Wsrtjtyk

I'm trying to use RSA encryption with KeyStore and I need to specify Parameters for KeyPairGenerator and I'm lost here. KeyPairGeneratorPair is kinda straightforward, but I don't understand KeyGenParameterSpec for API>=23

That's what I did, I think I got everything in else part, but now I'm confused about KeyGenParameterSpec

What exactly public exponent in RSAKeyGenParameterSpec is?

What Digests in .setDigests should i specify?

There's also .setBlockMode() method to call, and since I'm using RSA and RSA/None/OAEPWithSHA1AndMGF1Paddingwhich block mode to set? ECB, CBC?

if(Build.VERSION.SDK_INT >= Build.VERSION_CODES.M) {

                generator.initialize(new KeyGenParameterSpec.Builder("PrivateKey", KeyProperties.PURPOSE_ENCRYPT | KeyProperties.PURPOSE_DECRYPT)

                        .setAlgorithmParameterSpec(new RSAKeyGenParameterSpec(2048, RSAKeyGenParameterSpec.F4))

                        .setDigests(KeyProperties.DIGEST_SHA1,

                                KeyProperties.DIGEST_SHA256)

                        .setEncryptionPaddings(KeyProperties.ENCRYPTION_PADDING_RSA_OAEP)

                        .setCertificateSerialNumber(BigInteger.ONE)

                        .setCertificateSubject(new X500Principal("CN=" + "PrivateKey"))

                        .setCertificateNotBefore(calendar.getTime())

                        .setCertificateNotAfter(endCalendar.getTime())

                        .setKeySize(2048).build());

            } else {

                generator.initialize(new KeyPairGeneratorSpec.Builder(MainActivity.this)

                        .setAlias("PrivateKey")

                        .setSerialNumber(BigInteger.ONE)

                        .setSubject(new X500Principal("CN=" + "PrivateKey"))

                        .setStartDate(calendar.getTime())

                        .setEndDate(endCalendar.getTime())

                        .setKeySize(2048).build()      



 );

            }



Cipher cipher = Cipher.getInstance("RSA/None/OAEPWithSHA1AndMGF1Padding");

Method setDigests() sets digest method for your padding mode and setBlockMode() sets encryption mode which depends on your work.

I think you have set a lot of unnecessary field. For example I use this method to create my own RSA key:

public boolean createKey() {

    try {

        KeyPairGenerator keyPairGenerator = KeyPairGenerator.getInstance(

                KeyProperties.KEY_ALGORITHM_RSA,

                "AndroidKeyStore"

        );



        mKeyStore.load(null);

        KeyGenParameterSpec.Builder builder =

                new KeyGenParameterSpec.Builder(

                        MY_KEY,

                        KeyProperties.PURPOSE_DECRYPT).

                setKeySize(MY_KEYLEN).

                setEncryptionPaddings(KeyProperties.ENCRYPTION_PADDING_RSA_OAEP).

                setDigests(KeyProperties.DIGEST_SHA256);



        keyPairGenerator.initialize(builder.build());

        keyPairGenerator.generateKeyPair();

    } catch (NoSuchAlgorithmException | CertificateException | IOException |

            InvalidAlgorithmParameterException | NoSuchProviderException e) {

        return false;

    }



    return true;

}

I created this key to use with RSA/ECB/OAEPWithSHA-256AndMGF1Padding algorithm.