Expo Signature

expo-signature provides useful tools for digital signature on Android and iOS.

API documentation

Usage

import * as Signature from 'expo-signature';

const keyAlias = 'my_key_alias';

const encoder = new TextEncoder();
const data = encoder.encode('Data to sign');

Generate Key Pair

const keySpec: KeySpec = {
  alias: keyAlias,
  algorithm: 'EC', // or 'RSA'
  size: 256, // or 2048 for 'RSA'
};

const publicKey: PublicKey = await Signature.generateKeys(alias);

Retrieve an existing Public Key

const publicKey: PublicKey | null = await Signature.getPublicKey(alias);

Check for key presence

const keyExists: boolean = await Signature.isKeyPresentInKeychain(alias);

Delete Key Pair

const deleted: boolean = await Signature.deleteKey(alias);

Sign data

const info: SignaturePrompt = {
  title: 'User authentication',
  subtitle: 'Use biometry authentication to sign data',
  cancel: 'Cancel authentication',
};

const signature: Uint8Array = await Signature.signData(data, alias, info);

Verify data with key alias

const isValid: boolean = await Signature.verifyData(data, signature, alias);

Verify with Public Key

const publicKey: ECPublicKey = {
  x: '1234567890...',
  y: '0987654321...',
};

const isValid: boolean = await Signature.verifyWithKey(data, signature, publicKey);

Example

Check the example app for a full API usage.

Installation in managed Expo projects

For managed Expo projects, please follow the installation instructions in the API documentation for the latest stable release. If you follow the link and there is no documentation available then this library is not yet usable within managed projects — it is likely to be included in an upcoming Expo SDK release.

Installation in bare React Native projects

For bare React Native projects, you must ensure that you have installed and configured the expo package before continuing.

Add the package to your npm dependencies

npm install expo-signature

Configure for iOS

Add NSFaceIDUsageDescription key to your Info.plist:

<key>NSFaceIDUsageDescription</key>
<string>Allow $(PRODUCT_NAME) to use FaceID</string>

Run npx pod-install after installing the npm package.

Configure for Android

No additional set up necessary.

This module requires permissions to access the biometric data for authentication purposes. The USE_BIOMETRIC and USE_FINGERPRINT permissions are automatically added.

<!-- Added permissions -->
<uses-permission android:name="android.permission.USE_BIOMETRIC" />
<uses-permission android:name="android.permission.USE_FINGERPRINT" />

Config Plugin

This plugin is applied automatically in EAS Build, only add the config plugin if you want to pass in extra properties.

After installing this npm package, add the config plugin to the plugins array of your app.json or app.config.js:

{
  "expo": {
    "plugins": [
      [
        "expo-signature",
        {
          "faceIDPermission": "Allow $(PRODUCT_NAME) to use Face ID."
        }
      ]
    ]
  }
}

Running npx expo prebuild will generate the native project locally with the applied changes in your iOS Info.plist file.

Contributing

Contributions are very welcome! Please refer to guidelines described in the contributing guide.