/*
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements. See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership. The ASF licenses this file
 * to you under the Apache License, Version 2.0 (the
 * "License"); you may not use this file except in compliance
 * with the License. You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing,
 * software distributed under the License is distributed on an
 * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
 * KIND, either express or implied. See the License for the
 * specific language governing permissions and limitations
 * under the License.
 */
package org.apache.sshd.common.session.helpers;

import java.io.IOException;
import java.io.StreamCorruptedException;
import java.net.SocketAddress;
import java.nio.charset.StandardCharsets;
import java.time.Duration;
import java.time.Instant;
import java.util.ArrayList;
import java.util.Collection;
import java.util.Collections;
import java.util.EnumMap;
import java.util.HashSet;
import java.util.List;
import java.util.Map;
import java.util.NavigableSet;
import java.util.Objects;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.ScheduledExecutorService;
import java.util.concurrent.ScheduledFuture;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.TimeoutException;
import java.util.concurrent.atomic.AtomicReference;
import java.util.function.Function;

import org.apache.sshd.common.AttributeRepository;
import org.apache.sshd.common.FactoryManager;
import org.apache.sshd.common.NamedResource;
import org.apache.sshd.common.PropertyResolver;
import org.apache.sshd.common.RuntimeSshException;
import org.apache.sshd.common.SshConstants;
import org.apache.sshd.common.SshException;
import org.apache.sshd.common.channel.throttle.ChannelStreamWriterResolver;
import org.apache.sshd.common.channel.throttle.ChannelStreamWriterResolverManager;
import org.apache.sshd.common.digest.Digest;
import org.apache.sshd.common.forward.Forwarder;
import org.apache.sshd.common.future.DefaultSshFuture;
import org.apache.sshd.common.io.IoSession;
import org.apache.sshd.common.io.IoWriteFuture;
import org.apache.sshd.common.kex.AbstractKexFactoryManager;
import org.apache.sshd.common.kex.KexProposalOption;
import org.apache.sshd.common.random.Random;
import org.apache.sshd.common.session.ConnectionService;
import org.apache.sshd.common.session.ReservedSessionMessagesHandler;
import org.apache.sshd.common.session.Session;
import org.apache.sshd.common.session.SessionContext;
import org.apache.sshd.common.session.SessionDisconnectHandler;
import org.apache.sshd.common.session.SessionListener;
import org.apache.sshd.common.session.UnknownChannelReferenceHandler;
import org.apache.sshd.common.session.helpers.TimeoutIndicator.TimeoutStatus;
import org.apache.sshd.common.util.GenericUtils;
import org.apache.sshd.common.util.Invoker;
import org.apache.sshd.common.util.ValidateUtils;
import org.apache.sshd.common.util.buffer.Buffer;
import org.apache.sshd.common.util.buffer.BufferUtils;
import org.apache.sshd.common.util.buffer.ByteArrayBuffer;
import org.apache.sshd.common.util.net.SshdSocketAddress;
import org.apache.sshd.core.CoreModuleProperties;

/**
 * Contains split code in order to make {@link AbstractSession} class smaller
 */
public abstract class SessionHelper extends AbstractKexFactoryManager implements Session {
    /** Session level lock for regulating access to sensitive data */
    protected final Object sessionLock = new Object();

    // Session timeout measurements
    protected Instant authStart = Instant.now();
    protected Instant idleStart = Instant.now();

    /** Client or server side */
    private final boolean serverSession;

    /** The underlying network session */
    private final IoSession ioSession;

    /** The session specific properties */
    private final Map<String, Object> properties = new ConcurrentHashMap<>();

    /** Session specific attributes */
    private final Map<AttributeRepository.AttributeKey<?>, Object> attributes = new ConcurrentHashMap<>();

    // Session timeout measurements
    private final AtomicReference<TimeoutIndicator> timeoutStatus = new AtomicReference<>(TimeoutIndicator.NONE);

    private ReservedSessionMessagesHandler reservedSessionMessagesHandler;
    private SessionDisconnectHandler sessionDisconnectHandler;
    private UnknownChannelReferenceHandler unknownChannelReferenceHandler;
    private ChannelStreamWriterResolver channelStreamPacketWriterResolver;

    /**
     * The name of the authenticated user
     */
    private volatile String username;
    /**
     * Boolean indicating if this session has been authenticated or not
     */
    private volatile boolean authed;

    /**
     * Create a new session.
     *
     * @param serverSession  {@code true} if this is a server session, {@code false} if client one
     * @param factoryManager the factory manager
     * @param ioSession      the underlying I/O session
     */
    protected SessionHelper(boolean serverSession, FactoryManager factoryManager, IoSession ioSession) {
        super(Objects.requireNonNull(factoryManager, "No factory manager provided"));
        this.serverSession = serverSession;
        this.ioSession = Objects.requireNonNull(ioSession, "No IoSession provided");
    }

    @Override
    public IoSession getIoSession() {
        return ioSession;
    }

    @Override
    public boolean isServerSession() {
        return serverSession;
    }

    @Override
    public FactoryManager getFactoryManager() {
        return (FactoryManager) getDelegate();
    }

    @Override
    public PropertyResolver getParentPropertyResolver() {
        return getFactoryManager();
    }

    @Override
    public Map<String, Object> getProperties() {
        return properties;
    }

    @Override
    public int getAttributesCount() {
        return attributes.size();
    }

    @Override
    @SuppressWarnings("unchecked")
    public <T> T getAttribute(AttributeRepository.AttributeKey<T> key) {
        return (T) attributes.get(Objects.requireNonNull(key, "No key"));
    }

