JSch: Fix NPE if unsupported cipher specified

More specifically, fail gracefully rather than throw a
NullPointerException if a cipher algorithm supported by the SSH server
but not the SSH client was specified using the Ciphers OpenSSH config
file keyword.

Fixes #441

ChangeLog.md

@@ -69,6 +69,11 @@ Viewer now advertises support for the pseudo-encoding.  (The next major release
 of the TurboVNC Server will forego using the extension unless the VNC viewer
 advertises support for it.)
 
+14. Fixed an issue whereby the TurboVNC Viewer's built-in SSH client threw a
+NullPointerException if a cipher algorithm supported by the SSH server but not
+by the SSH client was specified using the `Ciphers` OpenSSH config file
+keyword.
+
 
 2.2.9 ESR
 =========

java/com/jcraft/jsch/Session.java

@@ -1,7 +1,7 @@
 /* -*-mode:java; c-basic-offset:2; indent-tabs-mode:nil -*- */
 /*
 Copyright (c) 2002-2016 ymnk, JCraft,Inc. All rights reserved.
-Copyright (c) 2018 D. R. Commander. All rights reserved.
+Copyright (c) 2018, 2025 D. R. Commander. All rights reserved.
 
 Redistribution and use in source and binary forms, with or without
 modification, are permitted provided that the following conditions are met:
@@ -1153,6 +1153,8 @@ private void updateKeys(KeyExchange kex) throws Exception{
       String method;
 
       method=guess[KeyExchange.PROPOSAL_ENC_ALGS_STOC];
+      if(getConfig(method)==null)
+        throw new JSchException("Unsupported cipher "+method);
       c=Class.forName(getConfig(method));
       s2ccipher=(Cipher)(c.getDeclaredConstructor().newInstance());
       while(s2ccipher.getBlockSize()>Es2c.length){
@@ -1171,6 +1173,8 @@ private void updateKeys(KeyExchange kex) throws Exception{
       s2ccipher_size=s2ccipher.getIVSize();
 
       method=guess[KeyExchange.PROPOSAL_MAC_ALGS_STOC];
+      if(getConfig(method)==null)
+        throw new JSchException("Unsupported cipher "+method);
       c=Class.forName(getConfig(method));
       s2cmac=(MAC)(c.getDeclaredConstructor().newInstance());
       MACs2c = expandKey(buf, K, H, MACs2c, hash, s2cmac.getBlockSize());
@@ -1180,6 +1184,8 @@ private void updateKeys(KeyExchange kex) throws Exception{
       s2cmac_result2=new byte[s2cmac.getBlockSize()];
 
       method=guess[KeyExchange.PROPOSAL_ENC_ALGS_CTOS];
+      if(getConfig(method)==null)
+        throw new JSchException("Unsupported cipher "+method);
       c=Class.forName(getConfig(method));
       c2scipher=(Cipher)(c.getDeclaredConstructor().newInstance());
       while(c2scipher.getBlockSize()>Ec2s.length){
@@ -1198,6 +1204,8 @@ private void updateKeys(KeyExchange kex) throws Exception{
       c2scipher_size=c2scipher.getIVSize();
 
       method=guess[KeyExchange.PROPOSAL_MAC_ALGS_CTOS];
+      if(getConfig(method)==null)
+        throw new JSchException("Unsupported cipher "+method);
       c=Class.forName(getConfig(method));
       c2smac=(MAC)(c.getDeclaredConstructor().newInstance());
       MACc2s = expandKey(buf, K, H, MACc2s, hash, c2smac.getBlockSize());