package gnu.crypto.key.rsa;

// ----------------------------------------------------------------------------
// $Id: RSAKeyPairGenerator.java,v 1.2 2005/10/06 04:24:16 rsdio Exp $
//
// Copyright (C) 2001, 2002, 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.
//
// You should have received a copy of the GNU General Public License
// along with this program; see the file COPYING.  If not, write to the
//
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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
// conditions of the GNU General Public License cover the whole
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// you permission to link this library with independent modules to
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// for each linked independent module, the terms and conditions of the
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// library, but you are not obligated to do so.  If you do not wish to
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// ----------------------------------------------------------------------------

import gnu.crypto.Registry;
import gnu.crypto.key.IKeyPairGenerator;
import gnu.crypto.util.Prime;
import gnu.crypto.util.PRNG;

import java.math.BigInteger;
import java.security.KeyPair;
import java.security.PrivateKey;
import java.security.PublicKey;
import java.security.SecureRandom;
import java.security.spec.RSAKeyGenParameterSpec;
import java.util.Map;

/**
 * <p>A key-pair generator for asymetric keys to use in conjunction with the RSA
 * scheme.</p>
 *
 * <p>Reference:</p>
 * <ol>
 *    <li><a href="http://www.cosic.esat.kuleuven.ac.be/nessie/workshop/submissions/rsa-pss.zip">
 *    RSA-PSS Signature Scheme with Appendix</a>, part B. Primitive
 *    specification and supporting documentation. Jakob Jonsson and Burt Kaliski.
 *    </li>
 *    <li><a href="http://www.cacr.math.uwaterloo.ca/hac/">Handbook of Applied
 *    Cryptography</a>, Alfred J. Menezes, Paul C. van Oorschot and Scott A.
 *    Vanstone. Section 11.3 RSA and related signature schemes.</li>
 * </ol>
 *
 * @version $Revision: 1.2 $
 */
public class RSAKeyPairGenerator implements IKeyPairGenerator {

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

   /** The BigInteger constant 1. */
   private static final BigInteger ONE = BigInteger.ONE;

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

   /** Property name of the length (Integer) of the modulus of an RSA key. */
   public static final String MODULUS_LENGTH = "gnu.crypto.rsa.L";

   /**
    * Property name of an optional {@link SecureRandom} instance to use. The
    * default is to use a classloader singleton from {@link PRNG}.
    */
   public static final String SOURCE_OF_RANDOMNESS = "gnu.crypto.rsa.prng";

   /**
    * Property name of an optional {@link RSAKeyGenParameterSpec} instance to
    * use for this generator's <code>n</code>, and <code>e</code> values. The
    * default is to generate <code>n</code> and use a fixed value for
    * <code>e</.code> (Fermat's F4 number).
    */
   public static final String RSA_PARAMETERS = "gnu.crypto.rsa.params";

   /** Default value for the modulus length. */
   private static final int DEFAULT_MODULUS_LENGTH = 1024;

   /** The desired bit length of the modulus. */
   private int L;

   /**
    * This implementation uses, by default, Fermat's F4 number as the public
    * exponent.
    */
   private BigInteger e = BigInteger.valueOf(65537L);

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

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

   // implicit 0-arguments constructor

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

   // gnu.crypto.key.IKeyPairGenerator interface implementation ---------------

   public String name() {
      return Registry.RSA_KPG;
   }

   /**
    * <p>Configures this instance.</p>
    *
    * @param attributes the map of name/value pairs to use.
    * @exception IllegalArgumentException if the designated MODULUS_LENGTH
    * value is less than 1024.
    */
   public void setup(Map attributes) {
      // do we have a SecureRandom, or should we use our own?
      rnd = (SecureRandom) attributes.get(SOURCE_OF_RANDOMNESS);

      // are we given a set of RSA params or we shall use our own?
      RSAKeyGenParameterSpec params =
         (RSAKeyGenParameterSpec) attributes.get(RSA_PARAMETERS);

      // find out the modulus length
      if (params != null) {
         L = params.getKeysize();
         e = params.getPublicExponent();
      } else {
         Integer l = (Integer) attributes.get(MODULUS_LENGTH);
         L = (l == null ? DEFAULT_MODULUS_LENGTH : l.intValue());
      }

      if (L < 1024) {
         throw new IllegalArgumentException(MODULUS_LENGTH);
      }
   }

   /**
    * <p>The algorithm used here is described in <i>nessie-pss-B.pdf</i>
    * document which is part of the RSA-PSS submission to NESSIE.</p>
    *
    * @return an RSA keypair.
    */
   public KeyPair generate() {
      BigInteger p, q, n, d;

      // 1. Generate a prime p in the interval [2**(M-1), 2**M - 1], where
      // M = CEILING(L/2), and such that GCD(p, e) = 1
      int M = (L+1)/2;
      BigInteger lower = TWO.pow(M-1);
      BigInteger upper = TWO.pow(M).subtract(ONE);
      byte[] kb = new byte[(M+7)/8]; // enough bytes to frame M bits
      step1: while (true) {
         nextRandomBytes(kb);
         p = new BigInteger(1, kb).setBit(0);
         if (     p.compareTo(lower) >= 0
               && p.compareTo(upper) <= 0
               && Prime.isProbablePrime(p)
               && p.gcd(e).equals(ONE)) {
            break step1;
         }
      }

      // 2. Generate a prime q such that the product of p and q is an L-bit
      // number, and such that GCD(q, e) = 1
      step2: while (true) {
         nextRandomBytes(kb);
         q = new BigInteger(1, kb).setBit(0);
         n = p.multiply(q);
         if (     n.bitLength() == L
               && Prime.isProbablePrime(q)
               && q.gcd(e).equals(ONE)) {
            break step2;
         }

         // TODO: test for p != q
      }

      // TODO: ensure p < q

      // 3. Put n = pq. The public key is (n, e).
      // 4. Compute the parameters necessary for the private key K (see
      // Section 2.2).
      BigInteger phi = p.subtract(ONE).multiply(q.subtract(ONE));
      d = e.modInverse(phi);

      // 5. Output the public key and the private key.
      PublicKey pubK = new GnuRSAPublicKey(n, e);
      PrivateKey secK = new GnuRSAPrivateKey(p, q, e, d);

      return new KeyPair(pubK, secK);
   }

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

   /**
    * <p>Fills the designated byte array with random data.</p>
    *
    * @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);
      }
   }
}
