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Editorial Team
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Asked: 2026-05-15T09:36:08+00:00

I know its not the usual thing to do. But the specification I’m implementing

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I know its not the usual thing to do. But the specification I’m implementing is discribed this way, and I cannot run out.

I was trying to encrypt the modulus and exponent of the private key, but the following test code raises an exception because the byte array is 1 byte larger then the maximum allowed by RSA block:

import java.security.KeyPair;
import java.security.KeyPairGenerator;
import java.security.NoSuchAlgorithmException;
import java.security.NoSuchProviderException;
import java.security.interfaces.RSAPrivateKey;
import java.security.interfaces.RSAPublicKey;

import javax.crypto.Cipher;

import org.apache.commons.lang.ArrayUtils;

public class TEST {

    public static KeyPair generateKeyPair() throws NoSuchAlgorithmException, NoSuchProviderException {
 KeyPairGenerator keyPairGenerator = KeyPairGenerator.getInstance("RSA", "BC");
 keyPairGenerator.initialize(1024);
 return keyPairGenerator.generateKeyPair();
    }

    public static void main(String[] args) throws Exception {

 KeyPair keyPair = generateKeyPair();
 RSAPrivateKey privateKey = (RSAPrivateKey) keyPair.getPrivate();

 System.out.println("Priv modulus len = " + privateKey.getModulus().bitLength());
 System.out.println("Priv exponent len = " + privateKey.getPrivateExponent().bitLength());
 System.out.println("Priv modulus toByteArray len = " + privateKey.getModulus().toByteArray().length);

 byte[] byteArray = privateKey.getModulus().toByteArray();
 // the byte at index 0 have no value (in every generation it is always zero)
 byteArray = ArrayUtils.subarray(byteArray, 1, byteArray.length);

 System.out.println("byteArray size: " + byteArray.length);

 RSAPublicKey publicKey = (RSAPublicKey) keyPair.getPublic();
 Cipher cipher = Cipher.getInstance("RSA", "BC");
 cipher.init(Cipher.ENCRYPT_MODE, publicKey);
 byte[] encryptedBytes = cipher.doFinal(byteArray);

 System.out.println("Success!");
    }

}

(obs. its just a test, i would never encrypt the private key with its pair public key)

The byte array is 128 bytes, the exactly maximum allowed by a RSA block, so why the exception? And how to fix it?

EDIT: if I initilize the key pair generator with 1023 (instead of 1024) it works, but isn’t the usual for RSA key size 1024?

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1 Answer

  1. Editorial Team

    The answer I was looking was (almost hidden) in the project’s specification. Its to encrypt the rsa key by parts, and concatenate the parts. The same to decrypt (of course the size will need to be a default size known by encryptor and decryptor). This way I can use RSA without any symmetric key (not that I agree that this is the best encryption method).

    Sorry trouble you guys (I tried to delete this question, but it cannot be deleted now).

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