Lab 10: RSA Encryption

In lecture 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

Snarf labs/10_RSAEncryption
You should find RSA,java and RSA.html files in your 10_RSAEncryption project.

Open the file RSA.html with a text editor (DO NOT double click on the RSA.html file. Doing so will freeze Eclipse. ) and add your name, NetID and resources used. 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 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 = (M^k)%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 = (C^d)%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 (x^y)%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.

Test your applet! :

Run your applet and test it for various inputs. 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 export your project.

Link to your applet

Your applet exists only in Eclipse work-space as of now. Besides submitting your RSA.java file and RSA.html file via Eclipse as shown below, you also need to export your project to your public_html/cps1 folder to ensure it works over the Internet.
- To Export your 10_RSAEncryption project, Right-click on the 10_RSAEncryption project and choose Export/li>
- In the next window, Choose File System�and click Next
- The next window will ask you to choose a Destination Folder. Hit the Browse button and navigate to P:\public_html\cps1\ folder and hit Finish
- This will create a new folder in the destination folder with the same name as the Eclipse project. (i.e. Eclipse will create a 10_RSAEncryption folder in P:\public_html\cps1 folder which will have both the RSA.java and the HTML file necessary to run the applet.
If you did everything correctly, you should be able to go to the URL
http://www.duke.edu/~login/cps1/10_RSAEncryption/RSA.html

and view your applet (replace login with your acpub login).
In Lab 1 we made a webpage for this class named index.html . Add a link to your applet on your CPS1 page. The easiest thing to do is to ADD this to the un-ordered list already present on your webpage. Add the following line to the HTML code for an unordered list which already exists in your lab1 webpage:

                <li> <a href = "http://www.duke.edu/~login/cps1/10_RSAEncryption/RSA.html"> Lab 10: RSA Encryption</a>

Submitting

Submit the files RSA.java and RSA.html using assignment name lab10. DON'T FORGET TO INCLUDE YOUR NAME AND LAB SECTION AT THE TOP OF ALL OF YOUR FILES!

Credit to Albert Meixner and Tammy Bailey for originally developing this assignment.