package gnu.crypto.key.dss;

// ----------------------------------------------------------------------------
// $Id: FIPS186.java,v 1.2 2005/10/06 04:24:16 rsdio Exp $
//
// Copyright (C) 2003 Free Software Foundation, Inc.
//
// This file is part of GNU Crypto.
//
// GNU Crypto 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, or (at your option)
// any later version.
//
// GNU Crypto 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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//
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//    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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// ----------------------------------------------------------------------------

import gnu.crypto.hash.Sha160;
import gnu.crypto.util.Prime;
import gnu.crypto.util.PRNG;

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

/**
 * <p>An implementation of the DSA parameters generation as described in
 * FIPS-186.</p>
 *
 * References:<br>
 * <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.
 *
 * @version $Revision: 1.2 $
 */
public class FIPS186 {

   // Constants and variables
   // -------------------------------------------------------------------------

   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 = new BigInteger("2");

   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;

   // Constructor(s)
   // -------------------------------------------------------------------------

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

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

   // Class methods
   // -------------------------------------------------------------------------

   // Instance methods
   // -------------------------------------------------------------------------

   /**
    * 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 gnu.crypto.hash.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 (Prime.isProbablePrime(q)) {
               break step1;
            }
         } // step1

         // 6. Let counter = 0 and offset = 2.
         counter = 0;
         offset = 2;
         step7: 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 (Prime.isProbablePrime(p)) {
                  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 };
   }

   // helper methods ----------------------------------------------------------

   /**
    * 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 {
         PRNG.nextBytes(buffer);
      }
   }
}
