Criptografia e descriptografia da API de impressão digital do Android

Question

Mar 14, 2016, 05:17 PM

android-keystore android-fingerprint-api encryption android

Criptografia e descriptografia da API de impressão digital do Android

Estou usando a API de impressão digital do Android M para permitir que os usuários acessem o aplicativo. Para fazer isso, eu precisaria armazenar o nome de usuário e a senha no dispositivo. Atualmente, tenho o login funcionando, bem como a API de impressão digital, mas o nome de usuário e a senha são armazenados como texto sem formatação. Gostaria de criptografar a senha antes de armazená-la e poder recuperá-la depois que o usuário se autenticar com a impressão digital.

Estou tendo muita dificuldade em fazer isso funcionar. Eu tenho tentado aplicar o que posso doAmostras de segurança do Android, mas cada exemplo parece lidar apenas com criptografia ou assinatura e nunca descriptografar.

O que tenho até agora é que tenho que obter uma instância doAndroidKeyStore, umaKeyPairGenerator e umCipher, usando criptografia assimétrica para permitir o uso do AndroidKeyGenParameterSpec.Builder().setUserAuthenticationRequired(true). O motivo da criptografia assimétrica é porque osetUserAuthenticationRequired método irá bloquearqualquer uso da chave se o usuário não estiver autenticado, mas:

Esta autorização se aplica apenas a operações de chave secreta e chave privada. Operações de chave pública não são restritas.

Isso deve permitir que eu criptografe a senha usando a chave pública antes que o usuário se autentique com a impressão digital e descriptografe usando a chave privada somente depois que o usuário for autenticado.

public KeyStore getKeyStore() {
    try {
        return KeyStore.getInstance("AndroidKeyStore");
    } catch (KeyStoreException exception) {
        throw new RuntimeException("Failed to get an instance of KeyStore", exception);
    }
}

public KeyPairGenerator getKeyPairGenerator() {
    try {
        return KeyPairGenerator.getInstance("EC", "AndroidKeyStore");
    } catch(NoSuchAlgorithmException | NoSuchProviderException exception) {
        throw new RuntimeException("Failed to get an instance of KeyPairGenerator", exception);
    }
}

public Cipher getCipher() {
    try {
        return Cipher.getInstance("EC");
    } catch(NoSuchAlgorithmException | NoSuchPaddingException exception) {
        throw new RuntimeException("Failed to get an instance of Cipher", exception);
    }
}

private void createKey() {
    try {
        mKeyPairGenerator.initialize(
                new KeyGenParameterSpec.Builder(KEY_ALIAS,
                        KeyProperties.PURPOSE_ENCRYPT | KeyProperties.PURPOSE_DECRYPT)
                        .setAlgorithmParameterSpec(new ECGenParameterSpec("secp256r1")
                        .setUserAuthenticationRequired(true)
                        .build());
        mKeyPairGenerator.generateKeyPair();
    } catch(InvalidAlgorithmParameterException exception) {
        throw new RuntimeException(exception);
    }
}

private boolean initCipher(int opmode) {
    try {
        mKeyStore.load(null);

        if(opmode == Cipher.ENCRYPT_MODE) {
            PublicKey key = mKeyStore.getCertificate(KEY_ALIAS).getPublicKey();
            mCipher.init(opmode, key);
        } else {
            PrivateKey key = (PrivateKey) mKeyStore.getKey(KEY_ALIAS, null);
            mCipher.init(opmode, key);
        }

        return true;
    } catch (KeyPermanentlyInvalidatedException exception) {
        return false;
    } catch(KeyStoreException | CertificateException | UnrecoverableKeyException
            | IOException | NoSuchAlgorithmException | InvalidKeyException
            | InvalidAlgorithmParameterException exception) {
        throw new RuntimeException("Failed to initialize Cipher", exception);
    }
}

private void encrypt(String password) {
    try {
        initCipher(Cipher.ENCRYPT_MODE);
        byte[] bytes = mCipher.doFinal(password.getBytes());
        String encryptedPassword = Base64.encodeToString(bytes, Base64.NO_WRAP);
        mPreferences.getString("password").set(encryptedPassword);
    } catch(IllegalBlockSizeException | BadPaddingException exception) {
        throw new RuntimeException("Failed to encrypt password", exception);
    }
}

private String decryptPassword(Cipher cipher) {
    try {
        String encryptedPassword = mPreferences.getString("password").get();
        byte[] bytes = Base64.decode(encryptedPassword, Base64.NO_WRAP);
        return new String(cipher.doFinal(bytes));
    } catch (IllegalBlockSizeException | BadPaddingException exception) {
        throw new RuntimeException("Failed to decrypt password", exception);
    }
}


To be honest, I am not sure if any of this is right, it is bits and pieces from anything I could find on the subject. Everything I change throws a different exception, and this particular build does not run because I cannot instantiate the Cipher, it throws a NoSuchAlgorithmException: No provider found for EC. I have tried switch to RSA as well, but I get similar errors.

So my question is basically this; how can I encrypt plaintext on Android, and make it available for decryption after the user is authenticated by the Fingerprint API? 



I have made some progress, mostly due to the discovery of the information on the KeyGenParameterSpec documentation page.

I have kept getKeyStore, encryptePassword, decryptPassword, getKeyPairGenerator and getCipher mostly the same, but I changed the KeyPairGenerator.getInstance and Cipher.getInstance to "RSA" and "RSA/ECB/OAEPWithSHA-256AndMGF1Padding" respectively. 

