/* 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.

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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.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;

/**
 * <p>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:</p>
 *
 * <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
 * ^    (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 ":" p))           (s is chosen randomly)
 * v = g^x                   (computes password verifier)
 *
 * The host then keeps {I, s, v} in its password database.
 *
 * The authentication protocol itself goes as follows:
 * User -> Host:  I, A = g^a         (identifies self, a = random number)
 * Host -> User:  s, B = 3v + g^b    (sends salt, b = random number)
 *
 * Both:  u = H(A, B)
 *
 * User:  x = H(s, p)                (user enters password)
 * User:  S = (B - 3g^x) ^ (a + ux)  (computes session key)
 * User:  K = H(S)
 *
 * Host:  S = (Av^u) ^ b             (computes session key)
 * Host:  K = H(S)
 * </pre>
 *
 * <p>Reference:</p>
 * <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
{

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

  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;

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

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

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

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

  // implementation of common abstract methods in BaseKeyAGreementParty ------

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

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

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

  protected BigInteger uValue(final BigInteger A, final BigInteger B)
  {
    //      IMessageDigest hash = (IMessageDigest) mda.clone();
    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());
  }
}
