Java: Generate a custom Private and Public key?

Question

Just jumped into security stuff in Java and was trying to use a digital signature. The thing is that I already generated my RSA keys manually and I would like to sign with them. Is that even possible?

This Is the code I wrote where sk is the servers privatekey, pk is the public server key and modulus is the servers module

public static byte[] sign(byte[] message, BigInteger sk, BigInteger pk, BigInteger modulus) throws NoSuchAlgorithmException, InvalidKeyException, SignatureException, InvalidKeySpecException, NoSuchProviderException{   
    //Initialize signature
    Signature sig = Signature.getInstance("MD5WithRSA");

    //Create public and private keys
    KeyFactory fact = KeyFactory.getInstance("RSA", "BC");
    RSAPrivateKeySpec skey = new RSAPrivateKeySpec(modulus, sk);
    RSAPrivateKey serverPrivateKey = (RSAPrivateKey)fact.generatePrivate(skey);
    RSAPublicKeySpec pkey = new RSAPublicKeySpec(modulus, pk);
    PublicKey serverPublicKey = fact.generatePublic(pkey);

    //We assign the key
    sig.initSign(serverPrivateKey);
    sig.update(message);
    byte[] signatureBytes = sig.sign();

    return signatureBytes;
}

After running it, I got the following error:

java.security.spec.InvalidKeySpecException: java.security.InvalidKeyException: RSA keys must be at least 512 bits long

Do you guys know how could I face this? I tried several ways of producing a Private / Public key out of my BigInteger values and there was no way.

Would apreciate any help/considerations.

Answer 1

Although the key is too small for practical use, you may still use it for educational purposes. Note that the key is so small that you cannot even use PKCS#1 padding modes, only "raw" RSA encryption (i.e. only the modular exponentiation part of RSA).

The following works perfectly well for the Bouncy Castle provider (where the key is a 64 bit key):

final Provider bc = new BouncyCastleProvider();

// generating the key from modulus & private exponent
KeyFactory rsaFactory = KeyFactory.getInstance("RSA", bc);
RSAPrivateKeySpec spec = new RSAPrivateKeySpec(key.getModulus(), key.getPrivateExponent());
RSAPrivateKey testKey = (RSAPrivateKey) rsaFactory.generatePrivate(spec);

// using it in a raw cipher
Cipher c= Cipher.getInstance("RSA/ECB/NoPadding", bc);
c.init(Cipher.DECRYPT_MODE, testKey);
c.doFinal(new byte[] {(byte) 0x7F, (byte) 0xFF, (byte) 0xFF, (byte) 0xFF, (byte) 0xFF, (byte) 0xFF, (byte) 0xFF, (byte) 0xFF, });