RSAActivity.java

package com.example.myapplication;

import android.content.Context;
import android.os.Build;
import android.os.Bundle;
import android.security.KeyPairGeneratorSpec;
import android.security.keystore.KeyGenParameterSpec;
import android.security.keystore.KeyProperties;
import android.util.Base64;
import android.util.Log;

import androidx.annotation.Nullable;
import androidx.annotation.RequiresApi;
import androidx.appcompat.app.AppCompatActivity;

import java.io.IOException;
import java.io.UnsupportedEncodingException;
import java.math.BigInteger;
import java.nio.charset.StandardCharsets;
import java.security.GeneralSecurityException;
import java.security.InvalidAlgorithmParameterException;
import java.security.InvalidKeyException;
import java.security.Key;
import java.security.KeyFactory;
import java.security.KeyPair;
import java.security.KeyPairGenerator;
import java.security.KeyStore;
import java.security.KeyStoreException;
import java.security.NoSuchAlgorithmException;
import java.security.NoSuchProviderException;
import java.security.PrivateKey;
import java.security.PublicKey;
import java.security.SecureRandom;
import java.security.Signature;
import java.security.SignatureException;
import java.security.UnrecoverableEntryException;
import java.security.cert.CertificateException;
import java.security.interfaces.ECPublicKey;
import java.security.spec.ECGenParameterSpec;
import java.security.spec.InvalidKeySpecException;
import java.security.spec.PKCS8EncodedKeySpec;
import java.security.spec.X509EncodedKeySpec;
import java.util.Arrays;
import java.util.Calendar;
import java.util.logging.Logger;

import javax.security.auth.x500.X500Principal;

public class Test extends AppCompatActivity {
    @RequiresApi(api = Build.VERSION_CODES.M)
    @Override
    protected void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        KeyPairGenerator kpg = null;
        try {
            kpg = KeyPairGenerator.getInstance(
                    KeyProperties.KEY_ALGORITHM_EC, "AndroidKeyStore");

            kpg.initialize(new KeyGenParameterSpec.Builder(
                    "alias",
                    KeyProperties.PURPOSE_SIGN | KeyProperties.PURPOSE_VERIFY)
                    .setDigests(KeyProperties.DIGEST_SHA256,
                            KeyProperties.DIGEST_SHA512)
                    .build());
            KeyPair kp = kpg.generateKeyPair();

            KeyStore ks = KeyStore.getInstance("AndroidKeyStore");
            ks.load(null);
            KeyStore.Entry entry = ks.getEntry("alias", null);
            if (!(entry instanceof KeyStore.PrivateKeyEntry)) {
                Log.w("ERROR", "Not an instance of a PrivateKeyEntry");
//                return null;
            }
            Signature s = Signature.getInstance("SHA256withECDSA");
            s.initSign(((KeyStore.PrivateKeyEntry) entry).getPrivateKey());
            s.update("data".getBytes(StandardCharsets.UTF_8));
            byte[] signature = s.sign();
        } catch (NoSuchAlgorithmException | NoSuchProviderException | InvalidAlgorithmParameterException | UnrecoverableEntryException | InvalidKeyException | CertificateException | KeyStoreException | IOException | SignatureException e) {
            e.printStackTrace();
        }

    }



}

working.java

package com.example.myapplication;

import android.content.Context;
import android.os.Build;
import android.os.Bundle;
import android.security.KeyPairGeneratorSpec;
import android.security.keystore.KeyGenParameterSpec;
import android.security.keystore.KeyProperties;
import android.util.Base64;
import android.util.Log;

import androidx.annotation.Nullable;
import androidx.annotation.RequiresApi;
import androidx.appcompat.app.AppCompatActivity;

