RSA Encryption Decryption in Android

Question

I am implementing a demo for RSA Encryption and Decryption in Android. I can Perform Encryption very well, but In Decryption I get an Exception: >>java.security.InvalidKeyException: unknown key type passed to RSA.

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

    public String RSAEncrypt (final 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 = new String(encryptedBytes);
        System.out.println("EEncrypted?????"+encrypted);
        return encrypted;

    }

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

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

    }

And I am calling the function from here:

encrypt.setOnClickListener(new OnClickListener()
        { 
            public void onClick(View arg0) 
            {
                    try
                    {
                        RSAEncrypt rsaencrypt=new RSAEncrypt();
                        rsaencrypt.RSAEncrypt(name);

                        result=rsaencrypt.RSAEncrypt(name);
                        Toast.makeText(getBaseContext(), result.toString(),Toast.LENGTH_SHORT).show();

                        System.out.println("Result:"+result);
                    }
                    catch(Exception e)
                    {
                        e.printStackTrace();
                        Toast.makeText(getBaseContext(), e.toString(),Toast.LENGTH_LONG).show();
                    }
            }
        });

        decrypt.setOnClickListener(new OnClickListener()
        { 
            public void onClick(View arg0) 
            {
                {
                    try
                    {
                        RSAEncrypt rsadecrypt=new RSAEncrypt();

                        rsadecrypt.RSADecrypt(result);

                        ans=rsadecrypt.RSADecrypt(result);
                        System.out.println("Result is"+ans);
                        Toast.makeText(getBaseContext(), ans.toString(),Toast.LENGTH_LONG).show();
                    }
                    catch(Exception e)
                    {
                        e.printStackTrace();
                        Toast.makeText(getBaseContext(), e.toString(),Toast.LENGTH_LONG).show();
                        System.out.println("Exception is>>"+e);
                    }
            }
        });

Answer 1

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); } } 

Answer 2

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; } 

Answer 3

Here is an example for Android of:

• generating a private/public RSA key pair
• encrypting a string
• decrypting the encrypted string

These methods deal with all the base 64 encoding/decoding.

    public void TestEncryptData(String dataToEncrypt) {
        // generate a new public/private key pair to test with (note. you should only do this once and keep them!)
        KeyPair kp = getKeyPair();

        PublicKey publicKey = kp.getPublic();
        byte[] publicKeyBytes = publicKey.getEncoded();
        String publicKeyBytesBase64 = new String(Base64.encode(publicKeyBytes, Base64.DEFAULT));

        PrivateKey privateKey = kp.getPrivate();
        byte[] privateKeyBytes = privateKey.getEncoded();
        String privateKeyBytesBase64 = new String(Base64.encode(privateKeyBytes, Base64.DEFAULT));

        // test encryption
        String encrypted = encryptRSAToString(dataToEncrypt, publicKeyBytesBase64);

        // test decryption
        String decrypted = decryptRSAToString(encrypted, privateKeyBytesBase64);
    }

    public static KeyPair getKeyPair() {
        KeyPair kp = null;
        try {
            KeyPairGenerator kpg = KeyPairGenerator.getInstance("RSA");
            kpg.initialize(2048);
            kp = kpg.generateKeyPair();
        } catch (Exception e) {
            e.printStackTrace();
        }

        return kp;
    }

    public static String encryptRSAToString(String clearText, String publicKey) {
        String encryptedBase64 = "";
        try {
            KeyFactory keyFac = KeyFactory.getInstance("RSA");
            KeySpec keySpec = new X509EncodedKeySpec(Base64.decode(publicKey.trim().getBytes(), Base64.DEFAULT));
            Key key = keyFac.generatePublic(keySpec);

            // get an RSA cipher object and print the provider
            final Cipher cipher = Cipher.getInstance("RSA/ECB/OAEPWITHSHA-256ANDMGF1PADDING");
            // encrypt the plain text using the public key
            cipher.init(Cipher.ENCRYPT_MODE, key);

            byte[] encryptedBytes = cipher.doFinal(clearText.getBytes("UTF-8"));
            encryptedBase64 = new String(Base64.encode(encryptedBytes, Base64.DEFAULT));
        } catch (Exception e) {
            e.printStackTrace();
        }

        return encryptedBase64.replaceAll("(\\r|\\n)", "");
    }

    public static String decryptRSAToString(String encryptedBase64, String privateKey) {

        String decryptedString = "";
        try {
            KeyFactory keyFac = KeyFactory.getInstance("RSA");
            KeySpec keySpec = new PKCS8EncodedKeySpec(Base64.decode(privateKey.trim().getBytes(), Base64.DEFAULT));
            Key key = keyFac.generatePrivate(keySpec);

            // get an RSA cipher object and print the provider
            final Cipher cipher = Cipher.getInstance("RSA/ECB/OAEPWITHSHA-256ANDMGF1PADDING");
            // encrypt the plain text using the public key
            cipher.init(Cipher.DECRYPT_MODE, key);

            byte[] encryptedBytes = Base64.decode(encryptedBase64, Base64.DEFAULT);
            byte[] decryptedBytes = cipher.doFinal(encryptedBytes);
            decryptedString = new String(decryptedBytes);
        } catch (Exception e) {
            e.printStackTrace();
        }

        return decryptedString;
    }

Answer 4

I think the problem is that you should use the same key pair to encrypt and decrypt the cipher. Referring to the JavaDoc:

 genKeyPair() This will generate a new key pair every time it is called.