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.

According to that link, the keysize = modulus size. I checked that value and its 8 Bytes long. Should not that fit that? — user3555923
– user3555923, Commented Apr 21, 2014 at 8:31
you're initializing with a keyset of 128 and RSA expects at least 512. — Asif Bhutto
– Asif Bhutto, Commented Apr 21, 2014 at 8:35
If the modulus is indeed 8 bytes then it is far too small to be secure - factoring an 8 byte (64 bit) number can be done in < 1 second. — Iridium
– Iridium, Commented Apr 21, 2014 at 18:06

Maarten Bodewes · Accepted Answer · 2014-04-22 00:09:36Z

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, });

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