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The Bouncy Castle Crypto package is a Java implementation of cryptographic algorithms. This jar contains JCE provider and lightweight API for the Bouncy Castle Cryptography APIs for JDK 1.4.

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package org.bouncycastle.util;

import java.math.BigInteger;
import java.security.SecureRandom;

/**
 * BigInteger utilities.
 */
public final class BigIntegers
{
    private static final int MAX_ITERATIONS = 1000;
    private static final BigInteger ZERO = BigInteger.valueOf(0);

    /**
     * Return the passed in value as an unsigned byte array.
     * 
     * @param value value to be converted.
     * @return a byte array without a leading zero byte if present in the signed encoding.
     */
    public static byte[] asUnsignedByteArray(
        BigInteger value)
    {
        byte[] bytes = value.toByteArray();
        
        if (bytes[0] == 0)
        {
            byte[] tmp = new byte[bytes.length - 1];
            
            System.arraycopy(bytes, 1, tmp, 0, tmp.length);
            
            return tmp;
        }
        
        return bytes;
    }

    /**
     * Return the passed in value as an unsigned byte array.
     *
     * @param value value to be converted.
     * @return a byte array without a leading zero byte if present in the signed encoding.
     */
    public static byte[] asUnsignedByteArray(
        int        length,
        BigInteger value)
    {
        byte[] bytes = value.toByteArray();

        if (bytes[0] == 0)
        {
            if (bytes.length - 1 > length)
            {
                throw new IllegalArgumentException("standard length exceeded for value");
            }

            byte[] tmp = new byte[length];

            System.arraycopy(bytes, 1, tmp, tmp.length - (bytes.length - 1), bytes.length - 1);

            return tmp;
        }
        else
        {
            if (bytes.length == length)
            {
                return bytes;
            }

            if (bytes.length > length)
            {
                throw new IllegalArgumentException("standard length exceeded for value");
            }

            byte[] tmp = new byte[length];

            System.arraycopy(bytes, 0, tmp, tmp.length - bytes.length, bytes.length);

            return tmp;
        }
    }

    /**
     * Return a random BigInteger not less than 'min' and not greater than 'max'
     * 
     * @param min the least value that may be generated
     * @param max the greatest value that may be generated
     * @param random the source of randomness
     * @return a random BigInteger value in the range [min,max]
     */
    public static BigInteger createRandomInRange(
        BigInteger      min,
        BigInteger      max,
        SecureRandom    random)
    {
        int cmp = min.compareTo(max);
        if (cmp >= 0)
        {
            if (cmp > 0)
            {
                throw new IllegalArgumentException("'min' may not be greater than 'max'");
            }

            return min;
        }

        if (min.bitLength() > max.bitLength() / 2)
        {
            return createRandomInRange(ZERO, max.subtract(min), random).add(min);
        }

        for (int i = 0; i < MAX_ITERATIONS; ++i)
        {
            BigInteger x = new BigInteger(max.bitLength(), random);
            if (x.compareTo(min) >= 0 && x.compareTo(max) <= 0)
            {
                return x;
            }
        }

        // fall back to a faster (restricted) method
        return new BigInteger(max.subtract(min).bitLength() - 1, random).add(min);
    }
}