/* RSAKeyPairGenerator.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.

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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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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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permission to link this library with independent modules to produce an
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package gnu.java.security.key.rsa;

import gnu.java.security.Configuration;
import gnu.java.security.Registry;
import gnu.java.security.key.IKeyPairGenerator;
import gnu.java.security.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;
import java.util.logging.Logger;

/**
 * A key-pair generator for asymetric keys to use in conjunction with the RSA
 * scheme.
 * <p>
 * Reference:
 * <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>
 */
public class RSAKeyPairGenerator
    implements IKeyPairGenerator
{
  private static final Logger log = Logger.getLogger(RSAKeyPairGenerator.class.getName());

  /** 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";

  /**
   * Property name of the preferred encoding format to use when externalizing
   * generated instance of key-pairs from this generator. The property is taken
   * to be an {@link Integer} that encapsulates an encoding format identifier.
   */
  public static final String PREFERRED_ENCODING_FORMAT = "gnu.crypto.rsa.encoding";

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

  /** Default encoding format to use when none was specified. */
  private static final int DEFAULT_ENCODING_FORMAT = Registry.RAW_ENCODING_ID;

  /** 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;

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

  /** Preferred encoding format of generated keys. */
  private int preferredFormat;

  // implicit 0-arguments constructor

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

  /**
   * Configures this instance.
   * 
   * @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)
  {
    if (Configuration.DEBUG)
      log.entering(this.getClass().getName(), "setup", 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);

    // what is the preferred encoding format
    Integer formatID = (Integer) attributes.get(PREFERRED_ENCODING_FORMAT);
    preferredFormat = formatID == null ? DEFAULT_ENCODING_FORMAT
                                       : formatID.intValue();
    if (Configuration.DEBUG)
      log.exiting(this.getClass().getName(), "setup");
  }

  /**
   * <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()
  {
    if (Configuration.DEBUG)
      log.entering(this.getClass().getName(), "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
            && p.isProbablePrime(80) && 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 && q.isProbablePrime(80) && 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(preferredFormat, n, e);
    PrivateKey secK = new GnuRSAPrivateKey(preferredFormat, p, q, e, d);
    KeyPair result = new KeyPair(pubK, secK);
    if (Configuration.DEBUG)
      log.exiting(this.getClass().getName(), "generate", result);
    return result;
  }

  /**
   * 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;
  }
}