[sshd] Implement SSH config KexAlgorithms

Make the used KEX algorithms configurable via the ssh config. Also
implement adding algorithms not in the default set: since sshd 2.6.0
deprecated SHA1-based algorithms, it is possible that the default set
has not all available algorithms, so adding algorithms makes sense.

This enables users who have to use a git server that only supports
old SHA1-based key exchange methods to enable those methods in the
ssh config:

  KexAlgorithms +diffie-hellman-group1-sha1

There are two more SHA1 algorithms that are not enabled by default:
diffie-hellman-group14-sha1 and diffie-hellman-group-exchange-sha1.
KeyAlgorithms accepts a comma-separated list of algorithm names.

Since adding algorithms is now supported, adapt the handling of
signature algorithms, too. Make sure that definitions for the KEX
exchange signature (HostKeyAlgorithms) don't conflict with the
definition for signatures for pubkey auth (PubkeyAcceptedAlgorithms).

HostKeyAlgorithms updates the signature factories set on the session
to include the default factories plus any that might have been added
via the SSH config. Move the handling of PubkeyAcceptedAlgorithms
from the client to the JGitPubkeyAuthentication, where it can be done
only if pubkey auth is attempted at all and where it can store its
adapted list of factories locally.

Bug: 574636
Change-Id: Ia5d5f174bbc8e5b41e10ec2c25216d861174e7c3
Signed-off-by: Thomas Wolf <thomas.wolf@paranor.ch>

2 files changed, 44 insertions, 0 deletions

diff --git a/org.eclipse.jgit.ssh.apache.test/META-INF/MANIFEST.MF b/org.eclipse.jgit.ssh.apache.test/META-INF/MANIFEST.MF
index ddb475dc13..1ac8faaa41 100644
--- a/org.eclipse.jgit.ssh.apache.test/META-INF/MANIFEST.MF
+++ b/org.eclipse.jgit.ssh.apache.test/META-INF/MANIFEST.MF
@@ -12,6 +12,7 @@ Import-Package: org.apache.sshd.client.config.hosts;version="[2.7.0,2.8.0)",
  org.apache.sshd.common.auth;version="[2.7.0,2.8.0)",
  org.apache.sshd.common.config.keys;version="[2.7.0,2.8.0)",
  org.apache.sshd.common.helpers;version="[2.7.0,2.8.0)",
+ org.apache.sshd.common.kex;version="[2.7.0,2.8.0)",
  org.apache.sshd.common.keyprovider;version="[2.7.0,2.8.0)",
  org.apache.sshd.common.session;version="[2.7.0,2.8.0)",
  org.apache.sshd.common.signature;version="[2.7.0,2.8.0)",
diff --git a/org.eclipse.jgit.ssh.apache.test/tst/org/eclipse/jgit/transport/sshd/ApacheSshTest.java b/org.eclipse.jgit.ssh.apache.test/tst/org/eclipse/jgit/transport/sshd/ApacheSshTest.java
index c56d2307c6..c1f5fef3cd 100644
--- a/org.eclipse.jgit.ssh.apache.test/tst/org/eclipse/jgit/transport/sshd/ApacheSshTest.java
+++ b/org.eclipse.jgit.ssh.apache.test/tst/org/eclipse/jgit/transport/sshd/ApacheSshTest.java
@@ -34,13 +34,18 @@ import java.util.stream.Collectors;
 
 import org.apache.sshd.client.config.hosts.KnownHostEntry;
 import org.apache.sshd.client.config.hosts.KnownHostHashValue;
+import org.apache.sshd.common.NamedFactory;
 import org.apache.sshd.common.config.keys.AuthorizedKeyEntry;
 import org.apache.sshd.common.config.keys.KeyUtils;
 import org.apache.sshd.common.config.keys.PublicKeyEntry;
 import org.apache.sshd.common.config.keys.PublicKeyEntryResolver;
+import org.apache.sshd.common.kex.BuiltinDHFactories;
+import org.apache.sshd.common.kex.DHFactory;
+import org.apache.sshd.common.kex.KeyExchangeFactory;
 import org.apache.sshd.common.session.Session;
 import org.apache.sshd.common.util.net.SshdSocketAddress;
 import org.apache.sshd.server.ServerAuthenticationManager;
+import org.apache.sshd.server.ServerBuilder;
 import org.apache.sshd.server.SshServer;
 import org.apache.sshd.server.forward.StaticDecisionForwardingFilter;
 import org.eclipse.jgit.api.Git;
@@ -702,4 +707,42 @@ public class ApacheSshTest extends SshTestBase {
 			session.disconnect();
 		}
 	}
+
+	/**
+	 * Tests that one can log in at an even poorer server that also only has the
+	 * SHA1 KEX methods available. Apparently this is the case for at least some
+	 * Microsoft TFS instances. The user has to enable the poor KEX methods in
+	 * the ssh config explicitly; we don't enable them by default.
+	 *
+	 * @throws Exception
+	 *             on failure
+	 */
+	@Test
+	public void testConnectOnlyRsaSha1() throws Exception {
+		try (SshServer oldServer = createServer(TEST_USER, publicKey1)) {
+			oldServer.setSignatureFactoriesNames("ssh-rsa");
+			List<DHFactory> sha1Factories = BuiltinDHFactories
+					.parseDHFactoriesList(
+							"diffie-hellman-group1-sha1,diffie-hellman-group14-sha1")
+					.getParsedFactories();
+			assertEquals(2, sha1Factories.size());
+			List<KeyExchangeFactory> kexFactories = NamedFactory
+					.setUpTransformedFactories(true, sha1Factories,
+							ServerBuilder.DH2KEX);
+			oldServer.setKeyExchangeFactories(kexFactories);
+			oldServer.start();
+			registerServer(oldServer);
+			installConfig("Host server", //
+					"HostName localhost", //
+					"Port " + oldServer.getPort(), //
+					"User " + TEST_USER, //
+					"IdentityFile " + privateKey1.getAbsolutePath(), //
+					"KexAlgorithms +diffie-hellman-group1-sha1");
+			RemoteSession session = getSessionFactory().getSession(
+					new URIish("ssh://server/doesntmatter"), null, FS.DETECTED,
+					10000);
+			assertNotNull(session);
+			session.disconnect();
+		}
+	}
 }