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    <p>Final ans</p> <p><code> public class MyEncrypt {</p> <pre><code>public static final int AES_Key_Size = 128; Cipher pkCipher, aesCipher; byte[] aesKey; SecretKeySpec aeskeySpec; public static void main(String[] args) throws Exception { //MyEncrypt.createRSAKeys(); //call to generated RSA keys MyEncrypt secure = new MyEncrypt(); // to encrypt a file secure.makeKey(); secure.saveKey(new File("keys/ase.key"), "keys/public.key"); secure.encrypt(new File("test/sample.pdf"), new File("test/encrypt_sample.pdf")); // to decrypt it again secure.loadKey(new File("keys/ase.key"), "keys/private.key"); secure.decrypt(new File("test/encrypt_sample.pdf"), new File("test/decrypted_sample.pdf")); } /** * Constructor: creates ciphers */ public MyEncrypt() throws GeneralSecurityException { // create RSA public key cipher pkCipher = Cipher.getInstance("RSA"); // create AES shared key cipher aesCipher = Cipher.getInstance("AES"); } public static void createRSAKeys() throws Exception { KeyPairGenerator kpg = KeyPairGenerator.getInstance("RSA"); kpg.initialize(512); KeyPair kp = kpg.genKeyPair(); RSAPublicKey publicKey = (RSAPublicKey) kp.getPublic(); RSAPrivateKey privateKey = (RSAPrivateKey) kp.getPrivate(); KeyFactory fact = KeyFactory.getInstance("RSA"); RSAPublicKeySpec pub = fact.getKeySpec(kp.getPublic(), RSAPublicKeySpec.class); RSAPrivateKeySpec priv = fact.getKeySpec(kp.getPrivate(), RSAPrivateKeySpec.class); saveToFile("keys/public.key", pub.getModulus(), pub.getPublicExponent()); saveToFile("keys/private.key", priv.getModulus(), priv.getPrivateExponent()); System.out.println("RSA public &amp; private keys are generated"); } private static void saveToFile(String fileName, BigInteger mod, BigInteger exp) throws IOException { ObjectOutputStream oout = new ObjectOutputStream(new BufferedOutputStream(new FileOutputStream(fileName))); try { oout.writeObject(mod); oout.writeObject(exp); } catch (Exception e) { throw new IOException("Unexpected error", e); } finally { oout.close(); } } /** * Creates a new AES key */ public void makeKey() throws NoSuchAlgorithmException { KeyGenerator kgen = KeyGenerator.getInstance("AES"); kgen.init(AES_Key_Size); SecretKey key = kgen.generateKey(); aesKey = key.getEncoded(); aeskeySpec = new SecretKeySpec(aesKey, "AES"); } /** * Encrypts the AES key to a file using an RSA public key */ public void saveKey(File out, String publicKeyFile) throws IOException, GeneralSecurityException { try { // read public key to be used to encrypt the AES key byte[] encodedKey = new byte[(int)publicKeyFile.length()]; new FileInputStream(publicKeyFile).read(encodedKey); // create public key //X509EncodedKeySpec publicKeySpec = new X509EncodedKeySpec(encodedKey); //KeyFactory kf = KeyFactory.getInstance("RSA"); //PublicKey pk = kf.generatePublic(publicKeySpec); PublicKey pk = this.readPublicKeyFromFile(publicKeyFile); // write AES key pkCipher.init(Cipher.ENCRYPT_MODE, pk); CipherOutputStream os = new CipherOutputStream(new FileOutputStream(out), pkCipher); os.write(aesKey); os.close(); } catch (Exception e) { //throw new Exception("Saving key exception", e); } } private PublicKey readPublicKeyFromFile(String keyFileName) throws Exception { InputStream in = new FileInputStream(keyFileName); ObjectInputStream oin = new ObjectInputStream(new BufferedInputStream(in)); try { BigInteger m = (BigInteger) oin.readObject(); BigInteger e = (BigInteger) oin.readObject(); RSAPublicKeySpec keySpec = new RSAPublicKeySpec(m, e); KeyFactory fact = KeyFactory.getInstance("RSA"); PublicKey pubKey = fact.generatePublic(keySpec); return pubKey; } catch (Exception e) { throw new RuntimeException("Spurious serialisation error", e); } finally { oin.close(); } } private PrivateKey readPrivateKeyFromFile(String keyFileName) throws Exception { InputStream in = new FileInputStream(keyFileName); ObjectInputStream oin = new ObjectInputStream(new BufferedInputStream(in)); try { BigInteger m = (BigInteger) oin.readObject(); BigInteger e = (BigInteger) oin.readObject(); RSAPrivateKeySpec keySpec = new RSAPrivateKeySpec(m, e); KeyFactory fact = KeyFactory.getInstance("RSA"); PrivateKey pubKey = fact.generatePrivate(keySpec); return pubKey; } catch (Exception e) { throw new RuntimeException("Spurious serialisation error", e); } finally { oin.close(); } } /** * Decrypts an AES key from a file using an RSA private key */ public void loadKey(File in, String privateKeyFile) throws GeneralSecurityException, IOException { try { // read private key to be used to decrypt the AES key byte[] encodedKey = new byte[(int)privateKeyFile.length()]; new FileInputStream(privateKeyFile).read(encodedKey); // create private key //PKCS8EncodedKeySpec privateKeySpec = new PKCS8EncodedKeySpec(encodedKey); //KeyFactory kf = KeyFactory.getInstance("RSA"); //PrivateKey pk = kf.generatePrivate(privateKeySpec); PrivateKey pk = this.readPrivateKeyFromFile(privateKeyFile); // read AES key pkCipher.init(Cipher.DECRYPT_MODE, pk); aesKey = new byte[AES_Key_Size/8]; CipherInputStream is = new CipherInputStream(new FileInputStream(in), pkCipher); is.read(aesKey); aeskeySpec = new SecretKeySpec(aesKey, "AES"); } catch (Exception e) { } } /** * Encrypts and then copies the contents of a given file. */ public void encrypt(File in, File out) throws IOException, InvalidKeyException { aesCipher.init(Cipher.ENCRYPT_MODE, aeskeySpec); FileInputStream is = new FileInputStream(in); CipherOutputStream os = new CipherOutputStream(new FileOutputStream(out), aesCipher); copy(is, os); os.close(); } /** * Decrypts and then copies the contents of a given file. */ public void decrypt(File in, File out) throws IOException, InvalidKeyException { aesCipher.init(Cipher.DECRYPT_MODE, aeskeySpec); CipherInputStream is = new CipherInputStream(new FileInputStream(in), aesCipher); FileOutputStream os = new FileOutputStream(out); copy(is, os); is.close(); os.close(); } /** * Copies a stream. */ private void copy(InputStream is, OutputStream os) throws IOException { int i; byte[] b = new byte[1024]; while((i=is.read(b))!=-1) { os.write(b, 0, i); } } </code></pre> <p>} </code></p>
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