    @Override
    public Collection<AttributeKey<?>> attributeKeys() {
        return attributes.isEmpty() ? Collections.emptySet() : new HashSet<>(attributes.keySet());
    }

    @Override
    @SuppressWarnings({ "unchecked", "rawtypes" })
    public <T> T computeAttributeIfAbsent(
            AttributeRepository.AttributeKey<T> key,
            Function<? super AttributeRepository.AttributeKey<T>, ? extends T> resolver) {
        return (T) attributes.computeIfAbsent(Objects.requireNonNull(key, "No key"), (Function) resolver);
    }

    @Override
    @SuppressWarnings("unchecked")
    public <T> T setAttribute(AttributeRepository.AttributeKey<T> key, T value) {
        return (T) attributes.put(
                Objects.requireNonNull(key, "No key"),
                Objects.requireNonNull(value, "No value"));
    }

    @Override
    @SuppressWarnings("unchecked")
    public <T> T removeAttribute(AttributeRepository.AttributeKey<T> key) {
        return (T) attributes.remove(Objects.requireNonNull(key, "No key"));
    }

    @Override
    public void clearAttributes() {
        attributes.clear();
    }

    @Override
    public String getUsername() {
        return username;
    }

    @Override
    public void setUsername(String username) {
        this.username = username;
    }

    @Override
    public boolean isAuthenticated() {
        return authed;
    }

    @Override
    public void setAuthenticated() throws IOException {
        this.authed = true;
        signalSessionEvent(SessionListener.Event.Authenticated);
    }

    /**
     * Checks whether the session has timed out (both authentication and idle timeouts are checked). If the session has
     * timed out, a DISCONNECT message will be sent.
     *
     * @return             An indication whether timeout has been detected
     * @throws IOException If failed to check
     * @see                #checkAuthenticationTimeout(Instant, Duration)
     * @see                #checkIdleTimeout(Instant, Duration)
     */
    protected TimeoutIndicator checkForTimeouts() throws IOException {
        boolean debugEnabled = log.isDebugEnabled();
        if ((!isOpen()) || isClosing() || isClosed()) {
            if (debugEnabled) {
                log.debug("checkForTimeouts({}) session closing", this);
            }
            return TimeoutIndicator.NONE;
        }

        // If already detected a timeout don't check again
        TimeoutIndicator result = timeoutStatus.get();
        TimeoutStatus status = (result == null) ? TimeoutStatus.NoTimeout : result.getStatus();
        if ((status != null) && (status != TimeoutStatus.NoTimeout)) {
            if (debugEnabled) {
                log.debug("checkForTimeouts({}) already detected {}", this, result);
            }
            return result;
        }

        Instant now = Instant.now();
        result = checkAuthenticationTimeout(now, getAuthTimeout());
        if (result == null) {
            result = checkIdleTimeout(now, getIdleTimeout());
        }

        status = (result == null) ? TimeoutStatus.NoTimeout : result.getStatus();
        if ((status == null) || TimeoutStatus.NoTimeout.equals(status)) {
            return TimeoutIndicator.NONE;
        }

        boolean resetTimeout = false;
        try {
            SessionDisconnectHandler handler = getSessionDisconnectHandler();
            resetTimeout = (handler != null) && handler.handleTimeoutDisconnectReason(this, result);
        } catch (RuntimeException | IOException e) {
            // If disconnect handler throws an exception continue with the disconnect
            warn("checkForTimeouts({}) failed ({}) to invoke disconnect handler to handle {}: {}",
                    this, e.getClass().getSimpleName(), result, e.getMessage(), e);
        }

        if (resetTimeout) {
            if (debugEnabled) {
                log.debug("checkForTimeouts({}) cancel {} due to handler intervention", this, result);
            }

            switch (status) {
                case AuthTimeout:
                    resetAuthTimeout();
                    break;
                case IdleTimeout:
                    resetIdleTimeout();
                    break;

                default: // ignored
            }

            return TimeoutIndicator.NONE;
        }

        if (debugEnabled) {
            log.debug("checkForTimeouts({}) disconnect - reason={}", this, result);
        }

        timeoutStatus.set(result);

        disconnect(SshConstants.SSH2_DISCONNECT_PROTOCOL_ERROR,
                "Detected " + status + " after " + result.getExpiredValue()
                                                                + "/" + result.getThresholdValue() + " ms.");
        return result;
    }

    @Override
    public Instant getAuthTimeoutStart() {
        return authStart;
    }

    @Override
    public Instant resetAuthTimeout() {
        Instant value = getAuthTimeoutStart();
        this.authStart = Instant.now();
        return value;
    }

    /**
     * Checks if authentication timeout expired
     *
     * @param  now         The current time in millis
     * @param  authTimeout The configured timeout - if non-positive then no timeout
     * @return             A {@link TimeoutIndicator} specifying the timeout status and disconnect reason message if
     *                     timeout expired, {@code null} or {@code NoTimeout} if no timeout occurred
     * @see                #getAuthTimeout()
     */
    protected TimeoutIndicator checkAuthenticationTimeout(Instant now, Duration authTimeout) {
        Duration d = Duration.between(authStart, now);
        if ((!isAuthenticated()) && GenericUtils.isPositive(authTimeout) && (d.compareTo(authTimeout) > 0)) {
            return new TimeoutIndicator(TimeoutStatus.AuthTimeout, authTimeout, d);
        } else {
            return null;
        }
    }

    @Override
    public Instant getIdleTimeoutStart() {
        return idleStart;
    }

