Establishing Keys and Message Exchange - Introduction to Cryptography with Java Applets - page 281

//Start by listening on port 11111
ServerSocket ss=new ServerSocket(11111);
//Wait for a connection
Socket socket=ss.accept();
//Open input and output streams on the socket
BufferedReader in=new BufferedReader(new
InputStreamReader(socket.getInputStream()));
PrintStream out=new PrintStream(socket.getOutputStream());
//Capture p,g,gtox values from client
BigInteger p=new BigInteger(in.readLine());
BigInteger g=new BigInteger(in.readLine());
BigInteger gtox=new BigInteger(in.readLine());
//Produce your own secret exponent
SecureRandom sr=new SecureRandom();
BigInteger y=new BigInteger(p.bitLength()-1,sr);
//Raise g to this power
BigInteger gtoy=g.modPow(y,p);
//Send this to client
out.println(gtoy);
//Raise gtox to y power-this is the secret key
BigInteger key=gtox.modPow(y,p);
System.out.println
(“The secret key with “+socket.getInetAddress().toString()+” is:\n”+key);
int c=System.in.read();
}
}
The client side of this connection is equally simple:
import java.security.*;
import java.math.*;
import java.net.*;
import java.io.*;
public class DiffieHellmanInitiator {
static BufferedReader k=new BufferedReader(new InputStreamReader(System.in));
public static void main(String[] args) throws IOException {
//Make a safe prime and generator
SecureRandom sr=new SecureRandom();
PrimeGenerator pg=new PrimeGenerator(1025,10,sr);
BigInteger[] pandg=pg.getSafePrimeAndGenerator();
//Make your secret exponent
BigInteger x=new BigInteger(pandg[0].bitLength()-1,sr);