flutter_apksig

A Flutter plugin for verifying and signing Android APK files, powered by the apksig-android library — an Android port of AOSP's apksig tool.

Supports JAR signing (v1), APK Signature Scheme v2, v3, and v4.

Features

  • ✅ Verify APK signatures (v1 / v2 / v3 / v3.1 / v4)
  • ✅ Sign APKs with a Java keystore (.jks / .p12)
  • ✅ Compare a certificate SHA-256 fingerprint to confirm APK authenticity
  • ✅ Retrieve per-signer certificate details (subject, issuer, SHA-256, validity)
  • ✅ In-place signing (input path == output path)
  • ✅ Runs all I/O on a background thread — never blocks the UI

Platform support

Platform Support
Android ✅
iOS ❌ (Android-only library)

Installation

dependencies:
  flutter_apksig: ^0.0.1

flutter pub get

Usage

import 'package:flutter_apksig/flutter_apksig.dart';

final apksig = FlutterApksig();

Verify an APK

final result = await apksig.verifyApk(
  '/sdcard/Download/app.apk',
  // Optional: bound the Android SDK range to verify against
  // minSdkVersion: 24,
  // maxSdkVersion: 34,
);

if (result.verified) {
  for (final signer in result.signers.whereType<ApkSignerInfo>()) {
    final cert = signer.certificates.firstOrNull;
    print('Subject : ${cert?.subjectDn}');
    print('SHA-256 : ${cert?.sha256Fingerprint}');
    print('Schemes : '
        '${signer.hasV1Signature ? 'v1 ' : ''}'
        '${signer.hasV2Signature ? 'v2 ' : ''}'
        '${signer.hasV3Signature ? 'v3 ' : ''}'
        '${signer.hasV4Signature ? 'v4' : ''}');
  }
} else {
  for (final e in result.errors.whereType<String>()) {
    print('Error: $e');
  }
}

Verify a certificate fingerprint

Confirm an APK was signed with a specific key by comparing its embedded certificate against a known SHA-256 fingerprint (e.g. the one from your release keystore).

final result = await apksig.verifyApkCertificate(
  '/sdcard/Download/app.apk',
  expectedSha256: 'AA:BB:CC:…', // colon-separated or plain hex, any case
);

if (result.isAuthentic) {
  print('Authentic — signed with the expected key.');
} else if (!result.verified) {
  print('Invalid signatures: ${result.verifyResult.errors}');
} else {
  print('Wrong certificate. Found: ${result.matchedFingerprints}');
}

To retrieve the fingerprint from an already-signed APK (useful for pinning):

final fingerprints = await apksig.getApkCertificateFingerprints(
  '/sdcard/Download/app.apk',
);
print(fingerprints); // ['AA:BB:CC:…']

Or get it directly from the keystore with keytool:

keytool -list -v \
  -keystore release.jks \
  -alias myKey \
  -storepass <keystorePassword>
# Look for the SHA256: line in the output.

Sign an APK

await apksig.signApk(
  ApkSignRequest(
    inputApkPath:     '/sdcard/Download/unsigned.apk',
    outputApkPath:    '/sdcard/Download/signed.apk',
    keystorePath:     '/sdcard/Download/release.jks',
    keystorePassword: 'keystorePassword',
    keyAlias:         'myKey',
    keyPassword:      'keyPassword',
    // Optional — library defaults are used when omitted:
    // v1SigningEnabled: true,
    // v2SigningEnabled: true,
    // v3SigningEnabled: true,
    // v4SigningEnabled: false,
    // minSdkVersion: 24,
  ),
);

In-place signing — outputApkPath may equal inputApkPath. The plugin writes to a temporary file and atomically renames it on success, so the original is never corrupted.

Keystore formats

Android does not include a JKS (Java KeyStore) provider at runtime, so .jks files created with older versions of Java must be converted to PKCS12 before use. Android Studio keystores are already PKCS12 internally (the .jks extension is kept for historical reasons), so they work without conversion.

Format Extension Support
PKCS12 .p12, .pfx, .jks (Android Studio) ✅
BKS (Bouncy Castle) .bks ✅
JKS (legacy Sun) .jks (old Java) ❌ — convert first

To check whether your .jks is PKCS12 or legacy JKS:

keytool -list -keystore release.jks -storepass <password>
# Output will say "Keystore type: PKCS12" or "Keystore type: JKS"

To convert a legacy JKS keystore to PKCS12:

keytool -importkeystore \
  -srckeystore  old.jks  -srcstoretype  JKS \
  -destkeystore new.p12  -deststoretype PKCS12 \
  -srcalias  <alias>     -destalias  <alias>

Error handling

All methods throw a PlatformException on failure (I/O error, bad keystore credentials, malformed APK, etc.):

try {
  await apksig.signApk(request);
} on PlatformException catch (e) {
  print('${e.code}: ${e.message}');
}

Android permissions

Verifying an APK requires no special permissions. Signing requires read access to the keystore and write access to the output path. For files on external storage, declare READ_EXTERNAL_STORAGE / WRITE_EXTERNAL_STORAGE (API ≤ 28) or use the permission_handler package.

How it works

Flutter (Dart)                      Android (Kotlin)
─────────────────────               ──────────────────────────────
FlutterApksig.verifyApk()           FlutterApksigPlugin
FlutterApksig.signApk()      ──►    (implements ApksigHostApi)
FlutterApksig.verifyApk            │
  Certificate()                    │  apksig-android (bundled JAR)
                                   ├─ ApkVerifier
                                   └─ ApkSigner

Platform channel code is generated with Pigeon from pigeons/messages.dart. All APK I/O runs on a background thread — the UI is never blocked.

License

Copyright 2024 flutter_apksig authors

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

    https://www.apache.org/licenses/LICENSE-2.0

The bundled apksig-android library is also Apache 2.0 — see apksig-android/LICENSE.

Libraries

flutter_apksig
Flutter plugin for verifying and signing Android APK files using the apksig-android library (an Android port of AOSP's apksig tool).