/* FIPS186.java --
   Copyright 2001, 2002, 2003, 2006 Free Software Foundation, Inc.

This file is a part of GNU Classpath.

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package gnu.java.security.key.dss;

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

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

/**
 * An implementation of the DSA parameters generation as described in FIPS-186.
 * <p>
 * References:
 * <p>
 * <a href="http://www.itl.nist.gov/fipspubs/fip186.htm">Digital Signature
 * Standard (DSS)</a>, Federal Information Processing Standards Publication
 * 186. National Institute of Standards and Technology.
 */
public class FIPS186
{
  public static final int DSA_PARAMS_SEED = 0;

  public static final int DSA_PARAMS_COUNTER = 1;

  public static final int DSA_PARAMS_Q = 2;

  public static final int DSA_PARAMS_P = 3;

  public static final int DSA_PARAMS_E = 4;

  public static final int DSA_PARAMS_G = 5;

  /** The BigInteger constant 2. */
  private static final BigInteger TWO = BigInteger.valueOf(2L);

  private static final BigInteger TWO_POW_160 = TWO.pow(160);

  /** The SHA instance to use. */
  private Sha160 sha = new Sha160();

  /** The length of the modulus of DSS keys generated by this instance. */
  private int L;

  /** The optional {@link SecureRandom} instance to use. */
  private SecureRandom rnd = null;

  /** Our default source of randomness. */
  private PRNG prng = null;

  public FIPS186(int L, SecureRandom rnd)
  {
    super();

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

  /**
   * This method generates the DSS <code>p</code>, <code>q</code>, and
   * <code>g</code> parameters only when <code>L</code> (the modulus length)
   * is not one of the following: <code>512</code>, <code>768</code> and
   * <code>1024</code>. For those values of <code>L</code>, this
   * implementation uses pre-computed values of <code>p</code>,
   * <code>q</code>, and <code>g</code> given in the document <i>CryptoSpec</i>
   * included in the security guide documentation of the standard JDK
   * distribution.
   * <p>
   * The DSS requires two primes , <code>p</code> and <code>q</code>,
   * satisfying the following three conditions:
   * <ul>
   * <li><code>2<sup>159</sup> &lt; q &lt; 2<sup>160</sup></code></li>
   * <li><code>2<sup>L-1</sup> &lt; p &lt; 2<sup>L</sup></code> for a
   * specified <code>L</code>, where <code>L = 512 + 64j</code> for some
   * <code>0 &lt;= j &lt;= 8</code></li>
   * <li>q divides p - 1.</li>
   * </ul>
   * The algorithm used to find these primes is as described in FIPS-186,
   * section 2.2: GENERATION OF PRIMES. This prime generation scheme starts by
   * using the {@link Sha160} and a user supplied <i>SEED</i> to construct a
   * prime, <code>q</code>, in the range 2<sup>159</sup> &lt; q &lt; 2<sup>160</sup>.
   * Once this is accomplished, the same <i>SEED</i> value is used to construct
   * an <code>X</code> in the range <code>2<sup>L-1
   * </sup> &lt; X &lt; 2<sup>L</sup>. The prime, <code>p</code>, is then
   * formed by rounding <code>X</code> to a number congruent to <code>1 mod
   * 2q</code>. In this implementation we use the same <i>SEED</i> value given
   * in FIPS-186, Appendix 5.
   */
  public BigInteger[] generateParameters()
  {
    int counter, offset;
    BigInteger SEED, alpha, U, q, OFFSET, SEED_PLUS_OFFSET, W, X, p, c, g;
    byte[] a, u;
    byte[] kb = new byte[20]; // to hold 160 bits of randomness

    // Let L-1 = n*160 + b, where b and n are integers and 0 <= b < 160.
    int b = (L - 1) % 160;
    int n = (L - 1 - b) / 160;
    BigInteger[] V = new BigInteger[n + 1];
    algorithm: while (true)
      {
        step1: while (true)
          {
            // 1. Choose an arbitrary sequence of at least 160 bits and
            // call it SEED.
            nextRandomBytes(kb);
            SEED = new BigInteger(1, kb).setBit(159).setBit(0);
            // Let g be the length of SEED in bits. here always 160
            // 2. Compute: U = SHA[SEED] XOR SHA[(SEED+1) mod 2**g]
            alpha = SEED.add(BigInteger.ONE).mod(TWO_POW_160);
            synchronized (sha)
              {
                a = SEED.toByteArray();
                sha.update(a, 0, a.length);
                a = sha.digest();
                u = alpha.toByteArray();
                sha.update(u, 0, u.length);
                u = sha.digest();
              }
            for (int i = 0; i < a.length; i++)
              a[i] ^= u[i];

            U = new BigInteger(1, a);
            // 3. Form q from U by setting the most significant bit (the
            // 2**159 bit) and the least significant bit to 1. In terms of
            // boolean operations, q = U OR 2**159 OR 1. Note that
            // 2**159 < q < 2**160.
            q = U.setBit(159).setBit(0);
            // 4. Use a robust primality testing algorithm to test whether
            // q is prime(1). A robust primality test is one where the
            // probability of a non-prime number passing the test is at
            // most 1/2**80.
            // 5. If q is not prime, go to step 1.
            if (q.isProbablePrime(80))
              break step1;
          } // step1
        // 6. Let counter = 0 and offset = 2.
        counter = 0;
        offset = 2;
        while (true)
          {
            OFFSET = BigInteger.valueOf(offset & 0xFFFFFFFFL);
            SEED_PLUS_OFFSET = SEED.add(OFFSET);
            // 7. For k = 0,...,n let V[k] = SHA[(SEED + offset + k) mod 2**g].
            synchronized (sha)
              {
                for (int k = 0; k <= n; k++)
                  {
                    a = SEED_PLUS_OFFSET
                        .add(BigInteger.valueOf(k & 0xFFFFFFFFL))
                        .mod(TWO_POW_160).toByteArray();
                    sha.update(a, 0, a.length);
                    V[k] = new BigInteger(1, sha.digest());
                  }
              }
            // 8. Let W be the integer:
            // V[0]+V[1]*2**160+...+V[n-1]*2**((n-1)*160)+(V[n]mod2**b)*2**(n*160)
            // and let : X = W + 2**(L-1).
            // Note that 0 <= W < 2**(L-1) and hence 2**(L-1) <= X < 2**L.
            W = V[0];
            for (int k = 1; k < n; k++)
              W = W.add(V[k].multiply(TWO.pow(k * 160)));

            W = W.add(V[n].mod(TWO.pow(b)).multiply(TWO.pow(n * 160)));
            X = W.add(TWO.pow(L - 1));
            // 9. Let c = X mod 2q and set p = X - (c - 1).
            // Note that p is congruent to 1 mod 2q.
            c = X.mod(TWO.multiply(q));
            p = X.subtract(c.subtract(BigInteger.ONE));
            // 10. If p < 2**(L-1), then go to step 13.
            if (p.compareTo(TWO.pow(L - 1)) >= 0)
              {
                // 11. Perform a robust primality test on p.
                // 12. If p passes the test performed in step 11, go to step 15.
                if (p.isProbablePrime(80))
                  break algorithm;
              }
            // 13. Let counter = counter + 1 and offset = offset + n + 1.
            counter++;
            offset += n + 1;
            // 14. If counter >= 4096 go to step 1, otherwise go to step 7.
            if (counter >= 4096)
              continue algorithm;
          } // step7
      } // 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;
  }
}