Question

Here are my constants

//Encryption fields
/** Algorithm=RSA Mode=ECB Padding=PKCS1Padding*/
public static final String ALGORITHM_MODE_PADDING = "RSA/ECB/PKCS1Padding";
/** Algorithm=RSA */
public static final String ALGORITHM = "RSA";
/** Provider=BouncyCastle */
public static final String PROVIDER = "BC";
/** Key size for the public and private keys */
public static final int KEY_SIZE = 1024;

I have made two public / private keys like this:

//Generate the keys
KeyPairGenerator kpg = KeyPairGenerator.getInstance(ALGORITHM,PROVIDER);
kpg.initialize(KEY_SIZE);
KeyPair kp = kpg.generateKeyPair();
PublicKey pubk = kp.getPublic();
PrivateKey prvk = kp.getPrivate();

I am decrypting like this:

byte[] privateKey = Base64.decodeBase64(pKey); //decode
PKCS8EncodedKeySpec keySpec = new PKCS8EncodedKeySpec(privateKey);
KeyFactory factory = KeyFactory.getInstance(ALGORITHM,PROVIDER);
PrivateKey privKey = factory.generatePrivate(keySpec);
Cipher cipher = Cipher.getInstance(ALGORITHM_MODE_PADDING);
cipher.init(Cipher.ENCRYPT_MODE, privKey);
return cipher.doFinal(data);

This works with small amounts of data, when when the data becomes larger such as 263 bytes if fails with an IllegalBlockSizeException. I thinks this is because the data is greater than 256 bytes but that is just an guess and I have no idea of how to fix it.

What am I doing wrong?

I changed it to use the update method, but still have the same problem:

// encryption pass
cipher.init(Cipher.ENCRYPT_MODE, privKey);
byte[] cipherText = new byte[cipher.getOutputSize(data.length)];
int ctLength = cipher.update(data, 0, data.length, cipherText, 0);
ctLength += cipher.doFinal(cipherText, ctLength);

I am trying to implement digital signatures by the way. The client has the public key and the server has the private key.

Answer 1

You cannot use RSA to encrypt more data than the size in bytes of the modulus – 11. This is probably what you are looking for.