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

@@ -115,6 +115,11 @@ in the system settings.
 23. The Windows TurboVNC Viewer now sends Alt-F4 keystrokes to the VNC server
 if keyboard grabbing is enabled.
 
+24. 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.
+
 
 3.0.3
 =====

java/com/jcraft/jsch/Session.java

@@ -1,7 +1,7 @@
 /* -*-mode:java; c-basic-offset:2; indent-tabs-mode:nil -*- */
 /*
 Copyright (c) 2002-2018 ymnk, JCraft,Inc. All rights reserved.
-Copyright (c) 2018, 2023-2024 D. R. Commander. All rights reserved.
+Copyright (c) 2018, 2023-2025 D. R. Commander. All rights reserved.
 Copyright (c) 2020-2021 Jeremy Norris. All rights reserved.
 
 Redistribution and use in source and binary forms, with or without
@@ -1155,6 +1155,8 @@ 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){
@@ -1173,6 +1175,8 @@ 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());
@@ -1182,6 +1186,8 @@ 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){
@@ -1200,6 +1206,8 @@ 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());