RSA encryption example for android

RSACipher.java

/**
 * Created by Awesometic
 * It's encrypt returns Base64 encoded, and also decrypt for Base64 encoded cipher
 * references: http://stackoverflow.com/questions/12471999/rsa-encryption-decryption-in-android
 */
import android.util.Base64;

import java.nio.charset.StandardCharsets;
import java.security.InvalidKeyException;
import java.security.KeyFactory;
import java.security.KeyPair;
import java.security.KeyPairGenerator;
import java.security.NoSuchAlgorithmException;
import java.security.PrivateKey;
import java.security.PublicKey;
import java.security.spec.InvalidKeySpecException;
import java.security.spec.X509EncodedKeySpec;

import javax.crypto.BadPaddingException;
import javax.crypto.Cipher;
import javax.crypto.IllegalBlockSizeException;
import javax.crypto.NoSuchPaddingException;

public class RSACipher {

    KeyPairGenerator kpg;
    KeyPair kp;
    PublicKey publicKey;
    PrivateKey privateKey;
    byte[] encryptedBytes, decryptedBytes;
    Cipher cipher, cipher1;
    String encrypted, decrypted;

    private final static String CRYPTO_METHOD = "RSA";
    private final static int CRYPTO_BITS = 2048;

    public RSACipher()
            throws NoSuchAlgorithmException,
            NoSuchPaddingException,
            InvalidKeyException,
            IllegalBlockSizeException,
            BadPaddingException {

        generateKeyPair();
    }

    private void generateKeyPair()
            throws NoSuchAlgorithmException,
            NoSuchPaddingException,
            InvalidKeyException,
            IllegalBlockSizeException,
            BadPaddingException {

        kpg = KeyPairGenerator.getInstance(CRYPTO_METHOD);
        kpg.initialize(CRYPTO_BITS);
        kp = kpg.genKeyPair();
        publicKey = kp.getPublic();
        privateKey = kp.getPrivate();
    }

    /**
     * Encrypt plain text to RSA encrypted and Base64 encoded string
     *
     * @param args
     *          args[0] should be plain text that will be encrypted
     *          If args[1] is be, it should be RSA public key to be used as encrypt public key
     * @return a encrypted string that Base64 encoded
     * @throws NoSuchAlgorithmException
     * @throws NoSuchPaddingException
     * @throws InvalidKeyException
     * @throws IllegalBlockSizeException
     * @throws BadPaddingException
     */
    public String encrypt(Object... args)
            throws NoSuchAlgorithmException,
            NoSuchPaddingException,
            InvalidKeyException,
            IllegalBlockSizeException,
            BadPaddingException {

        String plain = (String) args[0];
        PublicKey rsaPublicKey;

        if (args.length == 1) {
            rsaPublicKey = this.publicKey;
        } else {
            rsaPublicKey = (PublicKey) args[1];
        }

        cipher = Cipher.getInstance("RSA/ECB/OAEPWithSHA1AndMGF1Padding");
        cipher.init(Cipher.ENCRYPT_MODE, rsaPublicKey);
        encryptedBytes = cipher.doFinal(plain.getBytes(StandardCharsets.UTF_8));

        return Base64.encodeToString(encryptedBytes, Base64.DEFAULT);
    }

    public String decrypt(String result)
            throws NoSuchAlgorithmException,
            NoSuchPaddingException,
            InvalidKeyException,
            IllegalBlockSizeException,
            BadPaddingException {

        cipher1 = Cipher.getInstance("RSA/ECB/OAEPWithSHA1AndMGF1Padding");
        cipher1.init(Cipher.DECRYPT_MODE, privateKey);
        decryptedBytes = cipher1.doFinal(Base64.decode(result, Base64.DEFAULT));
        decrypted = new String(decryptedBytes);

        return decrypted;
    }

    public String getPublicKey(String option)
            throws NoSuchAlgorithmException,
            NoSuchPaddingException,
            InvalidKeyException,
            IllegalBlockSizeException,
            BadPaddingException {

        switch (option) {

            case "pkcs1-pem":
                String pkcs1pem = "-----BEGIN RSA PUBLIC KEY-----\n";
                pkcs1pem += Base64.encodeToString(publicKey.getEncoded(), Base64.DEFAULT);
                pkcs1pem += "-----END RSA PUBLIC KEY-----";

                return pkcs1pem;

            case "pkcs8-pem":
                String pkcs8pem = "-----BEGIN PUBLIC KEY-----\n";
                pkcs8pem += Base64.encodeToString(publicKey.getEncoded(), Base64.DEFAULT);
                pkcs8pem += "-----END PUBLIC KEY-----";

                return pkcs8pem;

            case "base64":
                return Base64.encodeToString(publicKey.getEncoded(), Base64.DEFAULT);

            default:
                return null;

