sshd: try all configured signature algorithms for a key

For RSA keys, there may be several configured signature algorithms:
rsa-sha2-512, rsa-sha2-256, and ssh-rsa. Upstream sshd has bug
SSHD-1105 [1] and always and unconditionally uses only the first
configured algorithm. With the default order, this means that it cannot
connect to a server that knows only ssh-rsa, like for instance Apache
MINA sshd servers older than 2.6.0.

This affects for instance bitbucket.org or also AWS Code Commit.

Re-introduce our own pubkey authenticator that fixes this.

Note that a server may impose a penalty (back-off delay) for subsequent
authentication attempts with signature algorithms unknown to the server.
In such cases, users can re-order the signature algorithm list via the
PubkeyAcceptedAlgorithms (formerly PubkeyAcceptedKeyTypes) ssh config.

[1] https://issues.apache.org/jira/browse/SSHD-1105

Bug: 572056
Change-Id: I7fb9c759ab6532e5f3b6524e9084085ddb2f30d6
Signed-off-by: Thomas Wolf <thomas.wolf@paranor.ch>

1 files changed, 38 insertions, 0 deletions

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 09d048b4fa..c56d2307c6 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
@@ -664,4 +664,42 @@ public class ApacheSshTest extends SshTestBase {
 			session.disconnect();
 		}
 	}
+
+	/**
+	 * Tests that one can log in to an old server that knows only the ssh-rsa
+	 * signature algorithm. The client has by default the list of signature
+	 * algorithms for RSA as "rsa-sha2-512,rsa-sha2-256,ssh-rsa". It should try
+	 * all three with the single key configured, and finally succeed.
+	 * <p>
+	 * The re-ordering mechanism (see
+	 * {@link #testConnectAuthSshRsaPubkeyAcceptedAlgorithms()}) is still
+	 * important; servers may impose a penalty (back-off delay) for subsequent
+	 * attempts with signature algorithms unknown to the server. So a user
+	 * connecting to such a server and noticing delays may still want to put
+	 * ssh-rsa first in the list for that host.
+	 * </p>
+	 *
+	 * @see <a href="https://bugs.eclipse.org/bugs/show_bug.cgi?id=572056">bug
+	 *      572056</a>
+	 * @throws Exception
+	 *             on failure
+	 */
+	@Test
+	public void testConnectAuthSshRsa() throws Exception {
+		try (SshServer oldServer = createServer(TEST_USER, publicKey1)) {
+			oldServer.setSignatureFactoriesNames("ssh-rsa");
+			oldServer.start();
+			registerServer(oldServer);
+			installConfig("Host server", //
+					"HostName localhost", //
+					"Port " + oldServer.getPort(), //
+					"User " + TEST_USER, //
+					"IdentityFile " + privateKey1.getAbsolutePath());
+			RemoteSession session = getSessionFactory().getSession(
+					new URIish("ssh://server/doesntmatter"), null, FS.DETECTED,
+					10000);
+			assertNotNull(session);
+			session.disconnect();
+		}
+	}
 }