A basic example of how to perform symmetric key encryption/decryption using AES and Java's cryptography API.

CryptoHelper.java

import java.security.Key;
import java.security.SecureRandom;

import javax.crypto.Cipher;
import javax.crypto.KeyGenerator;
import javax.crypto.SecretKey;
import javax.crypto.spec.IvParameterSpec;

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

public class CryptoHelper {

	public static void main( String [] args ) throws Exception {

		CryptoHelper crypto = new CryptoHelper();

		String plaintext = "This is a good secret.";
		System.out.println( plaintext );

		String ciphertext = crypto.encrypt( plaintext );
		System.out.println( ciphertext );

		String decrypted = crypto.decrypt( ciphertext );
		System.out.println( decrypted );

	}

	public String encrypt( String plaintext ) throws Exception {
		return encrypt( generateIV(), plaintext );
	}

	public String encrypt( byte [] iv, String plaintext ) throws Exception {

		byte [] decrypted = plaintext.getBytes();
		byte [] encrypted = encrypt( iv, decrypted );

		StringBuilder ciphertext = new StringBuilder();

		ciphertext.append( Base64.encodeBase64String( iv ) );
		ciphertext.append( ":" );
		ciphertext.append( Base64.encodeBase64String( encrypted ) );

		return ciphertext.toString();

	}

	public String decrypt( String ciphertext ) throws Exception {
		String [] parts = ciphertext.split( ":" );
		byte [] iv = Base64.decodeBase64( parts[0] );
		byte [] encrypted = Base64.decodeBase64( parts[1] );
		byte [] decrypted = decrypt( iv, encrypted );
		return new String( decrypted );
	}

	private Key key;

	public CryptoHelper( Key key ) {
		this.key = key;
	}

	public CryptoHelper() throws Exception {
		this( generateSymmetricKey() );
	}

	public Key getKey() {
		return key;
	}

	public void setKey( Key key ) {
		this.key = key;
	}

	public static byte [] generateIV() {
		SecureRandom random = new SecureRandom();
		byte [] iv = new byte [16];
		random.nextBytes( iv );
		return iv;
	}

	public static Key generateSymmetricKey() throws Exception {
		KeyGenerator generator = KeyGenerator.getInstance( "AES" );
		SecretKey key = generator.generateKey();
		return key;
	}

	public byte [] encrypt( byte [] iv, byte [] plaintext ) throws Exception {
		Cipher cipher = Cipher.getInstance( key.getAlgorithm() + "/CBC/PKCS5Padding" );
		cipher.init( Cipher.ENCRYPT_MODE, key, new IvParameterSpec( iv ) );
		return cipher.doFinal( plaintext );
	}

	public byte [] decrypt( byte [] iv, byte [] ciphertext ) throws Exception {
		Cipher cipher = Cipher.getInstance( key.getAlgorithm() + "/CBC/PKCS5Padding" );
		cipher.init( Cipher.DECRYPT_MODE, key, new IvParameterSpec( iv ) );
		return cipher.doFinal( ciphertext );
	}

}

how do you generate the encryption key

For symmetric keys, is signing same as encryption?