/* RFC2631.java --
   Copyright (C) 2003, 2006 Free Software Foundation, Inc.

This file is a part of GNU Classpath.

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it under the terms of the GNU General Public License as published by
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Linking this library statically or dynamically with other modules is
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package gnu.javax.crypto.key.dh;

import gnu.java.security.hash.Sha160;
import gnu.java.security.util.PRNG;

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

/**
 * An implementation of the Diffie-Hellman parameter generation as defined in
 * RFC-2631.
 * <p>
 * Reference:
 * <ol>
 * <li><a href="http://www.ietf.org/rfc/rfc2631.txt">Diffie-Hellman Key
 * Agreement Method</a><br>
 * Eric Rescorla.</li>
 * </ol>
 */
public class RFC2631
{
  public static final int DH_PARAMS_SEED = 0;
  public static final int DH_PARAMS_COUNTER = 1;
  public static final int DH_PARAMS_Q = 2;
  public static final int DH_PARAMS_P = 3;
  public static final int DH_PARAMS_J = 4;
  public static final int DH_PARAMS_G = 5;
  private static final BigInteger TWO = BigInteger.valueOf(2L);
  /** The SHA instance to use. */
  private Sha160 sha = new Sha160();
  /** Length of private modulus and of q. */
  private int m;
  /** Length of public modulus p. */
  private int L;
  /** The optional {@link SecureRandom} instance to use. */
  private SecureRandom rnd = null;
  /** Our default source of randomness. */
  private PRNG prng = null;

  public RFC2631(int m, int L, SecureRandom rnd)
  {
    super();

    this.m = m;
    this.L = L;
    this.rnd = rnd;
  }

  public BigInteger[] generateParameters()
  {
    int i, j, counter;
    byte[] u1, u2, v;
    byte[] seedBytes = new byte[m / 8];
    BigInteger SEED, U, q, R, V, W, X, p, g;
    // start by genrating p and q, where q is of length m and p is of length L
    // 1. Set m' = m/160 where / represents integer division with rounding
    //    upwards. I.e. 200/160 = 2.
    int m_ = (m + 159) / 160;
    // 2. Set L'=  L/160
    int L_ = (L + 159) / 160;
    // 3. Set N'=  L/1024
    int N_ = (L + 1023) / 1024;
    algorithm: while (true)
      {
        step4: while (true)
          {
            // 4. Select an arbitrary bit string SEED such that length of
            //    SEED >= m
            nextRandomBytes(seedBytes);
            SEED = new BigInteger(1, seedBytes).setBit(m - 1).setBit(0);
            // 5. Set U = 0
            U = BigInteger.ZERO;
            // 6. For i = 0 to m' - 1
            //    U = U + (SHA1[SEED + i] XOR SHA1[(SEED + m' + i)) * 2^(160 * i)
            //    Note that for m=160, this reduces to the algorithm of FIPS-186
            //    U = SHA1[SEED] XOR SHA1[(SEED+1) mod 2^160 ].
            for (i = 0; i < m_; i++)
              {
                u1 = SEED.add(BigInteger.valueOf(i)).toByteArray();
                u2 = SEED.add(BigInteger.valueOf(m_ + i)).toByteArray();
                sha.update(u1, 0, u1.length);
                u1 = sha.digest();
                sha.update(u2, 0, u2.length);
                u2 = sha.digest();
                for (j = 0; j < u1.length; j++)
                  u1[j] ^= u2[j];
                U = U.add(new BigInteger(1, u1).multiply(TWO.pow(160 * i)));
              }
            // 5. Form q from U by computing U mod (2^m) and setting the most
            //    significant bit (the 2^(m-1) bit) and the least significant
            //    bit to 1. In terms of boolean operations, q = U OR 2^(m-1) OR
            //    1. Note that 2^(m-1) < q < 2^m
            q = U.setBit(m - 1).setBit(0);
            // 6. Use a robust primality algorithm to test whether q is prime.
            // 7. If q is not prime then go to 4.
            if (q.isProbablePrime(80))
              break step4;
          }
        // 8. Let counter = 0
        counter = 0;
        while (true)
          {
            // 9. Set R = seed + 2*m' + (L' * counter)
            R = SEED
                .add(BigInteger.valueOf(2 * m_))
                .add(BigInteger.valueOf(L_ * counter));
            // 10. Set V = 0
            V = BigInteger.ZERO;
            // 12. For i = 0 to L'-1 do: V = V + SHA1(R + i) * 2^(160 * i)
            for (i = 0; i < L_; i++)
              {
                v = R.toByteArray();
                sha.update(v, 0, v.length);
                v = sha.digest();
                V = V.add(new BigInteger(1, v).multiply(TWO.pow(160 * i)));
              }
            // 13. Set W = V mod 2^L
            W = V.mod(TWO.pow(L));
            // 14. Set X = W OR 2^(L-1)
            //     Note that 0 <= W < 2^(L-1) and hence X >= 2^(L-1)
            X = W.setBit(L - 1);
            // 15. Set p = X - (X mod (2*q)) + 1
            p = X.add(BigInteger.ONE).subtract(X.mod(TWO.multiply(q)));
            // 16. If p > 2^(L-1) use a robust primality test to test whether p
            //     is prime. Else go to 18.
            // 17. If p is prime output p, q, seed, counter and stop.
            if (p.isProbablePrime(80))
              {
                break algorithm;
              }
            // 18. Set counter = counter + 1
            counter++;
            // 19. If counter < (4096 * N) then go to 8.
            // 20. Output "failure"
            if (counter >= 4096 * N_)
              continue algorithm;
          }
      }
    // compute g. from FIPS-186, Appendix 4:
    // 1. Generate p and q as specified in Appendix 2.
    // 2. Let e = (p - 1) / q
    BigInteger e = p.subtract(BigInteger.ONE).divide(q);
    BigInteger h = TWO;
    BigInteger p_minus_1 = p.subtract(BigInteger.ONE);
    g = TWO;
    // 3. Set h = any integer, where 1 < h < p - 1 and h differs from any
    //    value previously tried
    for (; h.compareTo(p_minus_1) < 0; h = h.add(BigInteger.ONE))
      {
        // 4. Set g = h**e mod p
        g = h.modPow(e, p);
        // 5. If g = 1, go to step 3
        if (! g.equals(BigInteger.ONE))
          break;
      }
    return new BigInteger[] { SEED, BigInteger.valueOf(counter), q, p, e, g };
  }

  /**
   * Fills the designated byte array with random data.
   *
   * @param buffer the byte array to fill with random data.
   */
  private void nextRandomBytes(byte[] buffer)
  {
    if (rnd != null)
      rnd.nextBytes(buffer);
    else
      getDefaultPRNG().nextBytes(buffer);
  }

  private PRNG getDefaultPRNG()
  {
    if (prng == null)
      prng = PRNG.getInstance();

    return prng;
  }
}