// CRYPTO LIBRARY FOR EXCHANGING KEYS
// USING THE DIFFIE-HELLMAN KEY EXCHANGE PROTOCOL
// The diffie-hellman can be used to securely exchange keys
// between parties, where a third party eavesdropper given
// the values being transmitted cannot determine the key.
// Implemented by Lee Griffiths, Jan 2004.
// This software is freeware, you may use it to your discretion,
// however by doing so you take full responsibility for any damage
// it may cause.
// Hope you find it useful, even if you just use some of the functions
// out of it like the prime number generator and the XtoYmodN function.
// It would be great if you could send me emails to: lee.griffiths@first4internet.co.uk
// with any suggestions, comments, or questions!
// Enjoy.
// Adopted to ms-logon for ultravnc and ported to Java by marscha, 2006.
package com.iiordanov.bVNC;
//import java.lang.Math;
public class DH {
public DH() {
maxNum = (((long) 1) << DH_MAX_BITS) - 1;
}
public DH(long generator, long modulus) throws Exception {
maxNum = (((long) 1) << DH_MAX_BITS) - 1;
if (generator >= maxNum || modulus >= maxNum)
throw new Exception("Modulus or generator too large.");
gen = generator;
mod = modulus;
}
private long rng(long limit) {
return (long) (java.lang.Math.random() * limit);
}
//Performs the miller-rabin primality test on a guessed prime n.
//trials is the number of attempts to verify this, because the function
//is not 100% accurate it may be a composite. However setting the trial
//value to around 5 should guarantee success even with very large primes
private boolean millerRabin (long n, int trials) {
long a = 0;
for (int i = 0; i < trials; i++) {
a = rng(n - 3) + 2;// gets random value in [2..n-1]
if (XpowYmodN(a, n - 1, n) != 1) return false; //n composite, return false
}
return true; // n probably prime
}
//Generates a large prime number by
//choosing a randomly large integer, and ensuring the value is odd
//then uses the miller-rabin primality test on it to see if it is prime
//if not the value gets increased until it is prime
private long generatePrime() {
long prime = 0;
do {
long start = rng(maxNum);
prime = tryToGeneratePrime(start);
} while (prime == 0);
return prime;
}
private long tryToGeneratePrime(long prime) {
//ensure it is an odd number
if ((prime & 1) == 0)
prime += 1;
long cnt = 0;
while (!millerRabin(prime, 25) && (cnt++ < DH_RANGE) && prime < maxNum) {
prime += 2;
if ((prime % 3) == 0) prime += 2;
}
return (cnt >= DH_RANGE || prime >= maxNum) ? 0 : prime;
}
//Raises X to the power Y in modulus N
//the values of X, Y, and N can be massive, and this can be
//achieved by first calculating X to the power of 2 then
//using power chaining over modulus N
private long XpowYmodN(long x, long y, long N) {
long result = 1;
final long oneShift63 = ((long) 1) << 63;
for (int i = 0; i < 64; y <<= 1, i++){
result = result * result % N;
if ((y & oneShift63) != 0)
result = result * x % N;
}
return result;
}
public void createKeys() {
gen = generatePrime();
mod = generatePrime();
if (gen > mod) {
long swap = gen;
gen = mod;
mod = swap;
}
}
public long createInterKey() {
priv = rng(maxNum);
return pub = XpowYmodN(gen,priv,mod);
}
public long createEncryptionKey(long interKey) throws Exception {
if (interKey >= maxNum){
throw new Exception("interKey too large");
}
return key = XpowYmodN(interKey,priv,mod);
}
public long getValue(int flags) {
switch (flags) {
case DH_MOD:
return mod;
case DH_GEN:
return gen;
case DH_PRIV:
return priv;
case DH_PUB:
return pub;
case DH_KEY:
return key;
default:
return (long) 0;
}
}
public int bits(long number){
for (int i = 0; i < 64; i++){
number /= 2;
if (number < 2) return i;
}
return 0;
}
public static byte[] longToBytes(long number) {
byte[] bytes = new byte[8];
for (int i = 0; i < 8; i++) {
bytes[i] = (byte) (0xff & (number >> (8 * (7 - i))));
}
return bytes;
}
public static long bytesToLong(byte[] bytes) {
long result = 0;
for (int i = 0; i < 8; i++) {
result <<= 8;
result += (byte) bytes[i];
}
return result;
}
private long gen;
private long mod;
private long priv;
private long pub;
private long key;
private long maxNum;
private static final int DH_MAX_BITS = 31;
private static final int DH_RANGE = 100;
private static final int DH_MOD = 1;
private static final int DH_GEN = 2;
private static final int DH_PRIV = 3;
private static final int DH_PUB = 4;
private static final int DH_KEY = 5;
}