How to read from a text file, a buffer at a time (specific bytes at a time)?

[harke baj]

Hi,

Could someone tell me how I can read a text file, a buffer at a time? I want to read certain bytes at a time.

I wanted to encrypt a text file using RSA ( just for knowledge sake ), using 1024 key size. But the size of the message to encrypt is limited. I can't go on increasing the key size as the process become super slow.

I was wondering if I could read certain bytes at a time, encrypt them and write the encrypted data to a file.

Like so:

start for loop(till end of file)
  read certain bytes from the file; 
  encrypt them;
  write the encrypted data to a new file;
end for loop

also should I use the doFinal() or the updata()? Can someone tell me a better way to do this, if there is another way?

Thank you.

regards,
harke

[Rob Spoor]

InputStream has read() methods that take a byte[]. They will read up to the array length in bytes, and return the number of bytes read. You can use those two (the array and the number of bytes read) in the update() method. doFinal() should be called when you've called update() with all read data already, to get the remainder.

[Rob Spoor]

harke baj wrote:should i put the doFinal() immediately after the update(), or after the loop finishes?

It's called doFinal; doesn't that say enough?

Also, when should I write the encoded data to a file.

I would do it in between. In pseudo code:

while (data is read)
{
    update();
    write results of update();
}
doFinal();
write results of doFinal();

[harke baj]

I have done it.

But not using the update(). I read the files in chunks and then used doFinal().

While(till the eof)
 read the data from buffer;
 encrypt using doFinal();
 write to destination file;
end while

But I still could not get, how to use the update().
Could you direct me to some examples?

thanks you

[Rob Spoor]

You're calling doFinal() each time so you are not encrypting the entire file; you are encrypting each chunk as if it were a separate entity. This will lead to problems if the cipher adds padding to make the final result of a certain length (like a multitude of 8 bytes). The update uses intermediate results, and does not add padding. You will only get that padding with doFinal(). In short:

byte[] buffer = new byte[1024]; // or any other size
int len;
while ((len = in.read(buffer)) != -1)
{
    // update with the read data; use len as upper bound in case you read less than buffer.length bytes
    byte[] result = cipher.update(buffer, 0, len);
    out.write(result);
}
// updated all, let doFinal finish up
byte[] result = cipher.doFinal();
out.write(result);
out.flush();

[harke baj]

this did not work.
I go the following error:

javax.crypto.IllegalBlockSizeException: Data must not be longer than 117 bytes

my code:

byte[] buffer = new byte[50];
            int len;
            byte[] ciperText;
            c1.init(Cipher.ENCRYPT_MODE, kp.getPublic());
            while ((len = bufin.read(buffer)) != -1) {
                ciperText = c1.update(buffer, 0, len);
                keyfos.write(ciperText);
            }
            ciperText = c1.doFinal();
            keyfos.write(ciperText);
            keyfos.flush();

The key size i used to generate the keys, is 1024.

Thanks,
harke

[harke baj]

hey i solved the problem. The problem was because i was using wrong byte size. I change the byte[] to 100 from 50. and it worked.\

Thanks.