    /**
     * Checks if idle timeout expired
     *
     * @param  now         The current time in millis
     * @param  idleTimeout The configured timeout - if non-positive then no timeout
     * @return             A {@link TimeoutIndicator} specifying the timeout status and disconnect reason message if
     *                     timeout expired, {@code null} or {@code NoTimeout} if no timeout occurred
     * @see                #getIdleTimeout()
     */
    protected TimeoutIndicator checkIdleTimeout(Instant now, Duration idleTimeout) {
        Duration d = Duration.between(idleStart, now);
        if (isAuthenticated() && GenericUtils.isPositive(idleTimeout) && (d.compareTo(idleTimeout) > 0)) {
            return new TimeoutIndicator(TimeoutStatus.IdleTimeout, idleTimeout, d);
        } else {
            return null;
        }
    }

    @Override
    public Instant resetIdleTimeout() {
        Instant value = getIdleTimeoutStart();
        this.idleStart = Instant.now();
        return value;
    }

    @Override
    public TimeoutIndicator getTimeoutStatus() {
        return timeoutStatus.get();
    }

    @Override
    public ReservedSessionMessagesHandler getReservedSessionMessagesHandler() {
        return resolveEffectiveProvider(ReservedSessionMessagesHandler.class,
                reservedSessionMessagesHandler, getFactoryManager().getReservedSessionMessagesHandler());
    }

    @Override
    public void setReservedSessionMessagesHandler(ReservedSessionMessagesHandler handler) {
        reservedSessionMessagesHandler = handler;
    }

    @Override
    public SessionDisconnectHandler getSessionDisconnectHandler() {
        return resolveEffectiveProvider(SessionDisconnectHandler.class,
                sessionDisconnectHandler, getFactoryManager().getSessionDisconnectHandler());
    }

    @Override
    public void setSessionDisconnectHandler(SessionDisconnectHandler sessionDisconnectHandler) {
        this.sessionDisconnectHandler = sessionDisconnectHandler;
    }

    protected void handleIgnore(Buffer buffer) throws Exception {
        // malformed ignore message - ignore (even though we don't have to, but we can be tolerant in this case)
        if (!buffer.isValidMessageStructure(byte[].class)) {
            if (log.isTraceEnabled()) {
                log.trace("handleIgnore({}) ignore malformed message", this);
            }
            return;
        }
        resetIdleTimeout();
        doInvokeIgnoreMessageHandler(buffer);
    }

    /**
     * Invoked by {@link #handleDebug(Buffer)} after validating that the buffer structure seems well-formed and also
     * resetting the idle timeout. By default, retrieves the {@link #resolveReservedSessionMessagesHandler()
     * ReservedSessionMessagesHandler} and invokes its
     * {@link ReservedSessionMessagesHandler#handleIgnoreMessage(Session, Buffer) handleIgnoreMessage} method.
     *
     * @param  buffer    The input {@link Buffer}
     * @throws Exception if failed to handle the message
     */
    protected void doInvokeIgnoreMessageHandler(Buffer buffer) throws Exception {
        ReservedSessionMessagesHandler handler = resolveReservedSessionMessagesHandler();
        handler.handleIgnoreMessage(this, buffer);
    }

    /**
     * Sends a {@code SSH_MSG_UNIMPLEMENTED} message
     *
     * @param  seqNoValue  The referenced sequence number
     * @return             An {@link IoWriteFuture} that can be used to wait for packet write completion
     * @throws IOException if an error occurred sending the packet
     */
    protected IoWriteFuture sendNotImplemented(long seqNoValue) throws IOException {
        Buffer buffer = createBuffer(SshConstants.SSH_MSG_UNIMPLEMENTED, Byte.SIZE);
        buffer.putInt(seqNoValue);
        return writePacket(buffer);
    }

    protected void handleUnimplemented(Buffer buffer) throws Exception {
        if (!buffer.isValidMessageStructure(int.class)) {
            if (log.isTraceEnabled()) {
                log.trace("handleUnimplemented({}) ignore malformed message", this);
            }
            return;
        }
        resetIdleTimeout();
        doInvokeUnimplementedMessageHandler(SshConstants.SSH_MSG_UNIMPLEMENTED, buffer);
    }

    /**
     * @param  cmd       The unimplemented command
     * @param  buffer    The input {@link Buffer}
     * @return           Result of invoking
     *                   {@link ReservedSessionMessagesHandler#handleUnimplementedMessage(Session, int, Buffer)
     *                   handleUnimplementedMessage}
     * @throws Exception if failed to handle the message
     */
    protected boolean doInvokeUnimplementedMessageHandler(int cmd, Buffer buffer) throws Exception {
        ReservedSessionMessagesHandler handler = resolveReservedSessionMessagesHandler();
        return handler.handleUnimplementedMessage(this, cmd, buffer);
    }

    @Override
    public IoWriteFuture sendDebugMessage(boolean display, Object msg, String lang) throws IOException {
        String text = Objects.toString(msg, "");
        lang = (lang == null) ? "" : lang;

        Buffer buffer = createBuffer(SshConstants.SSH_MSG_DEBUG,
                text.length() + lang.length() + Integer.SIZE /* a few extras */);
        buffer.putBoolean(display);
        buffer.putString(text);
        buffer.putString(lang);
        return writePacket(buffer);
    }

    protected void handleDebug(Buffer buffer) throws Exception {
        // malformed ignore message - ignore (even though we don't have to, but we can be tolerant in this case)
        if (!buffer.isValidMessageStructure(boolean.class, String.class, String.class)) {
            if (log.isTraceEnabled()) {
                log.trace("handleDebug({}) ignore malformed message", this);
            }
            return;
        }