I also changed the rest of the code to RSA instead of Elliptic Curve, because from what I understand, Java 1.7 (and therefore Android) does not support encryption and decryption with EC. I changed my createKeyPair method based on the "RSA key pair for encryption/decryption using RSA OAEP" example on the documentation page:

private void createKeyPair() {
    try {
        mKeyPairGenerator.initialize(
                new KeyGenParameterSpec.Builder(KEY_ALIAS, KeyProperties.PURPOSE_DECRYPT)
                        .setDigests(KeyProperties.DIGEST_SHA256, KeyProperties.DIGEST_SHA512)
                        .setEncryptionPaddings(KeyProperties.ENCRYPTION_PADDING_RSA_OAEP)
                        .setUserAuthenticationRequired(true)
                        .build());
        mKeyPairGenerator.generateKeyPair();
    } catch(InvalidAlgorithmParameterException exception) {
        throw new RuntimeException(exception);
    }
}


I also altered my initCipher method based on the known issue in the KeyGenParameterSpec documentation:


  A known bug in Android 6.0 (API Level 23) causes user authentication-related authorizations to be enforced even for public keys. To work around this issue extract the public key material to use outside of Android Keystore.


private boolean initCipher(int opmode) {
    try {
        mKeyStore.load(null);

        if(opmode == Cipher.ENCRYPT_MODE) {
            PublicKey key = mKeyStore.getCertificate(KEY_ALIAS).getPublicKey();

            PublicKey unrestricted = KeyFactory.getInstance(key.getAlgorithm())
                    .generatePublic(new X509EncodedKeySpec(key.getEncoded()));

            mCipher.init(opmode, unrestricted);
        } else {
            PrivateKey key = (PrivateKey) mKeyStore.getKey(KEY_ALIAS, null);
            mCipher.init(opmode, key);
        }

        return true;
    } catch (KeyPermanentlyInvalidatedException exception) {
        return false;
    } catch(KeyStoreException | CertificateException | UnrecoverableKeyException
            | IOException | NoSuchAlgorithmException | InvalidKeyException
            | InvalidAlgorithmParameterException exception) {
        throw new RuntimeException("Failed to initialize Cipher", exception);
    }
}


Now I can encrypt the password, and save the encrypted password. But when I obtain the encrypted password and attempt to decrypt, I get a KeyStoreException Unknown error...

03-15 10:06:58.074 14702-14702/com.example.app E/LoginFragment: Failed to decrypt password
        javax.crypto.IllegalBlockSizeException
            at android.security.keystore.AndroidKeyStoreCipherSpiBase.engineDoFinal(AndroidKeyStoreCipherSpiBase.java:486)
            at javax.crypto.Cipher.doFinal(Cipher.java:1502)
            at com.example.app.ui.fragment.util.LoginFragment.onAuthenticationSucceeded(LoginFragment.java:251)
            at com.example.app.ui.controller.FingerprintCallback.onAuthenticationSucceeded(FingerprintCallback.java:21)
            at android.support.v4.hardware.fingerprint.FingerprintManagerCompat$Api23FingerprintManagerCompatImpl$1.onAuthenticationSucceeded(FingerprintManagerCompat.java:301)
            at android.support.v4.hardware.fingerprint.FingerprintManagerCompatApi23$1.onAuthenticationSucceeded(FingerprintManagerCompatApi23.java:96)
            at android.hardware.fingerprint.FingerprintManager$MyHandler.sendAuthenticatedSucceeded(FingerprintManager.java:805)
            at android.hardware.fingerprint.FingerprintManager$MyHandler.handleMessage(FingerprintManager.java:757)
            at android.os.Handler.dispatchMessage(Handler.java:102)
            at android.os.Looper.loop(Looper.java:148)
            at android.app.ActivityThread.main(ActivityThread.java:5417)
            at java.lang.reflect.Method.invoke(Native Method)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)
        Caused by: android.security.KeyStoreException: Unknown error
            at android.security.KeyStore.getKeyStoreException(KeyStore.java:632)
            at android.security.keystore.KeyStoreCryptoOperationChunkedStreamer.doFinal(KeyStoreCryptoOperationChunkedStreamer.java:224)
            at android.security.keystore.AndroidKeyStoreCipherSpiBase.engineDoFinal(AndroidKeyStoreCipherSpiBase.java:473)
            at javax.crypto.Cipher.doFinal(Cipher.java:1502)&nbsp;
            at com.example.app.ui.fragment.util.LoginFragment.onAuthenticationSucceeded(LoginFragment.java:251)&nbsp;
            at com.example.app.ui.controller.FingerprintCallback.onAuthenticationSucceeded(FingerprintCallback.java:21)&nbsp;
            at android.support.v4.hardware.fingerprint.FingerprintManagerCompat$Api23FingerprintManagerCompatImpl$1.onAuthenticationSucceeded(FingerprintManagerCompat.java:301)&nbsp;
            at android.support.v4.hardware.fingerprint.FingerprintManagerCompatApi23$1.onAuthenticationSucceeded(FingerprintManagerCompatApi23.java:96)&nbsp;
            at android.hardware.fingerprint.FingerprintManager$MyHandler.sendAuthenticatedSucceeded(FingerprintManager.java:805)&nbsp;
            at android.hardware.fingerprint.FingerprintManager$MyHandler.handleMessage(FingerprintManager.java:757)&nbsp;
            at android.os.Handler.dispatchMessage(Handler.java:102)&nbsp;
            at android.os.Looper.loop(Looper.java:148)&nbsp;
            at android.app.ActivityThread.main(ActivityThread.java:5417)&nbsp;
            at java.lang.reflect.Method.invoke(Native Method)&nbsp;
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)&nbsp;
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)