        }

    }

    public static PublicKey stringToPublicKey(String publicKeyString)
            throws NoSuchAlgorithmException,
            NoSuchPaddingException,
            InvalidKeyException,
            IllegalBlockSizeException,
            BadPaddingException {

        try {
            if (publicKeyString.contains("-----BEGIN PUBLIC KEY-----") || publicKeyString.contains("-----END PUBLIC KEY-----"))
                publicKeyString = publicKeyString.replace("-----BEGIN PUBLIC KEY-----", "").replace("-----END PUBLIC KEY-----", "");
            byte[] keyBytes = Base64.decode(publicKeyString, Base64.DEFAULT);
            X509EncodedKeySpec spec = new X509EncodedKeySpec(keyBytes);
            KeyFactory keyFactory = KeyFactory.getInstance("RSA");

            return keyFactory.generatePublic(spec);

        } catch (NoSuchAlgorithmException | InvalidKeySpecException e) {
            e.printStackTrace();

            return null;
        }
    }
}

when i run below :

                   String result = null;
                   String g = null;
                   try {
                       result = new Pgp().encrypt("slkdfjlks");
                   } catch (NoSuchAlgorithmException e) {
                       e.printStackTrace();
                   } catch (NoSuchPaddingException e) {
                       e.printStackTrace();
                   } catch (InvalidKeyException e) {
                       e.printStackTrace();
                   } catch (IllegalBlockSizeException e) {
                       e.printStackTrace();
                   } catch (BadPaddingException e) {
                       e.printStackTrace();
                   }
                   try {
                       g = new Pgp().decrypt(result);
                   } catch (NoSuchAlgorithmException e) {
                       e.printStackTrace();
                   } catch (NoSuchPaddingException e) {
                       e.printStackTrace();
                   } catch (InvalidKeyException e) {
                       e.printStackTrace();
                   } catch (IllegalBlockSizeException e) {
                       e.printStackTrace();
                   } catch (BadPaddingException e) {
                       e.printStackTrace();
                   }

i get this error :
javax.crypto.BadPaddingException: data hash wrong
and exception is because of this line in decrypt function
decryptedBytes = cipher1.doFinal(Base64.decode(result, Base64.DEFAULT));

public String decrypt(String result)
            throws NoSuchAlgorithmException,
            NoSuchPaddingException,
            InvalidKeyException,
            IllegalBlockSizeException,
            BadPaddingException {

        cipher1 = Cipher.getInstance("RSA/ECB/OAEPWithSHA1AndMGF1Padding");
        cipher1.init(Cipher.DECRYPT_MODE, privateKey);
        decryptedBytes = cipher1.doFinal(Base64.decode(result, Base64.DEFAULT));
        decrypted = new String(decryptedBytes);

        return decrypted;
}

If you are using a hardcoded private string then you must check that
start/begin is same as pkcs8Pem = pkcs8Pem.replace("-----BEGIN PRIVATE KEY-----", "");
and end/completes with the pkcs8Pem = pkcs8Pem.replace("-----END PRIVATE KEY-----", "");

i get this error on decryption :

com.android.org.bouncycastle.jcajce.provider.util.BadBlockException: unable to decrypt block
at com.android.org.bouncycastle.jcajce.provider.asymmetric.rsa.CipherSpi.getOutput(CipherSpi.java:553)
at com.android.org.bouncycastle.jcajce.provider.asymmetric.rsa.CipherSpi.engineDoFinal(CipherSpi.java:498)
at javax.crypto.Cipher.doFinal(Cipher.java:2056)
at com.example.magicchatapp.utils.security.RSACipher.decrypt(RSACipher.java:106)
at com.example.magicchatapp.presentation.fragment.SingleChatThreadFragment