Lab 7: RSA Demo

In Lecture 16 you learned everything you needed to implement RSA key generation in Java using the fields and methods of the BigInteger class. In your Prelab you wrote the Java code for the keyGenerate() subroutine. In lab today you will add your subroutine to the RSA demo applet code and implement the RSA encryption and decryption subroutines. Once your code is working, you will have an applet just like this:

If your browser supported Java you would see an applet here

Press the Generate RSA Keys button to compute public and private key pairs. Enter your plaintext message in the box indicated and press Encrypt. Your encrypted text is shown. Press the Decrypt button to decrypt the ciphertext. You can generate a new set of keys for the same message by pressing the Generate RSA Keys button again. Press Clear to clear all areas of text.

Preliminaries

Download lab7.zip and extract its contents to the cps001 subdirectory of your public_html folder. You should now have a folder in your cps001 directory named lab7. Open the lab7 folder. It should contain two files, RSA.java and RSA.html.

Starting Emacs: On the desktop of your computer there is a folder named Class Applications. Open this folder and find the link to Emacs. Double click on the link to start Emacs.

Open the file RSA.html in Emacs and add your name and today's date. Save the file.

Open RSA.java. Look for the following:

/* ********************************************************************* */ /* ********************************************************************* */ /* ****************** Add your code below this line ******************** */ /* ****************** Add your code above this line ******************** */ /* ********************************************************************* */ /* ********************************************************************* */

You should type both your subroutines in the space between these comment lines. You do not need to modify any other code in the applet.

keyGenerate

Type your prelab code for the keyGenerate subroutine.

crypt

The only thing left to do before you can compile is to add a subroutine to perform encryption and decryption. In Lecture 16 we discussed how one subroutine could be used to perform both encryption and decryption, as they both perform the same computation.

Recall that RSA encyption takes as input a numeric value for the plaintext message P and the public key pair (k,n). The numeric ciphertext message C is then computed via the equation

C = (Mk)%n

Similarly, RSA decryption takes as input a numeric value for the ciphertext message C and the private key pair (d,n) and decrypts the ciphertext via the equation

D = (Cd)%n

All the values used for encryption and decryption are of type BigInteger. We can use the modPow method of the BigInteger class to perform the above computations for us. Specifically, given BigInteger values x, y and z, the value of (xy)%z is the BigInteger value

x.modPow(y,z)

We can write a subroutine crypt to perform both encryption and decryption as follows. The input parameters to crypt will be three variables of type BigInteger. The first variable, say M, represents the numeric text message. The second and third variables, say pkey and modkey, represent the RSA key pair for the desired operation - pkey being the unique public or private key and modkey being the shared modulus key. The crypt subroutine returns a value of type BigInteger, which is computed using the modPow method.

Type your code for the crypt subroutine below the code you just added for keyGenerate.

Compile and test

Compile RSA.java in Emacs by selecting Tools->Compile from the drop-down menu.

Emacs note: If you want to get back to one window, go to File->Unsplit Windows on the drop-down menu.

Once your applet has compiled, open RSA.html in a web browser and test it. If everything runs as it should -- that is, you can generate keys, enter a message, encrypt it and get your original message back by decrypting -- move on to the next section and add a link to your applet.

Add a link

Add a link to your RSA Demo applet on your CPS1 page