Here's the basic structure of the GenSig program. Place it in a file called GenSig.java.

import java.io.*;
import java.security.*;

class GenSig {

    public static void main(String[] args) {

        /* Generate a DSA signature */

        if (args.length != 1) {
            System.out.println("Usage: GenSig nameOfFileToSign");
            }
        else try {

        // the rest of the code goes here

        } catch (Exception e) {
            System.err.println("Caught exception " + e.toString());
        }
    }

}

Notes:

• The methods for signing data are in the java.security package, so the program imports everything from that package. The program also imports the java.io package, which contains the methods needed to input the file data to be signed.

• A single argument is expected, specifying the data file to be signed.

• The code written in subsequent steps will go between the try and the catch blocks.