sshd: implement server-sig-algs SSH extension (client side)

Apache MINA sshd has an implementation of this, but it doesn't comply
to RFC 8308 [1] and it is buggy. (See SSHD-1141 [2].)

Add a simpler KexExtensionHandler and if the server sends extension
server-sig-algs, use its value to re-order the chosen signature
algorithms such that the algorithms the server announced as supported
are at the front.

If the server didn't tell us anything, don't do anything. RFC 8308
suggests for RSA to default to ssh-rsa, but says once rsa-sha2-* was
"widely enough" adopted, defaulting to that might be OK.

Currently we seem to be in a transition phase; Fedora 33 has already
disabled ssh-rsa by default, and openssh is about to do so. Whatever
we might do without info from the server, it'd be good for some servers
and bad for others. So don't do anything and let the user re-order via
ssh config PubkeyAcceptedAlgorithms on a case-by-case basis.

[1] https://tools.ietf.org/html/rfc8308
[2] https://issues.apache.org/jira/browse/SSHD-1141

Bug: 572056
Change-Id: I59aa691a030ffe0fae54289df00ca5c6e165817b
Signed-off-by: Thomas Wolf <thomas.wolf@paranor.ch>

1 files changed, 3 insertions, 0 deletions

diff --git a/org.eclipse.jgit.ssh.apache/resources/org/eclipse/jgit/internal/transport/sshd/SshdText.properties b/org.eclipse.jgit.ssh.apache/resources/org/eclipse/jgit/internal/transport/sshd/SshdText.properties
index 16b5738332..9c604f214f 100644
--- a/org.eclipse.jgit.ssh.apache/resources/org/eclipse/jgit/internal/transport/sshd/SshdText.properties
+++ b/org.eclipse.jgit.ssh.apache/resources/org/eclipse/jgit/internal/transport/sshd/SshdText.properties
@@ -76,6 +76,9 @@ proxySocksPasswordTooLong=Password for proxy {0} must be at most 255 bytes long,
 proxySocksUnexpectedMessage=Unexpected message received from SOCKS5 proxy {0}; client state {1}: {2}
 proxySocksUnexpectedVersion=Expected SOCKS version 5, got {0}
 proxySocksUsernameTooLong=User name for proxy {0} must be at most 255 bytes long, is {1} bytes: {2}
+pubkeyAuthWrongCommand=Public key authentication received unknown SSH command {0} from {1} ({2})
+pubkeyAuthWrongKey=Public key authentication received wrong key; sent {0}, got back {1} from {2} ({3})
+pubkeyAuthWrongSignatureAlgorithm=Public key authentication requested signature type {0} but got back {1} from {2} ({3})
 serverIdNotReceived=No server identification received within {0} bytes
 serverIdTooLong=Server identification is longer than 255 characters (including line ending): {0}
 serverIdWithNul=Server identification contains a NUL character: {0}