Encrypt long String with RSA (Java)

Question

I'm having problems with an RSA application I have to do in Java.

I have to read a string from a file, encrypt it, and then save the encrypted string in a new file.

My RSA key is 1024 bits long.

The code's part where the problem is, is the following:

            readBytes = in.read(bytesToBeEncoded, 0, bytesToBeEncoded.length);

            while(readBytes != -1){
                encodedContent = ciph.update(bytesToBeEncoded, 0, readBytes);
                out.write(encodedContent);
                bytesToBeEncoded= new byte[(KEY_SIZE/8)-11];
                readBytes = in.read(bytesToBeEncoded, 0, bytesToBeEncoded.length);                  
            }

            encodedContent = ciph.doFinal();
            out.write(encodedContent);

Where the variables are defined like this:

        byte[] bytesToBeEncoded = new byte[(KEY_SIZE/8)-11]; 

        FileInputStream in = new FileInputStream(new File(file1));
        FileOutputStream out = new FileOutputStream(new File(file2));
        int readBytes;

The point is that when I encrypt a less-than-117 bytes string, it works perfectly (encrypt and then decrypt well), but when the size is larger, the application throws this exception:

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

Thrown by:

encodedContent = ciph.doFinal();

I don't know where the problem is and what I have to do.

Could anyone help me? Thank you.

Sorry about my english.

Do you really need to encrypt your data with RSA? The standard pattern is to encrypt a random symmetric key with RSA, and the actual data with that symmetric key. — CodesInChaos
– CodesInChaos, Commented Mar 11, 2012 at 16:33
You should listen to CodeInChaos and not directly encrypt data with an RSA key. It's against all good practices. Furthermore, your buffer size should not be related to the input size at all. — Maarten Bodewes
– Maarten Bodewes, Commented Mar 13, 2012 at 23:49

Steven Mastandrea · Accepted Answer · 2012-03-11 14:55:36Z

4

The problem is with how you are initializing the byte array that you are reading your input. You are setting the size based on the size of the RSA key instead of by the expected size of your input stream.

byte[] bytesToBeEncoded = new byte[(KEY_SIZE/8)-11];

The key size is 1024 bits, so 1024/8 -11 = 128 - 11 = 117 bytes. This is the amount of data that you can read from the stream, and should not be related to the size of your RSA key.

You should initialize the byte array to be the maximum size of data that you'll need to read in, which will avoid the Exception. For example:

byte[] bytesToBeEncoded = 1024;

would allow a maximum input string of 1024 characters.

answered Mar 11, 2012 at 14:55

Steven Mastandrea

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2 Comments

Shuli Over a year ago

Hi Steven, thanks for your fast answer. I have donde that (byte[] bytesToBeEncoded = new byte[1024]), but the problem persists. :S The program continues throwing the same exception. Thanks anyway. :)

Shuli Over a year ago

Avoid my comment, I didn't see that what you wrote was an example. Now I initialized the array to the size of the content to be read and modified my code a little bit and it worked. ^^ Thank you so much!

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