RSA Encryption

An example using a hybrid cryptosystem consisting of OAEP and GCM

The following example encrypts data by using a hybrid cryptosystem consisting of AES GCM and OAEP, using their default parameter sizes and an AES key size of 128 bits.

OAEP is less vulnerable to padding oracle attacks than PKCS#1 v1.5 padding. GCM is also protected against padding oracle attacks.

Decryption can be performed by first retrieving the length of the encapsulated key and then by retrieving the encapsulated key. The encapsulated key can then be decrypted using the RSA private key that forms a key pair with the public key. After that the AES/GCM encrypted ciphertext can be decrypted to the original plaintext.

The protocol consists of:

a length field for the wrapped key (RSAPrivateKey misses a getKeySize() method);
the wrapped/encapsulated key, of the same size as the RSA key size in bytes;
the GCM ciphertext and 128 bit authentication tag (automatically added by Java).

Notes:

To correctly use this code you should supply an RSA key of at least 2048 bits, bigger is better (but slower, especially during decryption);
To use AES-256 you should install the unlimited cryptography policy files first;
Instead creating your own protocol you might want to use a container format such as the Cryptographic Message Syntax (CMS / PKCS#7) or PGP instead.

So here’s the example:

/**
 * Encrypts the data using a hybrid crypto-system which uses GCM to encrypt the data and OAEP to encrypt the AES key.
 * The key size of the AES encryption will be 128 bit.
 * All the default parameter choices are used for OAEP and GCM.
 * 
 * @param publicKey the RSA public key used to wrap the AES key
 * @param plaintext the plaintext to be encrypted, not altered
 * @return the ciphertext
 * @throws InvalidKeyException if the key is not an RSA public key
 * @throws NullPointerException if the plaintext is null
 */
public static byte[] encryptData(PublicKey publicKey, byte[] plaintext)
        throws InvalidKeyException, NullPointerException {

    // --- create the RSA OAEP cipher ---

    Cipher oaep;
    try {
        // SHA-1 is the default and not vulnerable in this setting
        // use OAEPParameterSpec to configure more than just the hash
        oaep = Cipher.getInstance("RSA/ECB/OAEPwithSHA1andMGF1Padding");
    } catch (NoSuchAlgorithmException e) {
        throw new RuntimeException(
                "Runtime doesn't have support for RSA cipher (mandatory algorithm for runtimes)", e);
    } catch (NoSuchPaddingException e) {
        throw new RuntimeException(
                "Runtime doesn't have support for OAEP padding (present in the standard Java runtime sinze XX)", e);
    }
    oaep.init(Cipher.WRAP_MODE, publicKey);

    // --- wrap the plaintext in a buffer
    
    // will throw NullPointerException if plaintext is null
    ByteBuffer plaintextBuffer = ByteBuffer.wrap(plaintext);

    // --- generate a new AES secret key ---

    KeyGenerator aesKeyGenerator;
    try {
        aesKeyGenerator = KeyGenerator.getInstance("AES");
    } catch (NoSuchAlgorithmException e) {
        throw new RuntimeException(
                "Runtime doesn't have support for AES key generator (mandatory algorithm for runtimes)", e);
    }
    // for AES-192 and 256 make sure you've got the rights (install the
    // Unlimited Crypto Policy files)
    aesKeyGenerator.init(128);
    SecretKey aesKey = aesKeyGenerator.generateKey();
    
    // --- wrap the new AES secret key ---
    
    byte[] wrappedKey;
    try {
        wrappedKey = oaep.wrap(aesKey);
    } catch (IllegalBlockSizeException e) {
        throw new RuntimeException(
                "AES key should always fit OAEP with normal sized RSA key", e);
    }

    // --- setup the AES GCM cipher mode ---
    
    Cipher aesGCM;
    try {
        aesGCM = Cipher.getInstance("AES/GCM/Nopadding");
        // we can get away with a zero nonce since the key is randomly generated
        // 128 bits is the recommended (maximum) value for the tag size
        // 12 bytes (96 bits) is the default nonce size for GCM mode encryption
        GCMParameterSpec staticParameterSpec = new GCMParameterSpec(128, new byte[12]);
        aesGCM.init(Cipher.ENCRYPT_MODE, aesKey, staticParameterSpec);
    } catch (NoSuchAlgorithmException e) {
        throw new RuntimeException(
                "Runtime doesn't have support for AES cipher (mandatory algorithm for runtimes)", e);
    } catch (NoSuchPaddingException e) {
        throw new RuntimeException(
                "Runtime doesn't have support for GCM (present in the standard Java runtime sinze XX)", e);
    } catch (InvalidAlgorithmParameterException e) {
        throw new RuntimeException(
                "IvParameterSpec not accepted by this implementation of GCM", e);
    }

    // --- create a buffer of the right size for our own protocol ---
    
    ByteBuffer ciphertextBuffer = ByteBuffer.allocate(
            Short.BYTES
            + oaep.getOutputSize(128 / Byte.SIZE)
            + aesGCM.getOutputSize(plaintext.length));
    
    // - element 1: make sure that we know the size of the wrapped key
    ciphertextBuffer.putShort((short) wrappedKey.length);
    
    // - element 2: put in the wrapped key
    ciphertextBuffer.put(wrappedKey);

    // - element 3: GCM encrypt into buffer
    try {
        aesGCM.doFinal(plaintextBuffer, ciphertextBuffer);
    } catch (ShortBufferException | IllegalBlockSizeException | BadPaddingException e) {
        throw new RuntimeException("Cryptographic exception, AES/GCM encryption should not fail here", e);
    }

    return ciphertextBuffer.array();
}

Of course, encryption is not very useful without decryption. Note that this will return minimal information if decryption fails.

