KeyPairGeneratorSpec replacement with KeyGenParameterSpec.Builder equivalents - Keystore operation failed

Question

The following method is deprecated

KeyPairGenerator generator = KeyPairGenerator.getInstance("RSA", "AndroidKeyStore");

KeyPairGeneratorSpec spec = new KeyPairGeneratorSpec.Builder(this)
                            .setAlias(alias)
                            .setSubject(new X500Principal("CN=Sample Name, O=Android Authority"))
                            .setSerialNumber(BigInteger.ONE)
                            .setStartDate(start.getTime())
                            .setEndDate(end.getTime())
                            .build();

generator.initialize(spec);

The replacement I came upon looks like this

KeyPairGenerator generator = KeyPairGenerator.getInstance("RSA", "AndroidKeyStore");

generator.initialize(new KeyGenParameterSpec.Builder
                            (alias, KeyProperties.PURPOSE_SIGN)
                            .setDigests(KeyProperties.DIGEST_SHA256)
                            .setSignaturePaddings(KeyProperties.SIGNATURE_PADDING_RSA_PKCS1)
                            .build());

Although I am able to use this to generate a keypair entry and encrypt the value, I am unable to decrypt it

 public void encryptString(String alias) {
        try {
            KeyStore.PrivateKeyEntry privateKeyEntry = (KeyStore.PrivateKeyEntry)keyStore.getEntry(alias, null);
            RSAPublicKey publicKey = (RSAPublicKey) privateKeyEntry.getCertificate().getPublicKey();

            String initialText = startText.getText().toString();
            if(initialText.isEmpty()) {
                Toast.makeText(this, "Enter text in the 'Initial Text' widget", Toast.LENGTH_LONG).show();
                return;
            }

            //Security.getProviders();

            Cipher inCipher = Cipher.getInstance("RSA/ECB/PKCS1Padding", "AndroidKeyStoreBCWorkaround");
            inCipher.init(Cipher.ENCRYPT_MODE, publicKey);

            ByteArrayOutputStream outputStream = new ByteArrayOutputStream();
            CipherOutputStream cipherOutputStream = new CipherOutputStream(
                    outputStream, inCipher);
            cipherOutputStream.write(initialText.getBytes("UTF-8"));
            cipherOutputStream.close();

            byte [] vals = outputStream.toByteArray();
            encryptedText.setText(Base64.encodeToString(vals, Base64.DEFAULT));
        } catch (Exception e) {
            Toast.makeText(this, "Exception " + e.getMessage() + " occured", Toast.LENGTH_LONG).show();
            Log.e(TAG, Log.getStackTraceString(e));
        }
    }

public void decryptString(String alias) {
        try {
            KeyStore.PrivateKeyEntry privateKeyEntry = (KeyStore.PrivateKeyEntry)keyStore.getEntry(alias, null);

            Cipher output = Cipher.getInstance("RSA/ECB/PKCS1Padding", "AndroidKeyStoreBCWorkaround");
            output.init(Cipher.DECRYPT_MODE, privateKeyEntry.getPrivateKey());

            String cipherText = encryptedText.getText().toString();
            CipherInputStream cipherInputStream = new CipherInputStream(
                    new ByteArrayInputStream(Base64.decode(cipherText, Base64.DEFAULT)), output);
            ArrayList<Byte> values = new ArrayList<>();
            int nextByte;
            while ((nextByte = cipherInputStream.read()) != -1) {
                values.add((byte)nextByte);
            }

            byte[] bytes = new byte[values.size()];
            for(int i = 0; i < bytes.length; i++) {
                bytes[i] = values.get(i).byteValue();
            }

            String finalText = new String(bytes, 0, bytes.length, "UTF-8");
            decryptedText.setText(finalText);

        } catch (Exception e) {
            Toast.makeText(this, "Exception " + e.getMessage() + " occured", Toast.LENGTH_LONG).show();
            Log.e(TAG, Log.getStackTraceString(e));
        }

in the decrypt method, the following command fails:

 Cipher output = Cipher.getInstance("RSA/ECB/PKCS1Padding", "AndroidKeyStoreBCWorkaround");
                output.init(Cipher.DECRYPT_MODE, privateKeyEntry.getPrivateKey());

with

java.security.InvalidKeyException: Keystore operation failed

I think it has to do with the KeyGenParamaterSpec.Builder has incorrect conditions, similarly that the encrypt Cipher types are incorrect strings, same thing in the decrypt function.

But this can all be traced back to the use of the new KeygenParameterSpec.Builder, as using the older deprecated method allows me to encrypt and decrypt.

How to fix?

Answer 1

As Alex mentioned one missing piece is KeyProperties.PURPOSE_DECRYPT other one is setSignaturePaddings instead for that you have to use setEncryptionPaddings method. Here is the sample snippet.

    new KeyGenParameterSpec.Builder(ALIAS, KeyProperties.PURPOSE_ENCRYPT | KeyProperties.PURPOSE_DECRYPT)
            .setEncryptionPaddings(KeyProperties.ENCRYPTION_PADDING_RSA_PKCS1)
            // other options
           .build()

Refer documentation for more information.