Java - Encrypt String with existing public key file

Question

I've been researching this for the past 4-5 hours now and can't seem to find an answer that actually works despite finding 'answers' that used everything from a few methods to an entire ~100 line class. I can't imagine that there isn't some simple function to do such a trivial thing :P

I have a pre-existing set of public / private keys (actually, two sets - one generated by ssh-keygen and another by openssl so .. whatever format works is cool).

All I am after is a simple java equivalent to what I write in python like -

key_object = someModule.KeyObject(nameOfPublicKeyFile)

def encrypt (SomePlainText) :
  return someOtherModule.encrypt(key_object, SomePlainText)

Any help would be awesome!

Answer 1

These openssl commands in the shell create an RSA key pair and write the public and private keys to DER formatted files.

Here, the private key file is not password-protected (-nocrypt) to keep things simple.

$ openssl genrsa -out keypair.pem 2048
Generating RSA private key, 2048 bit long modulus
............+++
................................+++
e is 65537 (0x10001)
$ openssl rsa -in keypair.pem -outform DER -pubout -out public.der
writing RSA key
$ openssl pkcs8 -topk8 -nocrypt -in keypair.pem -outform DER -out private.der

Now that you have the DER files, you can read them in Java and use KeySpec and KeyFactory to create PublicKey and PrivateKey objects.

public byte[] readFileBytes(String filename) throws IOException
{
    Path path = Paths.get(filename);
    return Files.readAllBytes(path);        
}

public PublicKey readPublicKey(String filename) throws IOException, NoSuchAlgorithmException, InvalidKeySpecException
{
    X509EncodedKeySpec publicSpec = new X509EncodedKeySpec(readFileBytes(filename));
    KeyFactory keyFactory = KeyFactory.getInstance("RSA");
    return keyFactory.generatePublic(publicSpec);       
}

public PrivateKey readPrivateKey(String filename) throws IOException, NoSuchAlgorithmException, InvalidKeySpecException
{
    PKCS8EncodedKeySpec keySpec = new PKCS8EncodedKeySpec(readFileBytes(filename));
    KeyFactory keyFactory = KeyFactory.getInstance("RSA");
    return keyFactory.generatePrivate(keySpec);     
}

With the public and private keys, you can encrypt and decrypt small amounts of data (that fit within your RSA modulus.) I recommend OAEP padding.

public byte[] encrypt(PublicKey key, byte[] plaintext) throws NoSuchAlgorithmException, NoSuchPaddingException, InvalidKeyException, IllegalBlockSizeException, BadPaddingException
{
    Cipher cipher = Cipher.getInstance("RSA/ECB/OAEPWithSHA1AndMGF1Padding");   
    cipher.init(Cipher.ENCRYPT_MODE, key);  
    return cipher.doFinal(plaintext);
}

public byte[] decrypt(PrivateKey key, byte[] ciphertext) throws NoSuchAlgorithmException, NoSuchPaddingException, InvalidKeyException, IllegalBlockSizeException, BadPaddingException
{
    Cipher cipher = Cipher.getInstance("RSA/ECB/OAEPWithSHA1AndMGF1Padding");   
    cipher.init(Cipher.DECRYPT_MODE, key);  
    return cipher.doFinal(ciphertext);
}

Here it is tied together with a simple encryption and decryption:

public void Hello()
{
    try
    {
        PublicKey publicKey = readPublicKey("public.der");
        PrivateKey privateKey = readPrivateKey("private.der");
        byte[] message = "Hello World".getBytes("UTF8");
        byte[] secret = encrypt(publicKey, message);
        byte[] recovered_message = decrypt(privateKey, secret);
        System.out.println(new String(recovered_message, "UTF8"));
    }
    catch (Exception e)
    {
        e.printStackTrace();
    }
}