/**
 * Decrypts the data using a hybrid crypto-system which uses GCM to encrypt
 * the data and OAEP to encrypt the AES key. All the default parameter
 * choices are used for OAEP and GCM.
 * 
 * @param privateKey
 *            the RSA private key used to unwrap the AES key
 * @param ciphertext
 *            the ciphertext to be encrypted, not altered
 * @return the plaintext
 * @throws InvalidKeyException
 *             if the key is not an RSA private key
 * @throws NullPointerException
 *             if the ciphertext is null
 * @throws IllegalArgumentException
 *             with the message "Invalid ciphertext" if the ciphertext is invalid (minimize information leakage)
 */
public static byte[] decryptData(PrivateKey privateKey, byte[] ciphertext)
        throws InvalidKeyException, NullPointerException {

    // --- create the RSA OAEP cipher ---

    Cipher oaep;
    try {
        // SHA-1 is the default and not vulnerable in this setting
        // use OAEPParameterSpec to configure more than just the hash
        oaep = Cipher.getInstance("RSA/ECB/OAEPwithSHA1andMGF1Padding");
    } catch (NoSuchAlgorithmException e) {
        throw new RuntimeException(
                "Runtime doesn't have support for RSA cipher (mandatory algorithm for runtimes)",
                e);
    } catch (NoSuchPaddingException e) {
        throw new RuntimeException(
                "Runtime doesn't have support for OAEP padding (present in the standard Java runtime sinze XX)",
                e);
    }
    oaep.init(Cipher.UNWRAP_MODE, privateKey);

    // --- wrap the ciphertext in a buffer

    // will throw NullPointerException if ciphertext is null
    ByteBuffer ciphertextBuffer = ByteBuffer.wrap(ciphertext);

    // sanity check #1
    if (ciphertextBuffer.remaining() < 2) {
        throw new IllegalArgumentException("Invalid ciphertext");
    }
    // - element 1: the length of the encapsulated key
    int wrappedKeySize = ciphertextBuffer.getShort() & 0xFFFF;
    // sanity check #2
    if (ciphertextBuffer.remaining() < wrappedKeySize + 128 / Byte.SIZE) {
        throw new IllegalArgumentException("Invalid ciphertext");
    }

    // --- unwrap the AES secret key ---

    byte[] wrappedKey = new byte[wrappedKeySize];
    // - element 2: the encapsulated key
    ciphertextBuffer.get(wrappedKey);
    SecretKey aesKey;
    try {
        aesKey = (SecretKey) oaep.unwrap(wrappedKey, "AES",
                Cipher.SECRET_KEY);
    } catch (NoSuchAlgorithmException e) {
        throw new RuntimeException(
                "Runtime doesn't have support for AES cipher (mandatory algorithm for runtimes)",
                e);
    } catch (InvalidKeyException e) {
        throw new RuntimeException(
                "Invalid ciphertext");
    }

    // --- setup the AES GCM cipher mode ---

    Cipher aesGCM;
    try {
        aesGCM = Cipher.getInstance("AES/GCM/Nopadding");
        // we can get away with a zero nonce since the key is randomly
        // generated
        // 128 bits is the recommended (maximum) value for the tag size
        // 12 bytes (96 bits) is the default nonce size for GCM mode
        // encryption
        GCMParameterSpec staticParameterSpec = new GCMParameterSpec(128,
                new byte[12]);
        aesGCM.init(Cipher.DECRYPT_MODE, aesKey, staticParameterSpec);
    } catch (NoSuchAlgorithmException e) {
        throw new RuntimeException(
                "Runtime doesn't have support for AES cipher (mandatory algorithm for runtimes)",
                e);
    } catch (NoSuchPaddingException e) {
        throw new RuntimeException(
                "Runtime doesn't have support for GCM (present in the standard Java runtime sinze XX)",
                e);
    } catch (InvalidAlgorithmParameterException e) {
        throw new RuntimeException(
                "IvParameterSpec not accepted by this implementation of GCM",
                e);
    }

    // --- create a buffer of the right size for our own protocol ---

    ByteBuffer plaintextBuffer = ByteBuffer.allocate(aesGCM
            .getOutputSize(ciphertextBuffer.remaining()));

    // - element 3: GCM ciphertext
    try {
        aesGCM.doFinal(ciphertextBuffer, plaintextBuffer);
    } catch (ShortBufferException | IllegalBlockSizeException
            | BadPaddingException e) {
        throw new RuntimeException(
                "Invalid ciphertext");
    }

    return plaintextBuffer.array();
}