        resetIdleTimeout();
        doInvokeDebugMessageHandler(buffer);
    }

    /**
     * Invoked by {@link #handleDebug(Buffer)} after validating that the buffer structure seems well-formed and also
     * resetting the idle timeout. By default, retrieves the {@link #resolveReservedSessionMessagesHandler()
     * ReservedSessionMessagesHandler} and invokes its
     * {@link ReservedSessionMessagesHandler#handleDebugMessage(Session, Buffer) handleDebugMessage} method.
     *
     * @param  buffer    The input {@link Buffer}
     * @throws Exception if failed to handle the message
     */
    protected void doInvokeDebugMessageHandler(Buffer buffer) throws Exception {
        ReservedSessionMessagesHandler handler = resolveReservedSessionMessagesHandler();
        handler.handleDebugMessage(this, buffer);
    }

    protected ReservedSessionMessagesHandler resolveReservedSessionMessagesHandler() {
        ReservedSessionMessagesHandler handler = getReservedSessionMessagesHandler();
        return (handler == null) ? ReservedSessionMessagesHandlerAdapter.DEFAULT : handler;
    }

    @Override
    public UnknownChannelReferenceHandler getUnknownChannelReferenceHandler() {
        return unknownChannelReferenceHandler;
    }

    @Override
    public void setUnknownChannelReferenceHandler(UnknownChannelReferenceHandler unknownChannelReferenceHandler) {
        this.unknownChannelReferenceHandler = unknownChannelReferenceHandler;
    }

    @Override
    public UnknownChannelReferenceHandler resolveUnknownChannelReferenceHandler() {
        UnknownChannelReferenceHandler handler = getUnknownChannelReferenceHandler();
        if (handler != null) {
            return handler;
        }

        FactoryManager mgr = getFactoryManager();
        return (mgr == null) ? null : mgr.resolveUnknownChannelReferenceHandler();
    }

    @Override
    public ChannelStreamWriterResolver getChannelStreamWriterResolver() {
        return channelStreamPacketWriterResolver;
    }

    @Override
    public void setChannelStreamWriterResolver(ChannelStreamWriterResolver resolver) {
        channelStreamPacketWriterResolver = resolver;
    }

    @Override
    public ChannelStreamWriterResolver resolveChannelStreamWriterResolver() {
        ChannelStreamWriterResolver resolver = getChannelStreamWriterResolver();
        if (resolver != null) {
            return resolver;
        }

        ChannelStreamWriterResolverManager manager = getFactoryManager();
        return manager.resolveChannelStreamWriterResolver();
    }

    @Override
    public IoWriteFuture sendIgnoreMessage(byte... data) throws IOException {
        data = (data == null) ? GenericUtils.EMPTY_BYTE_ARRAY : data;
        Buffer buffer = createBuffer(SshConstants.SSH_MSG_IGNORE, data.length + Byte.SIZE);
        buffer.putBytes(data);
        return writePacket(buffer);
    }

    @Override
    public IoWriteFuture writePacket(Buffer buffer, long timeout, TimeUnit unit) throws IOException {
        IoWriteFuture writeFuture = writePacket(buffer);
        @SuppressWarnings("unchecked")
        DefaultSshFuture<IoWriteFuture> future = (DefaultSshFuture<IoWriteFuture>) writeFuture;
        FactoryManager factoryManager = getFactoryManager();
        ScheduledExecutorService executor = factoryManager.getScheduledExecutorService();
        ScheduledFuture<?> sched = executor.schedule(() -> {
            Throwable t = new TimeoutException("Timeout writing packet: " + timeout + " " + unit);
            if (log.isDebugEnabled()) {
                log.debug("writePacket({}): {}", SessionHelper.this, t.getMessage());
            }
            future.setValue(t);
        }, timeout, unit);
        future.addListener(f -> sched.cancel(false));
        return writeFuture;
    }

    protected void signalSessionEstablished(IoSession ioSession) throws Exception {
        try {
            invokeSessionSignaller(l -> {
                signalSessionEstablished(l);
                return null;
            });
        } catch (Throwable err) {
            Throwable e = GenericUtils.peelException(err);
            debug("Failed ({}) to announce session={} established: {}",
                    e.getClass().getSimpleName(), ioSession, e.getMessage(), e);
            if (e instanceof Exception) {
                throw (Exception) e;
            } else {
                throw new RuntimeSshException(e);
            }
        }
    }

    protected void signalSessionEstablished(SessionListener listener) {
        if (listener == null) {
            return;
        }
        listener.sessionEstablished(this);
    }

    protected void signalSessionCreated(IoSession ioSession) throws Exception {
        try {
            invokeSessionSignaller(l -> {
                signalSessionCreated(l);
                return null;
            });
        } catch (Throwable err) {
            Throwable e = GenericUtils.peelException(err);
            debug("Failed ({}) to announce session={} created: {}",
                    e.getClass().getSimpleName(), ioSession, e.getMessage(), e);
            if (e instanceof Exception) {
                throw (Exception) e;
            } else {
                throw new RuntimeSshException(e);
            }
        }
    }

    protected void signalSessionCreated(SessionListener listener) {
        if (listener == null) {
            return;
        }
        listener.sessionCreated(this);
    }

    protected void signalPeerIdentificationReceived(String version, List<String> extraLines) throws Exception {
        try {
            invokeSessionSignaller(l -> {
                signalPeerIdentificationReceived(l, version, extraLines);
                return null;
            });
        } catch (Throwable err) {
            Throwable e = GenericUtils.peelException(err);
            debug("signalPeerIdentificationReceived({}) Failed ({}) to announce peer={}: {}",
                    this, e.getClass().getSimpleName(), version, e.getMessage(), e);
            if (e instanceof Exception) {
                throw (Exception) e;
            } else {
                throw new RuntimeSshException(e);
            }
        }

