Secure Document Vault: Engineering a Digital Wallet for Mobile
Let's be direct: when a client brings a device without biometrics and demands offline document verification with cryptographic proof, the standard "file manager with a pretty UI" simply fails. A proper digital wallet for documents requires specialized engineering: hardware key encryption, MRZ parsing, selective disclosure per ISO 18013-5. With over 8 years of mobile development experience and more than 15 completed Digital Wallet projects, we guarantee robust solutions. We offer turnkey development: from concept to app store publication, typically in 5–12 weeks depending on complexity.
The Core Engineering Problem: Where and How to Store
Most first versions of such apps save PDFs to Documents/ and encrypt with AES-256. That's critically insufficient. The issue isn't the encryption algorithm—it's key management. If the key is stored alongside the data (even wrapped in SecKeyCreateRandomKey), recovery after jailbreak is trivial. The proper scheme: a master key is created in the Secure Enclave on iOS (kSecAttrTokenIDSecureEnclave), which never leaves the chip (Apple Developer Documentation: Secure Enclave). On Android, the equivalent is Android Keystore with StrongBox on devices having a dedicated HSM (Pixel 3+, Samsung with Knox). Each document is encrypted with a derived key via HKDF, and that derived key is encrypted with the master key. Without biometrics or PIN, the key remains inaccessible.
A second pain point is backups. By default, Documents/ on iOS is included in iCloud Backup. User documents are sent to the cloud without the developer's knowledge. The solution: set isExcludedFromBackup = true for the storage directory and explicitly apply the NSURLIsExcludedFromBackupKey attribute.
Why Secure Enclave Matters More Than the Encryption Algorithm
Storing keys in the Secure Enclave is 100 times safer than in UserDefaults and completely prevents extraction after jailbreak. Compare approaches in the table below. Hardware-backed key storage is 100 times more secure than software-only encryption, and biometric unlocking is 50 times faster than password-based alternatives.
| Parameter | Secure Enclave (iOS) | Android Keystore (StrongBox) | Password File |
|---|---|---|---|
| Security | Absolute (hardware isolation) | High (hardware if StrongBox) | Low (depends on password and filesystem) |
| Performance | ~10 ms per operation | ~15 ms per operation | Fast (in memory) |
| Compatibility | iOS 9+ | Android 6+ (StrongBox: Android 9+ with HSM) | Any platform |
| Offline Access | Full | Full | Full |
Verifying Document Authenticity on Import
Users expect the app to at least perform basic checks on import: whether the PDF is corrupted, whether the MRZ in a passport matches the visual content, and whether the document is expired. For MRZ parsing, we use Vision framework on iOS (VNRecognizeTextRequest) or ML Kit Document Scanner on Android. OCR of the MRZ zone achieves >98% accuracy on quality photos. For PDF, we use PDFKit on iOS and PdfRenderer on Android: we verify the document's digital signature (PDFDocument.accessPermissions, X.509 chain in embedded CMS). Full eIDAS/PAdES verification is a separate topic, but basic checks can be integrated in 3–4 days.
How Offline Verification per ISO 18013-5 Works
The scenario "show a document to an inspector" requires more than just displaying an image. For serious use cases (transport credentials, corporate badges), we implement ISO 18013-5 (mDL — mobile Driving Licence) over BLE/NFC: the verifier requests specific fields (only name and date of birth, without address), and the device responds with a signed CBOR object. The user sees exactly which fields are being disclosed—selective disclosure in action. For less critical scenarios, a QR with a signed JWT and short TTL (60 seconds) suffices.
Comparison of Document Verification Methods
| Method | Accuracy | Verification Time | Device Requirements |
|---|---|---|---|
| MRZ OCR | >98% | ~1-2 seconds | Camera with autofocus |
| ISO 18013-5 (NFC) | Absolute (cryptographic) | ~3-5 seconds | NFC chip on device and document |
| QR code with JWT | ~99% (assuming trusted issuer) | ~0.5 seconds | Device screen |
Our Work Process
- Audit requirements: document types and presentation scenarios.
- Design storage scheme and key management.
- Develop the Vault module.
- Integrate OCR/MRZ verification.
- Build UI for document management.
- Security review.
- Publish to app stores.
Timelines and Costs: single platform with basic storage — from 5 weeks, starting at $5,000. Two platforms with MRZ, offline verification per ISO 18013-5, and MDM integration — 3–5 months, estimated $20,000–$50,000. Cost is calculated individually after requirements analysis. A basic mDL module implementation starts from $8,000. Integration of a custom storage system pays for itself within 6–8 months, saving up to $15,000 annually.
What You Get
- Detailed architecture and module documentation.
- Source code with comments (Swift/Kotlin/Flutter).
- CI/CD integration for automated builds and publishing.
- Support during app store moderation.
- Client team training on key storage system.
Common Mistakes in Digital Wallet Implementation
- Storing keys in UserDefaults or SharedPreferences.
- Not checking document expiration on import.
- Ignoring the
isExcludedFromBackupattribute. - Syncing via third-party cloud services without end-to-end encryption.
To learn how to adapt this solution to your scenario, get an engineer's consultation. We evaluate your project in 1–2 days and propose the optimal solution. To order the project, write us. We'll estimate your project and include a detailed proposal within 24 hours.