Add option for client to verify negotiated key exchange algorithms.

src/main/java/net/schmizz/sshj/SSHClient.java

@@ -39,6 +39,7 @@
 import net.schmizz.sshj.transport.compression.DelayedZlibCompression;
 import net.schmizz.sshj.transport.compression.NoneCompression;
 import net.schmizz.sshj.transport.compression.ZlibCompression;
+import net.schmizz.sshj.transport.verification.AlgorithmsVerifier;
 import net.schmizz.sshj.transport.verification.HostKeyVerifier;
 import net.schmizz.sshj.transport.verification.OpenSSHKnownHosts;
 import net.schmizz.sshj.userauth.UserAuth;
@@ -158,10 +159,19 @@ public SSHClient(Config config) {
      * Add a {@link HostKeyVerifier} which will be invoked for verifying host key during connection establishment and
      * future key exchanges.
      *
-     * @param hostKeyVerifier {@link HostKeyVerifier} instance
+     * @param verifier {@link HostKeyVerifier} instance
      */
-    public void addHostKeyVerifier(HostKeyVerifier hostKeyVerifier) {
-        trans.addHostKeyVerifier(hostKeyVerifier);
+    public void addHostKeyVerifier(HostKeyVerifier verifier) {
+        trans.addHostKeyVerifier(verifier);
+    }
+
+    /**
+     * Add a {@link AlgorithmsVerifier} which will be invoked for verifying negotiated algorithms.
+     *
+     * @param verifier {@link AlgorithmsVerifier} instance
+     */
+    public void addAlgorithmsVerifier(AlgorithmsVerifier verifier) {
+        trans.addAlgorithmsVerifier(verifier);
     }
 
     /**

src/main/java/net/schmizz/sshj/transport/KeyExchanger.java

@@ -33,6 +33,8 @@
 import net.schmizz.sshj.transport.kex.KeyExchange;
 import net.schmizz.sshj.transport.mac.MAC;
 import net.schmizz.sshj.transport.verification.HostKeyVerifier;
+import net.schmizz.sshj.transport.verification.AlgorithmsVerifier;
+
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
@@ -68,6 +70,8 @@ private static enum Expected {
      */
     private final Queue<HostKeyVerifier> hostVerifiers = new LinkedList<HostKeyVerifier>();
 
+    private final Queue<AlgorithmsVerifier> algorithmVerifiers = new LinkedList<AlgorithmsVerifier>();
+
     private final AtomicBoolean kexOngoing = new AtomicBoolean();
 
     /** What we are expecting from the next packet */
@@ -108,6 +112,10 @@ synchronized void addHostKeyVerifier(HostKeyVerifier hkv) {
         hostVerifiers.add(hkv);
     }
 
+    synchronized void addAlgorithmsVerifier(AlgorithmsVerifier verifier) {
+        algorithmVerifiers.add(verifier);
+    }
+
     /**
      * Returns the session identifier computed during key exchange.
      *
@@ -218,6 +226,13 @@ private void gotKexInit(SSHPacket buf)
         final Proposal serverProposal = new Proposal(buf);
         negotiatedAlgs = clientProposal.negotiate(serverProposal);
         log.debug("Negotiated algorithms: {}", negotiatedAlgs);
+        for(AlgorithmsVerifier v: algorithmVerifiers) {
+            log.debug("Trying to verify algorithms with {}", v);
+            if(!v.verify(negotiatedAlgs)) {
+                throw new TransportException(DisconnectReason.KEY_EXCHANGE_FAILED,
+                        "Failed to verify negotiated algorithms `" + negotiatedAlgs + "`");
+            }
+        }
         kex = Factory.Named.Util.create(transport.getConfig().getKeyExchangeFactories(),
                                         negotiatedAlgs.getKeyExchangeAlgorithm());
         try {

src/main/java/net/schmizz/sshj/transport/Transport.java

@@ -20,6 +20,7 @@
 import net.schmizz.sshj.common.DisconnectReason;
 import net.schmizz.sshj.common.SSHPacket;
 import net.schmizz.sshj.common.SSHPacketHandler;
+import net.schmizz.sshj.transport.verification.AlgorithmsVerifier;
 import net.schmizz.sshj.transport.verification.HostKeyVerifier;
 
 import java.io.InputStream;
@@ -51,6 +52,13 @@ void init(String host, int port, InputStream in, OutputStream out)
      */
     void addHostKeyVerifier(HostKeyVerifier hkv);
 
+    /**
+     * Adds the specified verifier.
+     *
+     * @param verifier The verifier to call with negotiated algorithms
+     */
+    void addAlgorithmsVerifier(AlgorithmsVerifier verifier);
+
     /**
      * Do key exchange and algorithm negotiation. This can be the initial one or for algorithm renegotiation.
      *

src/main/java/net/schmizz/sshj/transport/TransportImpl.java

@@ -27,6 +27,7 @@
 import net.schmizz.sshj.common.Message;
 import net.schmizz.sshj.common.SSHException;
 import net.schmizz.sshj.common.SSHPacket;
+import net.schmizz.sshj.transport.verification.AlgorithmsVerifier;
 import net.schmizz.sshj.transport.verification.HostKeyVerifier;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
@@ -235,6 +236,11 @@ public void addHostKeyVerifier(HostKeyVerifier hkv) {
         kexer.addHostKeyVerifier(hkv);
     }
 
+    @Override
+    public void addAlgorithmsVerifier(AlgorithmsVerifier verifier) {
+        kexer.addAlgorithmsVerifier(verifier);
+    }
+
     @Override
     public void doKex()
             throws TransportException {

src/main/java/net/schmizz/sshj/transport/verification/AlgorithmsVerifier.java

@@ -0,0 +1,27 @@
+/**
+ * Copyright 2009 sshj contributors
+ *
+ * Licensed 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 net.schmizz.sshj.transport.verification;
+
+import net.schmizz.sshj.transport.NegotiatedAlgorithms;
+
+public interface AlgorithmsVerifier {
+
+    /**
+     * Callback is invoked when algorithms have been negotiated between client and server.
+     * @return False to interrupt the connection
+     */
+    boolean verify(NegotiatedAlgorithms algorithms);
+}