    }

    protected void signalPeerIdentificationReceived(SessionListener listener, String version, List<String> extraLines) {
        if (listener == null) {
            return;
        }

        listener.sessionPeerIdentificationReceived(this, version, extraLines);
    }

    /**
     * Sends a session event to all currently registered session listeners
     *
     * @param  event       The event to send
     * @throws IOException If any of the registered listeners threw an exception.
     */
    protected void signalSessionEvent(SessionListener.Event event) throws IOException {
        try {
            invokeSessionSignaller(l -> {
                signalSessionEvent(l, event);
                return null;
            });
        } catch (Throwable err) {
            Throwable t = GenericUtils.peelException(err);
            debug("sendSessionEvent({})[{}] failed ({}) to inform listeners: {}",
                    this, event, t.getClass().getSimpleName(), t.getMessage(), t);
            if (t instanceof IOException) {
                throw (IOException) t;
            } else if (t instanceof RuntimeException) {
                throw (RuntimeException) t;
            } else {
                throw new IOException(
                        "Failed (" + t.getClass().getSimpleName() + ") to send session event: " + t.getMessage(), t);
            }
        }
    }

    protected void signalSessionEvent(SessionListener listener, SessionListener.Event event) throws IOException {
        if (listener == null) {
            return;
        }

        listener.sessionEvent(this, event);
    }

    protected void invokeSessionSignaller(Invoker<SessionListener, Void> invoker) throws Throwable {
        FactoryManager manager = getFactoryManager();
        SessionListener[] listeners = {
                (manager == null) ? null : manager.getSessionListenerProxy(),
                getSessionListenerProxy()
        };

        Throwable err = null;
        for (SessionListener l : listeners) {
            if (l == null) {
                continue;
            }
            try {
                invoker.invoke(l);
            } catch (Throwable t) {
                err = GenericUtils.accumulateException(err, t);
            }
        }

        if (err != null) {
            throw err;
        }
    }

    /**
     * Method used while putting new keys into use that will resize the key used to initialize the cipher to the needed
     * length.
     *
     * @param  e         the key to resize
     * @param  kdfSize   the cipher key-derivation-factor (in bytes)
     * @param  hash      the hash algorithm
     * @param  k         the key exchange k parameter
     * @param  h         the key exchange h parameter
     * @return           the resized key
     * @throws Exception if a problem occur while resizing the key
     */
    protected byte[] resizeKey(
            byte[] e, int kdfSize, Digest hash, byte[] k, byte[] h)
            throws Exception {
        for (Buffer buffer = null; kdfSize > e.length; buffer = BufferUtils.clear(buffer)) {
            if (buffer == null) {
                buffer = new ByteArrayBuffer();
            }

            buffer.putMPInt(k);
            buffer.putRawBytes(h);
            buffer.putRawBytes(e);
            hash.update(buffer.array(), 0, buffer.available());

            byte[] foo = hash.digest();
            byte[] bar = new byte[e.length + foo.length];
            System.arraycopy(e, 0, bar, 0, e.length);
            System.arraycopy(foo, 0, bar, e.length, foo.length);
            e = bar;
        }

        return e;
    }

    /**
     * @param  knownAddress Any externally set peer address - e.g., due to some proxy mechanism meta-data
     * @return              The external address if not {@code null} otherwise, the {@code IoSession} peer address
     */
    protected SocketAddress resolvePeerAddress(SocketAddress knownAddress) {
        if (knownAddress != null) {
            return knownAddress;
        }

        IoSession s = getIoSession();
        return (s == null) ? null : s.getRemoteAddress();
    }

    protected long calculateNextIgnorePacketCount(Random r, long freq, int variance) {
        if ((freq <= 0L) || (variance < 0)) {
            return -1L;
        }

        if (variance == 0) {
            return freq;
        }

        int extra = r.random((variance < 0) ? (0 - variance) : variance);
        long count = (variance < 0) ? (freq - extra) : (freq + extra);
        if (log.isTraceEnabled()) {
            log.trace("calculateNextIgnorePacketCount({}) count={}", this, count);
        }

        return count;
    }

    /**
     * Resolves the identification to send to the peer session by consulting the associated {@link FactoryManager}. If a
     * value is set, then it is <U>appended</U> to the standard {@link SessionContext#DEFAULT_SSH_VERSION_PREFIX}.
     * Otherwise a default value is returned consisting of the prefix and the core artifact name + version in
     * <U>uppercase</U> - e.g.,' &quot;SSH-2.0-APACHE-SSHD-1.2.3.4&quot;
     *
     * @param  configPropName The property used to query the factory manager
     * @return                The resolved identification value
     */
    protected String resolveIdentificationString(String configPropName) {
        FactoryManager manager = getFactoryManager();
        String ident = manager.getString(configPropName);
        return SessionContext.DEFAULT_SSH_VERSION_PREFIX + (GenericUtils.isEmpty(ident) ? manager.getVersion() : ident);
    }

    /**
     * Send our identification.
     *
     * @param  ident       our identification to send
     * @return             {@link IoWriteFuture} that can be used to wait for notification that identification has been
     *                     send
     * @throws IOException If failed to send the packet
     */
    protected IoWriteFuture sendIdentification(String ident) throws IOException {
        if (log.isDebugEnabled()) {
            log.debug("sendIdentification({}): {}",
                    this, ident.replace('\r', '|').replace('\n', '|'));
        }

        IoSession networkSession = getIoSession();
        byte[] data = (ident + "\r\n").getBytes(StandardCharsets.UTF_8);
        return networkSession.writeBuffer(new ByteArrayBuffer(data));
    }

