnkey: from analysis to store deployment.

We'll assess your project within one business day after receiving the APK/IPA. Get in touch — we'll tell you which holes to close first. Schedule a consultation to discuss your mobile app security needs. Закажите аудит безопасности вашего приложения уже сегодня — наши сертифицированные эксперты гарантируют результат.