Ensure Jsch checks all configured algorithms

Jsch checks only for the availability of the algorithms given by
Jsch-internal config keys "CheckCiphers", "CheckKexes", and
"CheckSignatures". If the ssh config defines any algorithms
unknown to Jsch not listed in those keys, it'll still propose them
during the negotiation phase, and run into an NPE later on if the
server happens to propose such an algorithm and it gets chosen.

Jsch reads those "CheckCiphers" and the other values from either a
session-local config, or the global static Jsch config. It bypasses
~/.ssh/config for these values.

Therefore, copy these values from the config as read from
~/.ssh/config into the session-specific config. That makes Jsch
check _all_ configured algorithms up front, discarding any for
which it has no implementation. Thus it proposes only algorithms
it actually can handle.

Bug: 535672
Change-Id: I6a68e54f4d9a3267e895c536bcf3c58099826ad5
Signed-off-by: Thomas Wolf <thomas.wolf@paranor.ch>

org.eclipse.jgit/src/org/eclipse/jgit/transport/JschConfigSessionFactory.java

@@ -70,6 +70,7 @@ import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
 import com.jcraft.jsch.ConfigRepository;
+import com.jcraft.jsch.ConfigRepository.Config;
 import com.jcraft.jsch.JSch;
 import com.jcraft.jsch.JSchException;
 import com.jcraft.jsch.Session;
@@ -222,10 +223,30 @@ public abstract class JschConfigSessionFactory extends SshSessionFactory {
 			session.setUserInfo(new CredentialsProviderUserInfo(session,
 					credentialsProvider));
 		}
+		safeConfig(session, hc.getConfig());
 		configure(hc, session);
 		return session;
 	}
 
+	private void safeConfig(Session session, Config cfg) {
+		// Ensure that Jsch checks all configured algorithms, not just its
+		// built-in ones. Otherwise it may propose an algorithm for which it
+		// doesn't have an implementation, and then run into an NPE if that
+		// algorithm ends up being chosen.
+		copyConfigValueToSession(session, cfg, "Ciphers", "CheckCiphers"); //$NON-NLS-1$ //$NON-NLS-2$
+		copyConfigValueToSession(session, cfg, "KexAlgorithms", "CheckKexes"); //$NON-NLS-1$ //$NON-NLS-2$
+		copyConfigValueToSession(session, cfg, "HostKeyAlgorithms", //$NON-NLS-1$
+				"CheckSignatures"); //$NON-NLS-1$
+	}
+
+	private void copyConfigValueToSession(Session session, Config cfg,
+			String from, String to) {
+		String value = cfg.getValue(from);
+		if (value != null) {
+			session.setConfig(to, value);
+		}
+	}
+
 	private void setUserName(Session session, String userName) {
 		// Jsch 0.1.54 picks up the user name from the ssh config, even if an
 		// explicit user name was given! We must correct that if ~/.ssh/config