Dazed and confused by Java Security & BouncyCastle APIs

Question

I've been trying to make sense of the BouncyCastle cryptography APIs for Java. Unfortunately, I'm finding Java cryptography in general to be so obscured by service provider interfaces and jargon that I can't wrap my head around what anything actually does. I've tried reading the necessary documentation repeatedly but it just stays incomprehensible, introducing many concepts far beyond what I think should be needed.

All I really want is a class that does the following:

public class KeyPair {
    public byte[] public;
    public byte[] private;
}

public class RSACrypto {
    public static KeyPair generateRSAKeyPair() { /*implementation*/}
    public static byte[] encrypt(byte[] data, byte[] publicKey) { /*impl*/}
    public static byte[] decrypt(byte[] encryptedData, byte[] privateKey) { /*impl*/ }
}

Apologies if this is an incredibly complicated question to ask as "all I really want". Any pointers on where to read up on Java cryptography and BouncyCastle are very welcome. Any overviews of how the Java crypto systems actually are laid out are extremely welcome.

Answer 1

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

import javax.crypto.Cipher;

public class RSACrypto
{

  /* A 1024-bit key will encrypt messages up to 117 bytes long. */
  private static final int KEY_SIZE = 1024;

  private static final String XFORM = 
    "RSA/ECB/OAEPWITHSHA-256ANDMGF1PADDING";

  public static KeyPair generateRSAKeyPair()
    throws GeneralSecurityException
  {
    KeyPairGenerator gen = KeyPairGenerator.getInstance("RSA");
    gen.initialize(KEY_SIZE);
    return gen.generateKeyPair();
  }

  public static byte[] encrypt(byte[] plaintext, PublicKey pub)
    throws GeneralSecurityException
  {
    Cipher cipher = Cipher.getInstance(XFORM);
    cipher.init(Cipher.ENCRYPT_MODE, pub);
    return cipher.doFinal(plaintext);
  }

  public static byte[] decrypt(byte[] ciphertext, PrivateKey pvt)
    throws GeneralSecurityException
  {
    Cipher cipher = Cipher.getInstance(XFORM);
    cipher.init(Cipher.DECRYPT_MODE, pvt);
    return cipher.doFinal(ciphertext);
  }

  public static void main(String... argv)
    throws Exception
  {
    KeyPair pair = RSACrypto.generateRSAKeyPair();
    byte[] plaintext = "A short secret message.".getBytes("UTF-8");
    byte[] ciphertext = RSACrypto.encrypt(plaintext, pair.getPublic());
    byte[] recovered = RSACrypto.decrypt(ciphertext, pair.getPrivate());
    System.out.println(new String(recovered, "UTF-8"));
  }

}