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Homekit Accessory Protocol for Java
There is a newer version: 2.0.7
package io.github.hapjava.server.impl.pairing;

import com.nimbusds.srp6.SRP6ClientEvidenceContext;
import com.nimbusds.srp6.SRP6CryptoParams;
import com.nimbusds.srp6.SRP6Exception;
import com.nimbusds.srp6.SRP6Routines;
import com.nimbusds.srp6.SRP6ServerEvidenceContext;
import com.nimbusds.srp6.SRP6Session;
import com.nimbusds.srp6.URoutineContext;
import java.math.BigInteger;

/**
 * This is a slightly modified version of the SRP6ServerSession class included with nimbus. The only
 * change made for homekit compatability is a change to the size of the b key. HomeKit pairing fails
 * if b is not 3072 bytes.
 *
 * Stateful server-side Secure Remote Password (SRP-6a) authentication session. Handles the
 * computing and storing of SRP-6a variables between the protocol steps as well as timeouts.
 *
 * 
Usage:
 *
 * 

 *   Create a new SRP-6a server session for each client authentication attempt.
 *   
If you wish to use custom routines for the server evidence message 'M1' and / or the client
 *       evidence message 'M2' specify them at this point.
 *   
Proceed to {@link #step1 step one} on receiving a valid user identity 'I' from the
 *       authenticating client. Respond with the server public value 'B' and password salt 's'. If
 *       the SRP-6a crypto parameters 'N', 'g' and 'H' were not agreed in advance between server and
 *       client append them to the response.
 *   
Proceed to {@link #step2 step two} on receiving the public client value 'A' and evidence
 *       message 'M1'. If the client credentials are valid signal success and return the server
 *       evidence message 'M2'. The established session key 'S' may be {@link #getSessionKey
 *       retrieved} to encrypt further communication with the client. Else signal an authentication
 *       failure to the client.
 * 
 *
 * @author Vladimir Dzhuvinov
 */
public class HomekitSRP6ServerSession extends SRP6Session {

  /** Enumerates the states of a server-side SRP-6a authentication session. */
  public static enum State {

    /**
     * The session is initialised and ready to begin authentication, by proceeding to {@link
     * #STEP_1}.
     */
    INIT,

    /**
     * The user identity 'I' is received from the client and the server has returned its public
     * value 'B' based on the matching password verifier 'v'. The session is ready to proceed to
     * {@link #STEP_2}.
     */
    STEP_1,

    /**
     * The client public key 'A' and evidence message 'M1' are received and the server has replied
     * with its own evidence message 'M2'. The session is finished (authentication was successful or
     * failed).
     */
    STEP_2
  }

  /** Indicates a non-existing use identity and implies mock salt 's' and verifier 'v' values. */
  private boolean noSuchUserIdentity = false;

  /** The password verifier 'v'. */
  private BigInteger v = null;

  /** The server private value 'b'. */
  private BigInteger b = null;

  /** The current SRP-6a auth state. */
  private State state;

  /**
   * Creates a new server-side SRP-6a authentication session and sets its state to {@link
   * State#INIT}.
   *
   * @param config The SRP-6a crypto parameters configuration. Must not be {@code null}.
   * @param timeout The SRP-6a authentication session timeout in seconds. If the authenticating
   *     counterparty (server or client) fails to respond within the specified time the session will
   *     be closed. If zero timeouts are disabled.
   */
  public HomekitSRP6ServerSession(final SRP6CryptoParams config, final int timeout) {

    super(timeout);

    if (config == null)
      throw new IllegalArgumentException("The SRP-6a crypto parameters must not be null");

    this.config = config;

    digest = config.getMessageDigestInstance();

    if (digest == null)
      throw new IllegalArgumentException("Unsupported hash algorithm 'H': " + config.H);

    state = State.INIT;

    updateLastActivityTime();
  }

  /**
   * Creates a new server-side SRP-6a authentication session and sets its state to {@link
   * State#INIT}. Session timeouts are disabled.
   *
   * @param config The SRP-6a crypto parameters configuration. Must not be {@code null}.
   */
  public HomekitSRP6ServerSession(final SRP6CryptoParams config) {

    this(config, 0);
  }

  /**
   * Increments this SRP-6a authentication session to {@link State#STEP_1}.
   *
   * Argument origin:
   *
   * 

   *   From client: user identity 'I'.
   *   
From server database: matching salt 's' and password verifier 'v' values.
   * 
   *
   * @param userID The identity 'I' of the authenticating user. Must not be {@code null} or empty.
   * @param s The password salt 's'. Must not be {@code null}.
   * @param v The password verifier 'v'. Must not be {@code null}.
   * @return The server public value 'B'.
   * @throws IllegalStateException If the mehod is invoked in a state other than {@link State#INIT}.
   */
  public BigInteger step1(final String userID, final BigInteger s, final BigInteger v) {

