Cryptographic Applications - Introduction to Cryptography with Java Applets - page 345

//Generate two strong primes congruent to 3 mod 4
SecureRandom sr=new SecureRandom();
PrimeGenerator pg=new PrimeGenerator(513,10,sr);
BigInteger p=null,q=null;
do {
p=pg.getStrongPrime();
} while (!p.mod(four).equals(three));
do {
q=pg.getStrongPrime();
} while (!q.mod(four).equals(three));
//Form the modulus as the product of these primes
BigInteger modulus=p.multiply(q);
//Choose a random value t and square it modulo the modulus to form s
BigInteger t=new BigInteger(modulus.bitLength()-1,sr);
//s is your identifying number
BigInteger s=t.modPow(two,modulus);
//The values of s and the modulus should be made publicly available
//with a Trusted Third Party (TTP)
System.out.println(“Enter host name or IP address of challenger:”);
String host=k.readLine();
Socket socket=new Socket(host,12345);
PrintStream out=new PrintStream(socket.getOutputStream());
BufferedReader in=new BufferedReader(new
InputStreamReader(socket.getInputStream()));
//Send the values for the modulus, and s, to the challenger-we do not send t
//of course
out.println(modulus.toString());
out.println(s.toString());
String approved=”N”;
//The challenges begin
do {
System.out.println(“You have yet to be approved.”);
//Generate the random value r
BigInteger r=new BigInteger(modulus.bitLength()-1,sr);
//Compute z1 and z2
BigInteger z1=r.modPow(two,modulus);
BigInteger z2=s.multiply(z1.modInverse(modulus)).mod(modulus);
//Send z1 and z2 to the challenger
out.println(z1.toString());
out.println(z2.toString());