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Homekit Accessory Protocol for Java

There is a newer version: 2.0.7

package io.github.hapjava.server.impl.pairing;

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

/**
 * This class is modified from the nimbus SRP library to provide methods that are compatible with
 * some pecularities of HomeKit. Namely, the need for a 3072 bit private value
 *
 * @author Vladimir Dzhuvinov
 */
public class HomekitSRP6Routines {

  public static BigInteger generatePrivateValue(BigInteger N, SecureRandom random) {
    final int minBits = Math.min(3072, N.bitLength() / 2);

    BigInteger min = BigInteger.ONE.shiftLeft(minBits - 1);
    BigInteger max = N.subtract(BigInteger.ONE);

    return createRandomBigIntegerInRange(min, max, random);
  }

  /**
   * Returns a random big integer in the specified range [min, max].
   *
   * @param min The least value that may be generated. Must not be {@code null}.
   * @param max The greatest value that may be generated. Must not be {@code null}.
   * @param random Source of randomness. Must not be {@code null}.
   * @return A random big integer in the range [min, max].
   */
  protected static BigInteger createRandomBigIntegerInRange(
      final BigInteger min, final BigInteger max, final SecureRandom random) {

    final 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 createRandomBigIntegerInRange(BigInteger.ZERO, max.subtract(min), random).add(min);

    final int MAX_ITERATIONS = 1000;

    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);
  }
}