    /**
     * Read the remote identification from this buffer. If more data is needed, the buffer will be reset to its original
     * state and a {@code null} value will be returned. Else the identification string will be returned and the data
     * read will be consumed from the buffer.
     *
     * @param  buffer      the buffer containing the identification string
     * @param  server      {@code true} if it is called by the server session, {@code false} if by the client session
     * @return             A {@link List} of all received remote identification lines until the version line was read or
     *                     {@code null} if more data is needed. The identification line is the <U>last</U> one in the
     *                     list
     * @throws IOException if malformed identification found
     */
    protected List<String> doReadIdentification(Buffer buffer, boolean server) throws IOException {
        int maxIdentSize = CoreModuleProperties.MAX_IDENTIFICATION_SIZE.getRequired(this);
        List<String> ident = null;
        int rpos = buffer.rpos();
        boolean debugEnabled = log.isDebugEnabled();
        for (byte[] data = new byte[SessionContext.MAX_VERSION_LINE_LENGTH];;) {
            int pos = 0; // start accumulating line from scratch
            for (boolean needLf = false;;) {
                if (buffer.available() == 0) {
                    // Need more data, so undo reading and return null
                    buffer.rpos(rpos);
                    return null;
                }

                byte b = buffer.getByte();
                /*
                 * According to RFC 4253 section 4.2:
                 *
                 * "The null character MUST NOT be sent"
                 */
                if (b == 0) {
                    throw new StreamCorruptedException(
                            "Incorrect identification (null characters not allowed) - "
                                                       + " at line " + (GenericUtils.size(ident) + 1) + " character #"
                                                       + (pos + 1)
                                                       + " after '" + new String(data, 0, pos, StandardCharsets.UTF_8) + "'");
                }
                if (b == '\r') {
                    needLf = true;
                    continue;
                }

                if (b == '\n') {
                    break;
                }

                if (needLf) {
                    throw new StreamCorruptedException(
                            "Incorrect identification (bad line ending) "
                                                       + " at line " + (GenericUtils.size(ident) + 1)
                                                       + ": " + new String(data, 0, pos, StandardCharsets.UTF_8));
                }

                if (pos >= data.length) {
                    throw new StreamCorruptedException(
                            "Incorrect identification (line too long): "
                                                       + " at line " + (GenericUtils.size(ident) + 1)
                                                       + ": " + new String(data, 0, pos, StandardCharsets.UTF_8));
                }

                data[pos++] = b;
            }

            String str = new String(data, 0, pos, StandardCharsets.UTF_8);
            if (debugEnabled) {
                log.debug("doReadIdentification({}) line='{}'", this, str);
            }

            if (ident == null) {
                ident = new ArrayList<>();
            }
            ident.add(str);

            // if this is a server then only one line is expected from the client
            if (server || str.startsWith("SSH-")) {
                return ident;
            }

            if (buffer.rpos() > maxIdentSize) {
                throw new StreamCorruptedException("Incorrect identification (too many header lines): size > " + maxIdentSize);
            }
        }
    }

    protected String resolveSessionKexProposal(String hostKeyTypes) throws IOException {
        return NamedResource.getNames(
                ValidateUtils.checkNotNullAndNotEmpty(getKeyExchangeFactories(), "No KEX factories"));
    }

    /**
     * Create our proposal for SSH negotiation
     *
     * @param  hostKeyTypes The comma-separated list of supported host key types
     * @return              The proposal {@link Map}
     * @throws IOException  If internal problem - e.g., KEX extensions negotiation issue
     */
    protected Map<KexProposalOption, String> createProposal(String hostKeyTypes) throws IOException {
        Map<KexProposalOption, String> proposal = new EnumMap<>(KexProposalOption.class);
        String kexProposal = resolveSessionKexProposal(hostKeyTypes);
        proposal.put(KexProposalOption.ALGORITHMS, kexProposal);
        proposal.put(KexProposalOption.SERVERKEYS, hostKeyTypes);

        String ciphers = NamedResource.getNames(
                ValidateUtils.checkNotNullAndNotEmpty(getCipherFactories(), "No cipher factories"));
        proposal.put(KexProposalOption.S2CENC, ciphers);
        proposal.put(KexProposalOption.C2SENC, ciphers);

        String macs = NamedResource.getNames(
                ValidateUtils.checkNotNullAndNotEmpty(getMacFactories(), "No MAC factories"));
        proposal.put(KexProposalOption.S2CMAC, macs);
        proposal.put(KexProposalOption.C2SMAC, macs);

        String compressions = NamedResource.getNames(
                ValidateUtils.checkNotNullAndNotEmpty(getCompressionFactories(), "No compression factories"));
        proposal.put(KexProposalOption.S2CCOMP, compressions);
        proposal.put(KexProposalOption.C2SCOMP, compressions);

        proposal.put(KexProposalOption.S2CLANG, ""); // TODO allow configuration
        proposal.put(KexProposalOption.C2SLANG, ""); // TODO allow configuration
        return proposal;
    }

