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/* FIPS186.java --

   Copyright 2001, 2002, 2003, 2006 Free Software Foundation, Inc.

This file is a part of GNU Classpath.

GNU Classpath is free software; you can redistribute it and/or modify

it under the terms of the GNU General Public License as published by

the Free Software Foundation; either version 2 of the License, or (at

your option) any later version.

GNU Classpath is distributed in the hope that it will be useful, but

WITHOUT ANY WARRANTY; without even the implied warranty of

MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU

General Public License for more details.

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along with GNU Classpath; if not, write to the Free Software

Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301

USA

Linking this library statically or dynamically with other modules is

making a combined work based on this library.  Thus, the terms and

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As a special exception, the copyright holders of this library give you

permission to link this library with independent modules to produce an

executable, regardless of the license terms of these independent

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independent module, the terms and conditions of the license of that

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or based on this library.  If you modify this library, you may extend

this exception to your version of the library, but you are not

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exception statement from your version.  */

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;

  }

}