How to encrypt more than 117 bytes with RSA?

Question

Hi there,
I am struggling to encrypt more than 117 bytes of data with a 1024 bit RSA key.
If I try to encrypt (write to my OutputStreamWriter) 118 Bytes ("A"s) or more, the file I am writing to is just empty and I get an exception whe trying to read it.
I know the encryptable bytes (blocksize) are related to the key length: blocksize = keylength / 8 -11
I also know RSA encryption of large files is discouraged, because it is like 100x slower than for instance AES and was originally only intended to exchange symmetric keys.
Still I need to be able to asymmetrically encrypt at least 5kb of Data.
I am out of ideas here and have no understanding of why the block size is limited (i know from hours of "googling" that it is limited as described above though).
So, I would be very glad If somebody could point out a way how I could encrypt more than 117 bytes (without using a longer key of course).
import java.io.FileInputStream;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.io.OutputStream;
import java.io.FileOutputStream;
import java.io.Reader;
import java.io.Writer;
import java.io.OutputStreamWriter;
import java.security.PublicKey;
import java.io.IOException;
import java.security.InvalidKeyException;
import java.security.KeyPair;
import java.security.KeyPairGenerator;
import java.security.NoSuchAlgorithmException;
import java.security.PrivateKey;
import javax.crypto.*;
public class strchrbty {
     public static void main(String[] args) {
          //generate Keys
          PublicKey publicKey = null;
          PrivateKey privateKey = null;
          try {
               KeyPairGenerator kpg = KeyPairGenerator.getInstance("RSA");
               kpg.initialize(1024);
               KeyPair kp = kpg.generateKeyPair();
               publicKey = kp.getPublic();
               privateKey = kp.getPrivate();
          } catch (NoSuchAlgorithmException e) {
               e.printStackTrace();
          //------ENCODE------
          try {
              //initialize cipher
              Cipher cipher = Cipher.getInstance("RSA");  
               cipher.init(Cipher.ENCRYPT_MODE, publicKey);   
               OutputStream cos = new CipherOutputStream(new FileOutputStream("c:\\test.txt"), cipher);
              Writer out = new OutputStreamWriter(cos);
              // 1024 bit (key length) / 8 bytes - 11 bytes padding = 117 Bytes Data.
              //  for 118 Bytes (or more) Data: c:\\test.txt is empty annd no Data is read.
              for(int i = 0; i<117; i++) {
                   out.write("A");
              out.close();
              cos.close();
         } catch (InvalidKeyException e) {
              e.printStackTrace();      
         } catch (NoSuchPaddingException e) {
              e.printStackTrace();           
         } catch (NoSuchAlgorithmException e) {
              e.printStackTrace();      
         } catch (IOException e) {
              e.printStackTrace();
         //------DECODE------
         try {
              StringBuffer buf = new StringBuffer();
              Cipher cipher2 = Cipher.getInstance("RSA"); 
              cipher2.init(Cipher.DECRYPT_MODE, privateKey);
              //read char-wise
              InputStream cis = new CipherInputStream(new FileInputStream("c:\\test.txt"), cipher2);
              Reader in = new InputStreamReader(cis);
              for(int c = in.read(); c != -1; c = in.read()) {
                buf.append((char)c);
              //output
              System.out.println(buf.toString());
              in.close();
              cis.close();
          } catch (InvalidKeyException e) {
              e.printStackTrace();      
         } catch (NoSuchPaddingException e) {
              e.printStackTrace();           
         } catch (NoSuchAlgorithmException e) {
              e.printStackTrace();      
          } catch(IOException e) {
             e.printStackTrace();
}Regards.
Edited by: junghansmega on Sep 10, 2008 3:41 PM
Sorry about the bad autoformating.... It only occurrs in here, in my eclipse it looks fine =(

Answer 1

junghansmega wrote:
Hi there,
I am struggling to encrypt more than 117 bytes of data with a 1024 bit RSA key.
If I try to encrypt (write to my OutputStreamWriter) 118 Bytes ("A"s) or more, the file I am writing to is just empty and I get an exception whe trying to read it.
Good, it should be painful.
>
Still I need to be able to asymmetrically encrypt at least 5kb of Data.
In this forum, 99.999% of the time this kind of claim is due to a lack of understanding of the role of public key cryptography.
I am out of ideas here and have no understanding of why the block size is limited (i know from hours of "googling" that it is limited as described above though).
It should not be difficult to break up the input into 117 byte chunks and encrypt each chunk through a newly initted cipher object. Of course you will have to be very careful in properly encoding the output. You might naively think that the output is always 128 bytes, but is that true? Might it be 127 bytes sometimes? Maybe even 126 bytes?