File Signer

Android application for cryptographic file signing using ECDSA P-256. Signs any file on device and saves the detached signature alongside the original. Signatures can be verified at any time - either from the signing history or by selecting any file and its .sig independently via the standalone Verify flow.

Technical Summary

Property	Value
Algorithm	ECDSA (secp256r1 / NIST P-256)
Hash	SHA-256
Key Storage	Android KeyStore (StrongBox or TEE)
Signature Format	DER-encoded ECDSA, detached .sig file
Max File Size	500 MB
Min SDK	26 (Android 8.0)
Target SDK	35 (Android 15)

Architecture

Clean Architecture with MVVM. Three layers, strict dependency direction inward.

presentation/  ->  domain/  <-  data/
  MainActivity       UseCases      KeystoreDataSource
  MainViewModel      Models        FileDataSource
  Components         Repositories  RepositoryImpl

Dependency injection via Hilt. Coroutines for async operations. Jetpack Compose for UI.

Security

• Private keys never leave the hardware-backed secure enclave (StrongBox where available, TEE as fallback)
• No INTERNET permission. No network libraries. No analytics or telemetry
• android:allowBackup="false" and explicit data extraction exclusions
• cleartextTrafficPermitted="false" in network security config
• Debug logging uses SanitizedDebugTree that masks file URIs, paths, and names
• Streaming signature computation. Buffers zeroed after use
• ProGuard/R8 enabled for release builds

See docs/SECURITY.md for the full security design document.

Compliance

The application aligns with the following standards and frameworks:

Standard	Status	Notes
BSI TR-02102-1	Aligned	ECDSA P-256, SHA-256, hardware-backed keys
OWASP MASVS	90% coverage	All domains pass except optional resilience hardening
NIST SP 800-186	Aligned	Approved curve and hash
FIPS 186-4	Aligned	DSS-compliant algorithm selection
GDPR	Compliant	No data collection, no network, no PII processing

See docs/COMPLIANCE.md for the full compliance matrix.

Note: This application produces raw ECDSA signatures. It does not produce qualified electronic signatures under eIDAS Regulation (EU) 910/2014. For eIDAS compliance, signatures must be wrapped in CAdES/XAdES/PAdES format and issued through a Qualified Trust Service Provider (QTSP).

Build

# Debug
./gradlew assembleDebug

# Release
./gradlew assembleRelease

# Unit tests
./gradlew testDebugUnitTest

# Instrumented tests (requires emulator or device)
./gradlew connectedDebugAndroidTest

Project Structure

app/src/main/java/com/filesigner/
  data/
    repository/         FileRepositoryImpl, SigningRepositoryImpl
    source/             KeystoreDataSource, FileDataSource
  di/                   AppModule (Hilt)
  domain/
    model/              FileInfo, SigningResult, SigningHistoryEntry
    repository/         FileRepository, SigningRepository (interfaces)
    usecase/            SignFileUseCase, VerifyFileUseCase, GetFileInfoUseCase, GenerateKeyPairUseCase
  presentation/
    components/         AboutSheet, FilePickerButton, SignButton, StatusDisplay, SigningHistorySheet, VerifySheet, PermissionRationaleDialog
    theme/              Theme, Color
    MainActivity.kt
    MainViewModel.kt
    SigningUiState.kt
  util/
    SanitizedDebugTree  PII-masking Timber tree
    SamplePdfGenerator  Debug-only test file generation

Testing

Unit tests cover the domain and data layers. Instrumented tests cover UI components and cryptographic performance benchmarks. See docs/TESTING.md.

Accessibility

The application supports TalkBack screen reader navigation, dynamic font scaling, RTL layouts, dark theme, and Material You dynamic color on Android 12+. All interactive elements have content descriptions. Status changes are announced via live regions. Section headings use heading semantics for swipe navigation. See docs/ACCESSIBILITY.md.

Documentation

Document	Description
docs/SECURITY.md	Security architecture, cryptography design, threat model
docs/COMPLIANCE.md	BSI, OWASP MASVS, NIST, GDPR compliance matrix
docs/ARCHITECTURE.md	Architecture decisions, layer responsibilities, data flow
docs/TESTING.md	Test strategy, coverage areas, running tests
docs/ACCESSIBILITY.md	Accessibility features, TalkBack support, contrast
docs/ROADMAP.md	Planned improvements and future considerations
LEGAL.md	Legal notices, export controls, eIDAS, CRA, jurisdiction

License

Apache License 2.0 - see LICENSE.

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Compliance & Liability

IMPORTANT: By using, cloning, or forking this repository, you acknowledge and agree to the following:

Not a Certified Product

This application provides cryptographic file signing. It is NOT a FIPS 140-3 validated module, a qualified electronic signature tool under eIDAS (EU) 910/2014, or a certified product under any national or international scheme. Compliance alignment statements (BSI, OWASP, NIST) describe algorithm selection and architecture, not formal certification.

Export Controls

This software implements ECDSA P-256 cryptographic technology. Users are solely responsible for compliance with:

• EU: Dual-Use Regulation (EU 2021/821). Open-source exemptions may apply.
• US: Export Administration Regulations (EAR), ECCN 5D002. Publicly available open-source may qualify for License Exception TSR.
• Other: Local import/export and usage regulations for cryptographic software.

Jurisdiction Restrictions

Cryptographic software is restricted or regulated in certain jurisdictions. This software must not be used to circumvent any applicable laws or regulations. Users deploying in any jurisdiction assume full responsibility for legal compliance.

No Warranty

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY ARISING FROM THE USE OF THIS SOFTWARE.

See LEGAL.md for detailed legal notices covering eIDAS, EU Cyber Resilience Act, export controls, and jurisdiction.