JSch: Fix detection of supported RSA sig schemes

KeyExchange.guess() only returns the first server/client match for each
category, and the client algorithms are taken from the HostKeyAlgorithms
OpenSSH config file keyword rather than the PubkeyAcceptedAlgorithms
keyword.  Thus, fd34df2 effectively
made it so that an RSA signature scheme could only be used if it was
the first server-supported algorithm listed with the HostKeyAlgorithms
keyword.

Instead, set Session.supportedRSAMethods to the list of RSA signature
schemes that the server supports, and attempt to use the first one of
those algorithms that is specified with PubkeyAcceptedAlgorithms.  This
fulfills the intent of fd34df2 and
emulates the behavior of OpenSSH.

ChangeLog.md

@@ -20,6 +20,11 @@ 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.)
 
+5. Fixed an issue in the TurboVNC Viewer's built-in SSH client whereby the
+`ssh-rsa` and `rsa-sha2-512` signature schemes could not be used unless they
+were specified as the first argument for the `HostKeyAlgorithms` OpenSSH config
+file keyword or the server did not support `rsa-sha2-256`.
+
 
 3.1.2
 =====

java/com/jcraft/jsch/KeyExchange.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 D. R. Commander. All rights reserved.
+Copyright (c) 2018, 2024 D. R. Commander. All rights reserved.
 Copyright (c) 2020 Jeremy Norris. All rights reserved.
 
 Redistribution and use in source and binary forms, with or without
@@ -31,6 +31,8 @@ LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
 
 package com.jcraft.jsch;
 
+import java.util.Vector;
+
 public abstract class KeyExchange{
 
   static final int PROPOSAL_KEX_ALGS=0;
@@ -91,7 +93,7 @@ public String getKeyAlgorithName() {
     return key_alg_name;
   }
 
-  protected static String[] guess(byte[]I_S, byte[]I_C){
+  protected static String[] guess(Session session, byte[]I_S, byte[]I_C){
     String[] guess=new String[PROPOSAL_MAX];
     Buffer sb=new Buffer(I_S); sb.setOffSet(17);
     Buffer cb=new Buffer(I_C); cb.setOffSet(17);
@@ -115,6 +117,17 @@ protected static String[] guess(byte[]I_S, byte[]I_C){
       int j=0;
       int k=0;
 
+      if(i==PROPOSAL_SERVER_HOST_KEY_ALGS) {
+        String smethods=new String(sp);
+
+        if(smethods.matches("(^|.*,)ssh-rsa(,.*|$)"))
+          session.supportedRSAMethods.addElement("ssh-rsa");
+        if(smethods.matches("(^|.*,)rsa-sha2-256(,.*|$)"))
+          session.supportedRSAMethods.addElement("rsa-sha2-256");
+        if(smethods.matches("(^|.*,)rsa-sha2-512(,.*|$)"))
+          session.supportedRSAMethods.addElement("rsa-sha2-512");
+      }
+
       loop:
       while(j<cp.length){
 	while(j<cp.length && cp[j]!=',')j++; 

java/com/jcraft/jsch/Session.java

@@ -588,15 +588,10 @@ private KeyExchange receive_kexinit(Buffer buf) throws Exception {
      send_kexinit();
    }
 
-    guess=KeyExchange.guess(I_S, I_C);
+    guess=KeyExchange.guess(this, I_S, I_C);
     if(guess==null){
       throw new JSchException("Algorithm negotiation fail");
     }
-    for(String method : guess){
-      if(method.equals("ssh-rsa") || method.equals("rsa-sha2-256") || method.equals("rsa-sha2-512")){
-        supportedRSAMethods.addElement(method);
-      }
-    }
 
     if(!isAuthed &&
        (guess[KeyExchange.PROPOSAL_ENC_ALGS_CTOS].equals("none") ||

java/com/jcraft/jsch/UserAuthPublicKey.java

@@ -3,7 +3,7 @@
 Copyright (c) 2002-2018 ymnk, JCraft,Inc. All rights reserved.
 Copyright (c) 2020 Jeremy Norris. All rights reserved.
 Copyright (c) 2020-2021 Matthias Wiedemann. All rights reserved.
-Copyright (c) 2023 D. R. Commander. All rights reserved.
+Copyright (c) 2023-2024 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:
@@ -56,23 +56,20 @@ public boolean start(Session session) throws Exception{
         return false;
       }
 
-      String rsamethods=null;
+      String rsamethod=null;
       String nonrsamethods=null;
       for(int i=0; i<pkmethoda.length; i++){
         if(pkmethoda[i].equals("ssh-rsa") || pkmethoda[i].equals("rsa-sha2-256") || pkmethoda[i].equals("rsa-sha2-512")){
-          if(session.supportedRSAMethods.contains(pkmethoda[i])){
-            if(rsamethods==null) rsamethods=pkmethoda[i];
-            else rsamethods+=","+pkmethoda[i];
-          }
+          if(session.supportedRSAMethods.contains(pkmethoda[i]) &&
+             rsamethod==null)
+            rsamethod=pkmethoda[i];
         }
         else{
           if(nonrsamethods==null) nonrsamethods=pkmethoda[i];
           else nonrsamethods+=","+pkmethoda[i];
         }
       }
-      String[] rsamethoda=Util.split(rsamethods, ",");
       String[] nonrsamethoda=Util.split(nonrsamethods, ",");
-
       _username=Util.str2byte(username);
 
       iloop:
@@ -97,8 +94,8 @@ public boolean start(Session session) throws Exception{
 
         String ipkmethod=identity.getAlgName();
         String[] ipkmethoda=null;
-        if(ipkmethod.equals("ssh-rsa")){
-          ipkmethoda=rsamethoda;
+        if(ipkmethod.equals("ssh-rsa") && rsamethod!=null){
+          ipkmethoda=new String[]{rsamethod};
         }
         else if(nonrsamethoda!=null && nonrsamethoda.length>0){
           for(int j=0; j<nonrsamethoda.length; j++){