    // Check arguments

    if (userID == null || userID.trim().isEmpty())
      throw new IllegalArgumentException("The user identity 'I' must not be null or empty");

    this.userID = userID;

    if (s == null) throw new IllegalArgumentException("The salt 's' must not be null");

    this.s = s;

    if (v == null) throw new IllegalArgumentException("The verifier 'v' must not be null");

    this.v = v;

    // Check current state
    if (state != State.INIT)
      throw new IllegalStateException("State violation: Session must be in INIT state");

    // Generate server private and public values
    k = SRP6Routines.computeK(digest, config.N, config.g);
    digest.reset();

    b = HomekitSRP6Routines.generatePrivateValue(config.N, random);
    digest.reset();

    B = SRP6Routines.computePublicServerValue(config.N, config.g, k, v, b);

    state = State.STEP_1;

    updateLastActivityTime();

    return B;
  }

  /**
   * Increments this SRP-6a authentication session to {@link State#STEP_1} indicating a non-existing
   * user identity 'I' with mock (simulated) salt 's' and password verifier 'v' values.
   *
   * This method can be used to avoid informing the client at step one that the user identity is
   * bad and throw instead a guaranteed general "bad credentials" SRP-6a exception at step two.
   *
   * 
Argument origin:
   *
   * 

   *   From client: user identity 'I'.
   *   
Simulated by server, preferably consistently for the specified identity 'I': salt 's' and
   *       password verifier 'v' values.
   * 
   *
   * @param userID The identity 'I' of the authenticating user. Must not be {@code null} or empty.
   * @param s The password salt 's'. Must not be {@code null}.
   * @param v The password verifier 'v'. Must not be {@code null}.
   * @return The server public value 'B'.
   * @throws IllegalStateException If the method is invoked in a state other than {@link
   *     State#INIT}.
   */
  public BigInteger mockStep1(final String userID, final BigInteger s, final BigInteger v) {

    noSuchUserIdentity = true;

    return step1(userID, s, v);
  }

  /**
   * Increments this SRP-6a authentication session to {@link State#STEP_2}.
   *
   * Argument origin:
   *
   * 

   *   From client: public value 'A' and evidence message 'M1'.
   * 
   *
   * @param A The client public value. Must not be {@code null}.
   * @param M1 The client evidence message. Must not be {@code null}.
   * @return The server evidence message 'M2'.
   * @throws SRP6Exception If the session has timed out, the client public value 'A' is invalid or
   *     the user credentials are invalid.
   * @throws IllegalStateException If the method is invoked in a state other than {@link
   *     State#STEP_1}.
   */
  public BigInteger step2(final BigInteger A, final BigInteger M1) throws SRP6Exception {

    // Check arguments

    if (A == null)
      throw new IllegalArgumentException("The client public value 'A' must not be null");

    this.A = A;

    if (M1 == null)
      throw new IllegalArgumentException("The client evidence message 'M1' must not be null");

    this.M1 = M1;

    // Check current state
    if (state != State.STEP_1)
      throw new IllegalStateException("State violation: Session must be in STEP_1 state");

    // Check timeout
    if (hasTimedOut()) throw new SRP6Exception("Session timeout", SRP6Exception.CauseType.TIMEOUT);

    // Check A validity
    if (!SRP6Routines.isValidPublicValue(config.N, A))
      throw new SRP6Exception(
          "Bad client public value 'A'", SRP6Exception.CauseType.BAD_PUBLIC_VALUE);

    // Check for previous mock step 1
    if (noSuchUserIdentity)
      throw new SRP6Exception("Bad client credentials", SRP6Exception.CauseType.BAD_CREDENTIALS);

    if (hashedKeysRoutine != null) {
      URoutineContext hashedKeysContext = new URoutineContext(A, B);
      u = hashedKeysRoutine.computeU(config, hashedKeysContext);
    } else {
      u = SRP6Routines.computeU(digest, config.N, A, B);
      digest.reset();
    }

    S = SRP6Routines.computeSessionKey(config.N, v, u, A, b);

    // Compute the own client evidence message 'M1'
    BigInteger computedM1;

    if (clientEvidenceRoutine != null) {

      // With custom routine
      SRP6ClientEvidenceContext ctx = new SRP6ClientEvidenceContext(userID, s, A, B, S);
      computedM1 = clientEvidenceRoutine.computeClientEvidence(config, ctx);
    } else {
      // With default routine
      computedM1 = SRP6Routines.computeClientEvidence(digest, A, B, S);
      digest.reset();
    }

    if (!computedM1.equals(M1))
      throw new SRP6Exception("Bad client credentials", SRP6Exception.CauseType.BAD_CREDENTIALS);

    state = State.STEP_2;

    if (serverEvidenceRoutine != null) {

      // With custom routine
      SRP6ServerEvidenceContext ctx = new SRP6ServerEvidenceContext(A, M1, S);

      M2 = serverEvidenceRoutine.computeServerEvidence(config, ctx);
    }

    updateLastActivityTime();

    return M2;
  }

  /**
   * Returns the current state of this SRP-6a authentication session.
   *
   * @return The current state.
   */
  public State getState() {

    return state;
  }
}