Modular random number generator for the multiplicative subgroup. - Java java.util

Java examples for java.util:Random

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Description

Modular random number generator for the multiplicative subgroup.

Demo Code

// This program is free software; you can redistribute it and/or
//package com.java2s;
import java.util.Random;
import java.math.BigInteger;

public class Main {
    /**//from  w w  w .  jav  a2 s  . com
     * 
     * Modular random number generator for the multiplicative
     * subgroup. Generates an equally disctributed random number less
     * than {@code mod} that has a modular inverse modulo {@code mod}.
     * The bit length of the random number is at most 4 less than
     * {@code mod}.
     * 
     * @param rand Random number generator
     * @param mod modulus
     * @return random number less than {@code mod} with a modular
     * inverse 
     */
    public static BigInteger mod_rand_with_inverse(Random rand,
            BigInteger mod) {
        BigInteger res;
        do {
            res = mod_rand(rand, mod);
        } while (!coprime(res, mod));
        return res;
    }

    /**
     * 
     * Modular random number generator. Generate an equally
     * distributed random number less than {@code mod} with a bit
     * length at most 4 less than the bit length of {@code mod}.
     * 
     * @param rand Random number generator
     * @param mod modulus
     * @return a random number less than {@code mod}
     */
    public static BigInteger mod_rand(Random rand, BigInteger mod) {
        int bit_size = mod.bitLength();
        BigInteger res;
        do {
            res = new BigInteger(bit_size, rand);
        } while (res.bitLength() < bit_size - 4 || res.compareTo(mod) >= 0);
        return res;
    }

    /**
     * 
     * Coprime check. Tests whether the two arguments are coprime. Two
     * natural numbers are said to be coprime if the biggest factor
     * they share is 1, ie. if there greatest common divisor is 1.
     * 
     * @param a first BigInteger
     * @param b second BigInteger
     * @return true if {@code a} and {@code b} are coprime.
     */
    public static boolean coprime(BigInteger a, BigInteger b) {
        return a.gcd(b).compareTo(BigInteger.ONE) == 0;
    }
}

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