/* SRP6KeyAgreement.java -- 
   Copyright (C) 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
conditions of the GNU General Public License cover the whole
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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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package gnu.javax.crypto.key.srp6;

import gnu.java.security.Registry;
import gnu.java.security.hash.IMessageDigest;
import gnu.java.security.util.Util;

import gnu.javax.crypto.key.BaseKeyAgreementParty;
import gnu.javax.crypto.key.KeyAgreementException;
import gnu.javax.crypto.sasl.srp.SRP;

import java.math.BigInteger;

/**
 * The Secure Remote Password (SRP) key agreement protocol, also known as SRP-6,
 * is designed by Thomas J. Wu (see references). The protocol, and its elements
 * are described as follows:
 * <pre>
 *  N    A large safe prime (N = 2q+1, where q is prime)
 *       All arithmetic is done modulo N.
 *  g    A generator modulo N
 *  s    User's salt
 *  I    Username
 *  p    Cleartext Password
 *  H()  One-way hash function
 *  &circ;    (Modular) Exponentiation
 *  u    Random scrambling parameter
 *  a,b  Secret ephemeral values
 *  A,B  Public ephemeral values
 *  x    Private key (derived from p and s)
 *  v    Password verifier
 * 
 *  The host stores passwords using the following formula:
 *  x = H(s | H(I &quot;:&quot; p))           (s is chosen randomly)
 *  v = g&circ;x                         (computes password verifier)
 * 
 *  The host then keeps {I, s, v} in its password database.
 * 
 *  The authentication protocol itself goes as follows:
 *  User -&gt; Host:  I, A = g&circ;a         (identifies self, a = random number)
 *  Host -&gt; User:  s, B = 3v + g&circ;b    (sends salt, b = random number)
 * 
 *  Both:  u = H(A, B)
 * 
 *  User:  x = H(s, p)               (user enters password)
 *  User:  S = (B - 3g&circ;x) &circ; (a + ux) (computes session key)
 *  User:  K = H(S)
 * 
 *  Host:  S = (Av&circ;u) &circ; b            (computes session key)
 *  Host:  K = H(S)
 * </pre>
 * <p>
 * Reference:
 * <ol>
 * <li><a href="http://srp.stanford.edu/design.html">SRP Protocol Design</a><br>
 * Thomas J. Wu.</li>
 * </ol>
 */
public abstract class SRP6KeyAgreement
    extends BaseKeyAgreementParty
{
  public static final String SOURCE_OF_RANDOMNESS = "gnu.crypto.srp6.ka.prng";
  public static final String SHARED_MODULUS = "gnu.crypto.srp6.ka.N";
  public static final String GENERATOR = "gnu.crypto.srp6.ka.g";
  public static final String HASH_FUNCTION = "gnu.crypto.srp6.ka.H";
  public static final String USER_IDENTITY = "gnu.crypto.srp6.ka.I";
  public static final String USER_PASSWORD = "gnu.crypto.srp6.ka.p";
  public static final String HOST_PASSWORD_DB = "gnu.crypto.srp6.ka.password.db";
  protected static final BigInteger THREE = BigInteger.valueOf(3L);
  protected SRP srp;
  protected BigInteger N;
  protected BigInteger g;
  /** The shared secret key. */
  protected BigInteger K;

  protected SRP6KeyAgreement()
  {
    super(Registry.SRP6_KA);
  }

  protected byte[] engineSharedSecret() throws KeyAgreementException
  {
    return Util.trim(K);
  }

  protected void engineReset()
  {
    srp = null;
    N = null;
    g = null;
    K = null;
  }

  protected BigInteger uValue(final BigInteger A, final BigInteger B)
  {
    final IMessageDigest hash = srp.newDigest();
    byte[] b;
    b = Util.trim(A);
    hash.update(b, 0, b.length);
    b = Util.trim(B);
    hash.update(b, 0, b.length);
    return new BigInteger(1, hash.digest());
  }
}