RSA Cipher

I want to pass messages to and from the server/client and use a RSA cipher, but I can't figure out how to generate a pair, and pass the public key to the other, and then use it.

I am trying to modify this code that I found via Google:

import java.security.KeyPair;
import java.security.KeyPairGenerator;
import java.security.PublicKey;
import java.security.PrivateKey;

import javax.crypto.Cipher;

import org.apache.commons.codec.binary.Base64;

public class TestRsa2
{
	private KeyPair keyPair;

public TestRsa2() throws Exception
	{
		Initialize();
	}

public void Initialize() throws Exception
	{
		KeyPairGenerator keygen = KeyPairGenerator.getInstance("RSA");
		keygen.initialize(1024);
		keyPair = keygen.generateKeyPair();
	}

public String encrypt(String plaintext)  throws Exception
	{
		PublicKey key = keyPair.getPublic();
		Cipher cipher = Cipher.getInstance("RSA/ECB/PKCS1Padding");

cipher.init(Cipher.ENCRYPT_MODE, key);

byte[] ciphertext = cipher.doFinal(plaintext.getBytes("UTF8"));
		return encodeBASE64(ciphertext);
	}

public String decrypt(String ciphertext)  throws Exception
	{
		PrivateKey key = keyPair.getPrivate();
		Cipher cipher = Cipher.getInstance("RSA/ECB/PKCS1Padding");

cipher.init(Cipher.DECRYPT_MODE, key);

byte[] plaintext = cipher.doFinal(decodeBASE64(ciphertext));
		return new String(plaintext, "UTF8");
	}

private static String encodeBASE64(byte[] bytes)
    {
        Base64 b64 = new Base64();
        return b64.encodeToString(bytes);
    }

private static byte[] decodeBASE64(String text) throws Exception
    {
        Base64 b64 = new Base64();
        return b64.decode(text);
    }

public static void main(String[] args) throws Exception
	{
		TestRsa2 app = new TestRsa2();

System.out.println("Enter a line: ");
		java.io.InputStreamReader sreader = new java.io.InputStreamReader(System.in);
		java.io.BufferedReader breader = new java.io.BufferedReader(sreader);
		String input = breader.readLine();

System.out.println("Plaintext = " + input);

String ciphertext = app.encrypt(input);
		System.out.println("After Encryption Ciphertext = " + ciphertext);
		System.out.println("After Decryption Plaintext = " + app.decrypt(ciphertext));

}
}

The code already generates a key pair, and extracts both private and public key, no? The data to be used can be gotten from either key by calling the "getEncoded" method.

I know, but I want to specifically set a public key that coincides with the private key the server generates.

I can't figure out how to set a key...

Once you have the bytes that make up a key, you can create a Key object from it like this:

Well Ulf, I tried fiddling with it for a bit, and googling the error, and I can't seem to get it...

Here is what I have atm:

import java.security.KeyPair;
import java.security.KeyPairGenerator;

import javax.crypto.Cipher;
import javax.crypto.spec.SecretKeySpec;

import org.apache.commons.codec.binary.Base64;

public class TestRsa2
{
	private KeyPair keyPair;
	byte[] pk, prk;

public TestRsa2() throws Exception
	{
		Initialize();
	}

public void Initialize() throws Exception
	{
		KeyPairGenerator keygen = KeyPairGenerator.getInstance("RSA");
		keygen.initialize(1024);
		keyPair = keygen.generateKeyPair();
		pk = keyPair.getPublic().getEncoded();
		prk = keyPair.getPrivate().getEncoded();
	}

public String encrypt(String plaintext)  throws Exception
	{		
		SecretKeySpec keySpec = new SecretKeySpec(pk, "RSA");
		Cipher encrypt = Cipher.getInstance("RSA", "SunJCE");
		encrypt.init(Cipher.ENCRYPT_MODE, keySpec);
		byte[] ciphertext = encrypt.doFinal(plaintext.getBytes("UTF8"));
		return encodeBASE64(ciphertext);
	}

public String decrypt(String ciphertext)  throws Exception
	{
		SecretKeySpec keySpec = new SecretKeySpec(prk, "RSA");
		Cipher decrypt = Cipher.getInstance("RSA", "SunJCE");
		decrypt.init(Cipher.DECRYPT_MODE, keySpec);

byte[] plaintext = decrypt.doFinal(decodeBASE64(ciphertext));
		return new String(plaintext, "UTF8");
	}

private static String encodeBASE64(byte[] bytes)
    {
        Base64 b64 = new Base64();
        return b64.encodeToString(bytes);
    }

private static byte[] decodeBASE64(String text) throws Exception
    {
        Base64 b64 = new Base64();
        return b64.decode(text);
    }

public static void main(String[] args) throws Exception
	{
		TestRsa2 app = new TestRsa2();

System.out.println("Enter a line: ");
		java.io.InputStreamReader sreader = new java.io.InputStreamReader(System.in);
		java.io.BufferedReader breader = new java.io.BufferedReader(sreader);
		String input = breader.readLine();

System.out.println("Plaintext = " + input);

String ciphertext = app.encrypt(input);
		System.out.println("After Encryption Ciphertext = " + ciphertext);
		System.out.println("After Decryption Plaintext = " + app.decrypt(ciphertext));

}
}

And here is the error:
Exception in thread "main" java.security.InvalidKeyException: Neither a public nor a private key
at sun.security.rsa.RSAKeyFactory.engineTranslateKey(RSAKeyFactory.java:193)
at sun.security.rsa.RSAKeyFactory.toRSAKey(RSAKeyFactory.java:111)
at com.sun.crypto.provider.RSACipher.init(RSACipher.java:260)
at com.sun.crypto.provider.RSACipher.engineInit(RSACipher.java:205)
at javax.crypto.Cipher.init(Cipher.java:986)
at javax.crypto.Cipher.init(Cipher.java:935)
at TestRsa2.encrypt(TestRsa2.java:32)
at TestRsa2.main(TestRsa2.java:70)

Ulf Dittmer wrote:Once you have the bytes that make up a key, you can create a Key object from it like this:
SecretKeySpec keySpec = new SecretKeySpec(key, JCE_ALGORITHM); Cipher encrypt = Cipher.getInstance(JCE_ALGORITHM, JCE_PROVIDER); encrypt.init(Cipher.ENCRYPT_MODE, keySpec);

That is for a secret (symmetric) key. For a public key, you create an X509EncodedKeySpec from the result of PublicKey.getEncoded(). You supply the X509EncodedKeySpec instance to a KeyFactory instance. Using the getInstance() method to get an RSA instance. Then use the generatePublic() method and supply the X509EncodedKeySpec to it to get the public key.

Thanks Greg, I finally got it to work.