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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 1.5.
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 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);
}
}