import java.io.IOException;
import java.io.UnsupportedEncodingException;
import java.math.BigInteger;
import java.nio.charset.StandardCharsets;
import java.security.GeneralSecurityException;
import java.security.InvalidAlgorithmParameterException;
import java.security.InvalidKeyException;
import java.security.Key;
import java.security.KeyFactory;
import java.security.KeyPair;
import java.security.KeyPairGenerator;
import java.security.KeyStore;
import java.security.KeyStoreException;
import java.security.NoSuchAlgorithmException;
import java.security.NoSuchProviderException;
import java.security.PublicKey;
import java.security.Signature;
import java.security.SignatureException;
import java.security.UnrecoverableEntryException;
import java.security.cert.CertificateException;
import java.security.spec.X509EncodedKeySpec;
import java.util.Enumeration;


public class Test extends AppCompatActivity {
    @RequiresApi(api = Build.VERSION_CODES.M)
    @Override
    protected void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        KeyStore kss = null;


        KeyPairGenerator kpg = null;
        try {

//            kpg = KeyPairGenerator.getInstance(
//                    KeyProperties.KEY_ALGORITHM_EC, "AndroidKeyStore");
//
//            kpg.initialize(new KeyGenParameterSpec.Builder(
//                    "alias",
//                    KeyProperties.PURPOSE_SIGN | KeyProperties.PURPOSE_VERIFY)
//                    .setDigests(KeyProperties.DIGEST_SHA256,
//                            KeyProperties.DIGEST_SHA512)
//                    .build());
//            KeyPair kp =  kpg.generateKeyPair();

            KeyStore ks = KeyStore.getInstance("AndroidKeyStore");
            ks.load(null);
            KeyStore.Entry entry = ks.getEntry("alias", null);

            Signature s = Signature.getInstance("SHA256withECDSA");
            s.initSign(((KeyStore.PrivateKeyEntry) entry).getPrivateKey());
            s.update("datahh".getBytes(StandardCharsets.UTF_8));
            byte[] signature = s.sign();


            //check if alias is present, when true, public key has been generated, don't generate again
            Log.d("yeahssss", String.valueOf(ks.containsAlias("alias")));



            //Convert Public Key to String
            String  pubKey = publicKeyToString(((KeyStore.PrivateKeyEntry) entry).getCertificate().getPublicKey());

            Log.d("yeah", pubKey);


            PublicKey key = stringToPublicKey("MFkwEwYHKoZIzj0CAQYIKoZIzj0DAQcDQgAE0WwWRFeiTubqscASUDhNz9ObGc63e67EV/R49A7gzpugu5R6b2so3FXhWuUPmjHs6zaW5SVVNzU4pJT1EZzoHg==");


//           Convert Signature to Base64 string
            String signatureBase64 = Base64.encodeToString("datahh".getBytes(), Base64.DEFAULT);



            Signature signature1 = Signature.getInstance("SHA256withECDSA");
            signature1.initVerify(key);

            //Convert base64 String to signature.
            signature1.update(Base64.decode(signatureBase64, Base64.DEFAULT));
            boolean verified = signature1.verify(signature);
            Log.d("yeahp",  String.valueOf(verified));


        } catch (GeneralSecurityException e) {
            Log.d("yeah",  e.getMessage());

            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }


    }



    public  PublicKey stringToPublicKey(String publStr)  {

        PublicKey publicKey = null;
        try {
            byte[] data = Base64.decode(publStr, Base64.NO_WRAP);
            X509EncodedKeySpec spec = new X509EncodedKeySpec(data);
            KeyFactory fact = KeyFactory.getInstance("EC");
            publicKey = fact.generatePublic(spec);
        } catch (GeneralSecurityException e) {
            e.printStackTrace();
        }
        return publicKey;
    }






    public  String publicKeyToString(PublicKey publ) {
        String publicKeyString = null;
        try {
            KeyFactory fact = KeyFactory.getInstance("EC");
            X509EncodedKeySpec spec = fact.getKeySpec(publ,
                    X509EncodedKeySpec.class);
            publicKeyString = Base64.encodeToString(spec.getEncoded(), Base64.NO_WRAP);
        } catch (GeneralSecurityException e) {
            Log.d("yeahp", e.getMessage());
            e.printStackTrace();
        }
        return publicKeyString;
    }


}