upstream commit

Revise hostkeys@openssh.com hostkey learning extension.

The client will not ask the server to prove ownership of the private
halves of any hitherto-unseen hostkeys it offers to the client.

Allow UpdateHostKeys option to take an 'ask' argument to let the
user manually review keys offered.

ok markus@

1 files changed, 39 insertions, 14 deletions

diff --git a/PROTOCOL b/PROTOCOL
index 8150c577..f9560839 100644
--- a/PROTOCOL
+++ b/PROTOCOL
@@ -40,8 +40,8 @@ http://www.openssh.com/txt/draft-miller-secsh-compression-delayed-00.txt
      "ecdsa-sha2-nistp521-cert-v01@openssh.com"
 
 OpenSSH introduces new public key algorithms to support certificate
-authentication for users and hostkeys. These methods are documented in
-the file PROTOCOL.certkeys
+authentication for users and host keys. These methods are documented
+in the file PROTOCOL.certkeys
 
 1.4. transport: Elliptic Curve cryptography
 
@@ -283,26 +283,51 @@ by the client cancel the forwarding of a Unix domain socket.
 	string		socket path
 
 2.5. connection: hostkey update and rotation "hostkeys@openssh.com"
+and "hostkeys-prove@openssh.com"
 
 OpenSSH supports a protocol extension allowing a server to inform
-a client of all its protocol v.2 hostkeys after user-authentication
+a client of all its protocol v.2 host keys after user-authentication
 has completed.
 
 	byte		SSH_MSG_GLOBAL_REQUEST
 	string		"hostkeys@openssh.com"
 	string[]	hostkeys
 
-Upon receiving this message, a client may update its known_hosts
-file, adding keys that it has not seen before and deleting keys
-for the server host that are no longer offered.
+Upon receiving this message, a client should check which of the
+supplied host keys are present in known_hosts. For keys that are
+not present, it should send a "hostkeys-prove@openssh.com" message
+to request the server prove ownership of the private half of the
+key.
 
-This extension allows a client to learn key types that it had
-not previously encountered, thereby allowing it to potentially
-upgrade from weaker key algorithms to better ones. It also
-supports graceful key rotation: a server may offer multiple keys
-of the same type for a period (to give clients an opportunity to
-learn them using this extension) before removing the deprecated
-key from those offered.
+	byte		SSH_MSG_GLOBAL_REQUEST
+	string		"hostkeys-prove@openssh.com"
+	char		1 /* want-reply */
+	string[]	hostkeys
+
+When a server receives this message, it should generate a signature
+using each requested key over the following:
+
+	string		session identifier
+	string		"hostkeys-prove@openssh.com"
+	string		hostkey
+
+These signatures should be included in the reply, in the order matching
+the hostkeys in the request:
+
+	byte		SSH_MSG_REQUEST_SUCCESS
+	string[]	signatures
+
+When the client receives this reply (and not a failure), it should
+validate the signatures and may update its known_hosts file, adding keys
+that it has not seen before and deleting keys for the server host that
+are no longer offered.
+
+These extensions let a client learn key types that it had not previously
+encountered, thereby allowing it to potentially upgrade from weaker
+key algorithms to better ones. It also supports graceful key rotation:
+a server may offer multiple keys of the same type for a period (to
+give clients an opportunity to learn them using this extension) before
+removing the deprecated key from those offered.
 
 3. SFTP protocol changes
 
@@ -428,4 +453,4 @@ respond with a SSH_FXP_STATUS message.
 This extension is advertised in the SSH_FXP_VERSION hello with version
 "1".
 
-$OpenBSD: PROTOCOL,v 1.25 2015/01/26 03:04:45 djm Exp $
+$OpenBSD: PROTOCOL,v 1.26 2015/02/16 22:13:32 djm Exp $