RSA Encryption Decryption in Android

In RSA you should use the public key for encryption and the private key for decryption.

Your sample code uses for encryption and decryption the public key - this can not work.

Hence in the decryption part you should initialize the cipher this way:

cipher1.init(Cipher.DECRYPT_MODE, privateKey);

Furthermor your code has a second significant bug:

You are converting a byte array with binary content to a String.

Never ever convert binary data to a String!

Strings are for string characters, not binary data. If you want to pack binary data into a String encode it to printable characters for example using Hex or Base64.

The following example uses the hexadecimal encoder fro org.apache.common.codec package - a third party library with has to be installed.

public byte[] RSAEncrypt(final String plain) throws NoSuchAlgorithmException, NoSuchPaddingException,
        InvalidKeyException, IllegalBlockSizeException, BadPaddingException {
    kpg = KeyPairGenerator.getInstance("RSA");
    kpg.initialize(2048);
    kp = kpg.genKeyPair();
    publicKey = kp.getPublic();
    privateKey = kp.getPrivate();

    cipher = Cipher.getInstance("RSA");
    cipher.init(Cipher.ENCRYPT_MODE, publicKey);
    encryptedBytes = cipher.doFinal(plain.getBytes());
    System.out.println("EEncrypted?????" + new String(org.apache.commons.codec.binary.Hex.encodeHex(encryptedBytes)));
    return encryptedBytes;
}

public String RSADecrypt(final byte[] encryptedBytes) throws NoSuchAlgorithmException, NoSuchPaddingException,
        InvalidKeyException, IllegalBlockSizeException, BadPaddingException {

    cipher1 = Cipher.getInstance("RSA");
    cipher1.init(Cipher.DECRYPT_MODE, privateKey);
    decryptedBytes = cipher1.doFinal(encryptedBytes);
    decrypted = new String(decryptedBytes);
    System.out.println("DDecrypted?????" + decrypted);
    return decrypted;
}

My class:

package com.infovale.cripto;

import java.io.UnsupportedEncodingException;
import java.math.BigInteger;
import java.security.InvalidKeyException;
import java.security.KeyPair;
import java.security.KeyPairGenerator;
import java.security.NoSuchAlgorithmException;
import java.security.PrivateKey;
import java.security.PublicKey;
import java.util.Arrays;

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

public class RSA {

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

public String Encrypt (String plain) throws NoSuchAlgorithmException, NoSuchPaddingException, InvalidKeyException, IllegalBlockSizeException, BadPaddingException 
{
    kpg = KeyPairGenerator.getInstance("RSA");
    kpg.initialize(1024);
    kp = kpg.genKeyPair();
    publicKey = kp.getPublic();
    privateKey = kp.getPrivate();

    cipher = Cipher.getInstance("RSA");
    cipher.init(Cipher.ENCRYPT_MODE, publicKey);
    encryptedBytes = cipher.doFinal(plain.getBytes());

    encrypted = bytesToString(encryptedBytes);
    return encrypted;

}

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

    cipher1=Cipher.getInstance("RSA");
    cipher1.init(Cipher.DECRYPT_MODE, privateKey);
    decryptedBytes = cipher1.doFinal(stringToBytes(result));
    decrypted = new String(decryptedBytes);
    return decrypted;

}

public  String bytesToString(byte[] b) {
    byte[] b2 = new byte[b.length + 1];
    b2[0] = 1;
    System.arraycopy(b, 0, b2, 1, b.length);
    return new BigInteger(b2).toString(36);
}

public  byte[] stringToBytes(String s) {
    byte[] b2 = new BigInteger(s, 36).toByteArray();
    return Arrays.copyOfRange(b2, 1, b2.length);
}
}