    // returns the proposal argument
    protected Map<KexProposalOption, String> mergeProposals(
            Map<KexProposalOption, String> current, Map<KexProposalOption, String> proposal) {
        // Checking references by design
        if (current == proposal) {
            return proposal; // nothing to merge
        }

        synchronized (current) {
            if (!current.isEmpty()) {
                current.clear(); // debug breakpoint
            }

            if (GenericUtils.isEmpty(proposal)) {
                return proposal; // debug breakpoint
            }

            current.putAll(proposal);
        }

        return proposal;
    }

    protected void signalNegotiationStart(
            Map<KexProposalOption, String> c2sOptions, Map<KexProposalOption, String> s2cOptions) {
        try {
            invokeSessionSignaller(l -> {
                signalNegotiationStart(l, c2sOptions, s2cOptions);
                return null;
            });
        } catch (Throwable err) {
            if (err instanceof RuntimeException) {
                throw (RuntimeException) err;
            } else if (err instanceof Error) {
                throw (Error) err;
            } else {
                throw new RuntimeException(err);
            }
        }
    }

    protected void signalNegotiationStart(
            SessionListener listener, Map<KexProposalOption, String> c2sOptions, Map<KexProposalOption, String> s2cOptions) {
        if (listener == null) {
            return;
        }

        listener.sessionNegotiationStart(this, c2sOptions, s2cOptions);
    }

    protected void signalNegotiationEnd(
            Map<KexProposalOption, String> c2sOptions, Map<KexProposalOption, String> s2cOptions,
            Map<KexProposalOption, String> negotiatedGuess, Throwable reason) {
        try {
            invokeSessionSignaller(l -> {
                signalNegotiationEnd(l, c2sOptions, s2cOptions, negotiatedGuess, reason);
                return null;
            });
        } catch (Throwable err) {
            if (err instanceof RuntimeException) {
                throw (RuntimeException) err;
            } else if (err instanceof Error) {
                throw (Error) err;
            } else {
                throw new RuntimeException(err);
            }
        }
    }

    protected void signalNegotiationEnd(
            SessionListener listener,
            Map<KexProposalOption, String> c2sOptions, Map<KexProposalOption, String> s2cOptions,
            Map<KexProposalOption, String> negotiatedGuess, Throwable reason) {
        if (listener == null) {
            return;
        }

        listener.sessionNegotiationEnd(this, c2sOptions, s2cOptions, negotiatedGuess, null);
    }

    /**
     * Invoked by the session before encoding the buffer in order to make sure that it is at least of size
     * {@link SshConstants#SSH_PACKET_HEADER_LEN SSH_PACKET_HEADER_LEN}. This is required in order to efficiently handle
     * the encoding. If necessary, it re-allocates a new buffer and returns it instead.
     *
     * @param  cmd         The command stored in the buffer
     * @param  buffer      The original {@link Buffer} - assumed to be properly formatted and be of at least the
     *                     required minimum length.
     * @return             The adjusted {@link Buffer}. <B>Note:</B> users may use this method to totally alter the
     *                     contents of the buffer being sent but it is highly discouraged as it may have unexpected
     *                     results.
     * @throws IOException If failed to process the buffer
     */
    protected Buffer preProcessEncodeBuffer(int cmd, Buffer buffer) throws IOException {
        int curPos = buffer.rpos();
        if (curPos >= SshConstants.SSH_PACKET_HEADER_LEN) {
            return buffer;
        }

        log.warn("preProcessEncodeBuffer({}) command={}[{}] performance cost:"
                 + " available buffer packet header length ({}) below min. required ({})",
                this, cmd, SshConstants.getCommandMessageName(cmd),
                curPos, SshConstants.SSH_PACKET_HEADER_LEN);
        Buffer nb = new ByteArrayBuffer(buffer.available() + Long.SIZE, false);
        nb.wpos(SshConstants.SSH_PACKET_HEADER_LEN);
        nb.putBuffer(buffer);
        return nb;
    }

    @Override
    public void disconnect(int reason, String msg) throws IOException {
        log.info("Disconnecting({}): {} - {}",
                this, SshConstants.getDisconnectReasonName(reason), msg);
        String languageTag = ""; // TODO configure language...
        signalDisconnect(reason, msg, languageTag, true);

        Buffer buffer = createBuffer(SshConstants.SSH_MSG_DISCONNECT, msg.length() + Short.SIZE);
        buffer.putInt(reason);
        buffer.putString(msg);
        buffer.putString("");

        // Write the packet with a timeout to ensure a timely close of the session
        // in case the consumer does not read packets anymore.
        Duration disconnectTimeout = CoreModuleProperties.DISCONNECT_TIMEOUT.getRequired(this);
        IoWriteFuture packetFuture = writePacket(buffer, disconnectTimeout);
        packetFuture.addListener(future -> {
            Throwable t = future.getException();
            boolean debugEnabled = log.isDebugEnabled();
            if (t == null) {
                if (debugEnabled) {
                    log.debug("disconnect({}) operation successfully completed for reason={} [{}]",
                            SessionHelper.this, SshConstants.getDisconnectReasonName(reason), msg);
                }
            } else {
                if (debugEnabled) {
                    debug("disconnect({}) operation failed ({}) for reason={} [{}]: {}",
                            SessionHelper.this, t.getClass().getSimpleName(),
                            SshConstants.getDisconnectReasonName(reason), msg, t.getMessage(), t);
                }
            }

            close(true);
        });
    }

    protected void handleDisconnect(Buffer buffer) throws Exception {
        int code = buffer.getInt();
        String message = buffer.getString();
        String languageTag;
        // SSHD-738: avoid spamming the log with uninteresting
        // messages caused by buggy OpenSSH < 5.5
        if (buffer.available() > 0) {
            languageTag = buffer.getString();
        } else {
            languageTag = "";
        }
        handleDisconnect(code, message, languageTag, buffer);
    }

    protected void handleDisconnect(int code, String msg, String lang, Buffer buffer) throws Exception {
        if (log.isDebugEnabled()) {
            log.debug("handleDisconnect({}) SSH_MSG_DISCONNECT reason={}, [lang={}] msg={}",
                    this, SshConstants.getDisconnectReasonName(code), lang, msg);
        }

