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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 and up.
package org.bouncycastle.util;
import java.math.BigInteger;
import java.security.SecureRandom;
/**
* BigInteger utilities.
*/
public final class BigIntegers
{
public static final BigInteger ZERO = BigInteger.valueOf(0);
public static final BigInteger ONE = BigInteger.valueOf(1);
private static final BigInteger TWO = BigInteger.valueOf(2);
private static final BigInteger THREE = BigInteger.valueOf(3);
private static final int MAX_ITERATIONS = 1000;
/**
* 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.length == length)
{
return bytes;
}
int start = bytes[0] == 0 ? 1 : 0;
int count = bytes.length - start;
if (count > length)
{
throw new IllegalArgumentException("standard length exceeded for value");
}
byte[] tmp = new byte[length];
System.arraycopy(bytes, start, tmp, tmp.length - count, count);
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 = createRandomBigInteger(max.bitLength(), random);
if (x.compareTo(min) >= 0 && x.compareTo(max) <= 0)
{
return x;
}
}
// fall back to a faster (restricted) method
return createRandomBigInteger(max.subtract(min).bitLength() - 1, random).add(min);
}
public static BigInteger fromUnsignedByteArray(byte[] buf)
{
return new BigInteger(1, buf);
}
public static BigInteger fromUnsignedByteArray(byte[] buf, int off, int length)
{
byte[] mag = buf;
if (off != 0 || length != buf.length)
{
mag = new byte[length];
System.arraycopy(buf, off, mag, 0, length);
}
return new BigInteger(1, mag);
}
public static int intValueExact(BigInteger x)
{
// Since Java 1.8 could use BigInteger.intValueExact instead
if (x.bitLength() > 31)
{
throw new ArithmeticException("BigInteger out of int range");
}
return x.intValue();
}
public static long longValueExact(BigInteger x)
{
// Since Java 1.8 could use BigInteger.longValueExact instead
if (x.bitLength() > 63)
{
throw new ArithmeticException("BigInteger out of long range");
}
return x.longValue();
}
public static int getUnsignedByteLength(BigInteger n)
{
return (n.bitLength() + 7) / 8;
}
/**
* Return a positive BigInteger in the range of 0 to 2**bitLength - 1.
*
* @param bitLength maximum bit length for the generated BigInteger.
* @param random a source of randomness.
* @return a positive BigInteger
*/
public static BigInteger createRandomBigInteger(int bitLength, SecureRandom random)
{
return new BigInteger(1, createRandom(bitLength, random));
}
// Hexadecimal value of the product of the 131 smallest odd primes from 3 to 743
private static final BigInteger SMALL_PRIMES_PRODUCT = new BigInteger(
"8138e8a0fcf3a4e84a771d40fd305d7f4aa59306d7251de54d98af8fe95729a1f"
+ "73d893fa424cd2edc8636a6c3285e022b0e3866a565ae8108eed8591cd4fe8d2"
+ "ce86165a978d719ebf647f362d33fca29cd179fb42401cbaf3df0c614056f9c8"
+ "f3cfd51e474afb6bc6974f78db8aba8e9e517fded658591ab7502bd41849462f",
16);
private static final int MAX_SMALL = BigInteger.valueOf(743).bitLength(); // bitlength of 743 * 743
/**
* Return a prime number candidate of the specified bit length.
*
* @param bitLength bit length for the generated BigInteger.
* @param random a source of randomness.
* @return a positive BigInteger of numBits length
*/
public static BigInteger createRandomPrime(int bitLength, int certainty, SecureRandom random)
{
if (bitLength < 2)
{
throw new IllegalArgumentException("bitLength < 2");
}
BigInteger rv;
if (bitLength == 2)
{
return (random.nextInt() < 0) ? TWO : THREE;
}
do
{
byte[] base = createRandom(bitLength, random);
int xBits = 8 * base.length - bitLength;
byte lead = (byte)(1 << (7 - xBits));
// ensure top and bottom bit set
base[0] |= lead;
base[base.length - 1] |= 0x01;
rv = new BigInteger(1, base);
if (bitLength > MAX_SMALL)
{
while (!rv.gcd(SMALL_PRIMES_PRODUCT).equals(ONE))
{
rv = rv.add(TWO);
}
}
}
while (!rv.isProbablePrime(certainty));
return rv;
}
private static byte[] createRandom(int bitLength, SecureRandom random)
throws IllegalArgumentException
{
if (bitLength < 1)
{
throw new IllegalArgumentException("bitLength must be at least 1");
}
int nBytes = (bitLength + 7) / 8;
byte[] rv = new byte[nBytes];
random.nextBytes(rv);
// strip off any excess bits in the MSB
int xBits = 8 * nBytes - bitLength;
rv[0] &= (byte)(255 >>> xBits);
return rv;
}
}
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