A simple Miller-Rabin prime number test I wrote in Java.

Along with this method I implemented isWitness as well as the modulo power by bit shifting method.

Feel free to comment ;)

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Update: 

Someone requested for myBigRanNum and FactorOfTwo, I put them here as well, enjoy.

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[code]public static boolean MillerRabin(BigInteger n, int k){

        BigInteger a;
        BigInteger m = n.subtract(BigInteger.ONE);
        boolean isPrime = true;
       
        if (n.compareTo(BigInteger.ONE) == 0) return false;
        if (n.mod(BigInteger.valueOf(2)).intValue() == 0) return false;       
      
        for (int i = 0; i < k; i++) {
            a = myBigRanNum(MAX_BIT);
            if (a.compareTo(m) > 0) a = a.mod(m);
            if (a.compareTo(BigInteger.ZERO) == 0) a = BigInteger.ONE;
           
            if (isWitness(a, n)) return false;
        }
        return isPrime;

}

 

 

public static boolean isWitness(BigInteger a, BigInteger n) {
        int s;
        BigInteger apow;
        BigInteger firstEqua, secondEqua;
        BigInteger m = n.subtract(BigInteger.ONE);
        BigInteger d;
        s = FactorOfTwo(m);
        d = m.divide(BigInteger.valueOf((int)Math.pow(2, s)));  

        firstEqua = ModExpo(a, d, n);
        if (firstEqua.intValue() == 1 || firstEqua.compareTo(m) == 0 ) return false;      
      
        for (int r = 0; r < s; r++) {
            apow = d.multiply(BigInteger.valueOf((int)Math.pow(2, r)));
            secondEqua = ModExpo(a, apow, n);
            if (secondEqua.intValue() == -1 || secondEqua.compareTo(m) == 0) {              
                return false;
            }
        }
        return true;
}

 

public static BigInteger ModExpo(BigInteger aBigInt, BigInteger pow, BigInteger n) {
        BigInteger e = pow;
        BigInteger a = aBigInt;
        BigInteger result = BigInteger.ONE;
        while (e.compareTo(BigInteger.ZERO) > 0 ) {
            if (e.and(BigInteger.ONE).compareTo(BigInteger.ONE) == 0 ){              
                result = (result.multiply(a)).mod(n);
            }          
            e = e.shiftRight(1);
            a = (a.multiply(a)).mod(n);
        }
        return result;
}

 

 

public static int FactorOfTwo(BigInteger aNumber) {
        int i = 0;
        BigInteger myNumber = aNumber;
        BigInteger myRemainer;
        while (true) {          
            myRemainer = myNumber.mod(BigInteger.valueOf(2));
            if (myRemainer.compareTo(BigInteger.ZERO) != 0 ) break;
            myNumber = myNumber.divide(BigInteger.valueOf(2));
            i++;
        }
        return i;
}

 

 

public static BigInteger myBigRanNum(int numBits) {
      
        //We only deal with positive number here

        BigInteger aBigInterger = new BigInteger(numBits, aRand);
        aBigInterger = aBigInterger.abs();
        return aBigInterger;
}[/code]

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