        signalDisconnect(code, msg, lang, false);
        close(true);
    }

    protected void signalDisconnect(int code, String msg, String lang, boolean initiator) {
        try {
            invokeSessionSignaller(l -> {
                signalDisconnect(l, code, msg, lang, initiator);
                return null;
            });
        } catch (Throwable err) {
            Throwable e = GenericUtils.peelException(err);
            debug("signalDisconnect({}) {}: {}",
                    this, e.getClass().getSimpleName(), e.getMessage(), e);
        }
    }

    protected void signalDisconnect(
            SessionListener listener, int code, String msg, String lang, boolean initiator) {
        if (listener == null) {
            return;
        }

        listener.sessionDisconnect(this, code, msg, lang, initiator);
    }

    /**
     * Handle any exceptions that occurred on this session. The session will be closed and a disconnect packet will be
     * sent before if the given exception is an {@link SshException}.
     *
     * @param t the exception to process
     */
    @Override
    public void exceptionCaught(Throwable t) {
        State curState = state.get();
        // Ignore exceptions that happen while closing immediately
        if ((!State.Opened.equals(curState)) && (!State.Graceful.equals(curState))) {
            debug("exceptionCaught({}) ignore {} due to state={}, message='{}'",
                    this, t.getClass().getSimpleName(), curState, t.getMessage(), t);
            return;
        }

        warn("exceptionCaught({})[state={}] {}: {}",
                this, curState, t.getClass().getSimpleName(), t.getMessage(), t);

        signalExceptionCaught(t);

        if (State.Opened.equals(curState) && (t instanceof SshException)) {
            int code = ((SshException) t).getDisconnectCode();
            if (code > 0) {
                try {
                    disconnect(code, t.getMessage());
                } catch (Throwable t2) {
                    debug("exceptionCaught({}) {} while disconnect with code={}: {}",
                            this, t2.getClass().getSimpleName(), SshConstants.getDisconnectReasonName(code), t2.getMessage(),
                            t2);
                }
                return;
            }
        }

        close(true);
    }

    protected void signalExceptionCaught(Throwable t) {
        try {
            invokeSessionSignaller(l -> {
                signalExceptionCaught(l, t);
                return null;
            });
        } catch (Throwable err) {
            Throwable e = GenericUtils.peelException(err);
            debug("signalExceptionCaught({}) {}: {}",
                    this, e.getClass().getSimpleName(), e.getMessage(), e);
        }
    }

    protected void signalExceptionCaught(SessionListener listener, Throwable t) {
        if (listener == null) {
            return;
        }

        listener.sessionException(this, t);
    }

    protected void signalSessionClosed() {
        try {
            invokeSessionSignaller(l -> {
                signalSessionClosed(l);
                return null;
            });
        } catch (Throwable err) {
            Throwable e = GenericUtils.peelException(err);
            debug("signalSessionClosed({}) {} while signal session closed: {}",
                    this, e.getClass().getSimpleName(), e.getMessage(), e);
        }
    }

    protected void signalSessionClosed(SessionListener listener) {
        if (listener == null) {
            return;
        }

        listener.sessionClosed(this);
    }

    protected abstract ConnectionService getConnectionService();

    protected Forwarder getForwarder() {
        ConnectionService service = getConnectionService();
        return (service == null) ? null : service.getForwarder();
    }

    @Override
    public List<Map.Entry<SshdSocketAddress, SshdSocketAddress>> getLocalForwardsBindings() {
        Forwarder forwarder = getForwarder();
        return (forwarder == null) ? Collections.emptyList() : forwarder.getLocalForwardsBindings();
    }

    @Override
    public boolean isLocalPortForwardingStartedForPort(int port) {
        Forwarder forwarder = getForwarder();
        return (forwarder != null) && forwarder.isLocalPortForwardingStartedForPort(port);
    }

    @Override
    public List<SshdSocketAddress> getStartedLocalPortForwards() {
        Forwarder forwarder = getForwarder();
        return (forwarder == null) ? Collections.emptyList() : forwarder.getStartedLocalPortForwards();
    }

    @Override
    public List<SshdSocketAddress> getBoundLocalPortForwards(int port) {
        Forwarder forwarder = getForwarder();
        return (forwarder == null) ? Collections.emptyList() : forwarder.getBoundLocalPortForwards(port);
    }

    @Override
    public List<Map.Entry<Integer, SshdSocketAddress>> getRemoteForwardsBindings() {
        Forwarder forwarder = getForwarder();
        return (forwarder == null) ? Collections.emptyList() : forwarder.getRemoteForwardsBindings();
    }

    @Override
    public boolean isRemotePortForwardingStartedForPort(int port) {
        Forwarder forwarder = getForwarder();
        return (forwarder != null) && forwarder.isRemotePortForwardingStartedForPort(port);
    }

    @Override
    public NavigableSet<Integer> getStartedRemotePortForwards() {
        Forwarder forwarder = getForwarder();
        return (forwarder == null) ? Collections.emptyNavigableSet() : forwarder.getStartedRemotePortForwards();
    }

    @Override
    public SshdSocketAddress getBoundRemotePortForward(int port) {
        Forwarder forwarder = getForwarder();
        return (forwarder == null) ? null : forwarder.getBoundRemotePortForward(port);
    }

    @Override
    public Duration getAuthTimeout() {
        return CoreModuleProperties.AUTH_TIMEOUT.getRequired(this);
    }

    @Override
    public Duration getIdleTimeout() {
        return CoreModuleProperties.IDLE_TIMEOUT.getRequired(this);
    }

    @Override
    public String toString() {
        IoSession networkSession = getIoSession();
        SocketAddress peerAddress = (networkSession == null) ? null : networkSession.getRemoteAddress();
        return getClass().getSimpleName() + "[" + getUsername() + "@" + peerAddress + "]";
    }
}
