From b632a9ce0f0f64923e10f2795e3f3abf68a75e5e Mon Sep 17 00:00:00 2001
From: DRC <information@virtualgl.org>
Date: Tue, 15 Oct 2024 17:10:56 -0400
Subject: [PATCH] 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, fd34df2ca25f99288f0569511b68f107c289e7b1 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 fd34df2ca25f99288f0569511b68f107c289e7b1 and
emulates the behavior of OpenSSH.
---
 ChangeLog.md                                |  5 +++++
 java/com/jcraft/jsch/KeyExchange.java       | 17 +++++++++++++++--
 java/com/jcraft/jsch/Session.java           |  7 +------
 java/com/jcraft/jsch/UserAuthPublicKey.java | 17 +++++++----------
 4 files changed, 28 insertions(+), 18 deletions(-)

diff --git a/ChangeLog.md b/ChangeLog.md
index 2eb4dbb44..d975706ba 100644
--- a/ChangeLog.md
+++ b/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
 =====
diff --git a/java/com/jcraft/jsch/KeyExchange.java b/java/com/jcraft/jsch/KeyExchange.java
index 6334ef8df..9062f8d14 100644
--- a/java/com/jcraft/jsch/KeyExchange.java
+++ b/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++; 
diff --git a/java/com/jcraft/jsch/Session.java b/java/com/jcraft/jsch/Session.java
index eb3e9b71a..1fa9cd299 100644
--- a/java/com/jcraft/jsch/Session.java
+++ b/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") ||
diff --git a/java/com/jcraft/jsch/UserAuthPublicKey.java b/java/com/jcraft/jsch/UserAuthPublicKey.java
index b3c908f3e..4b65cce23 100644
--- a/java/com/jcraft/jsch/UserAuthPublicKey.java
+++ b/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++){