ssh: send (and rename) keyboard-interactive name field to the client

The server side implementation was not actually populating the
SSH_MSG_USERAUTH_INFO_REQUEST field with the
KeyboardInteractiveChallenge argument, although the client side was
deserializing it and passing it to the KeyboardInteractiveChallenge
callback.

Anyway, the first field of SSH_MSG_USERAUTH_INFO_REQUEST is "name", not
"user". Maybe the confusion was due to the first field of
SSH_MSG_USERAUTH_REQUEST being the user.

RFC 4256, Section 3.3, says this about it

   One possibility is to use the name field (possibly
   prefixed with the application's name) as the title of a dialog window
   in which the prompt(s) are presented.

and examples include "CRYPTOCard Authentication", "Password
Authentication", and "Password Expired".

Co-authored-by: Kevin Wallace <kevin@pentabarf.net>
Change-Id: Ic6ec0dfea2122704603c44f42898a980689a15c9
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/372234
Trust: Filippo Valsorda <filippo@golang.org>
Run-TryBot: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Roland Shoemaker <roland@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>

ssh/client_auth.go

diff --git a/ssh/client_auth.go b/ssh/client_auth.go
index c611aeb..affc01d 100644
--- a/ssh/client_auth.go
+++ b/ssh/client_auth.go
@@ -380,10 +380,10 @@
 // disabling echoing (e.g. for passwords), and return all the answers.
 // Challenge may be called multiple times in a single session. After
 // successful authentication, the server may send a challenge with no
-// questions, for which the user and instruction messages should be
+// questions, for which the name and instruction messages should be
 // printed.  RFC 4256 section 3.3 details how the UI should behave for
 // both CLI and GUI environments.
-type KeyboardInteractiveChallenge func(user, instruction string, questions []string, echos []bool) (answers []string, err error)
+type KeyboardInteractiveChallenge func(name, instruction string, questions []string, echos []bool) (answers []string, err error)
 
 // KeyboardInteractive returns an AuthMethod using a prompt/response
 // sequence controlled by the server.
@@ -465,7 +465,7 @@
 			return authFailure, nil, errors.New("ssh: extra data following keyboard-interactive pairs")
 		}
 
-		answers, err := cb(msg.User, msg.Instruction, prompts, echos)
+		answers, err := cb(msg.Name, msg.Instruction, prompts, echos)
 		if err != nil {
 			return authFailure, nil, err
 		}

ssh/messages.go

diff --git a/ssh/messages.go b/ssh/messages.go
index ac41a41..62f9330 100644
--- a/ssh/messages.go
+++ b/ssh/messages.go
@@ -180,11 +180,11 @@
 const msgUserAuthInfoResponse = 61
 
 type userAuthInfoRequestMsg struct {
-	User               string `sshtype:"60"`
-	Instruction        string
-	DeprecatedLanguage string
-	NumPrompts         uint32
-	Prompts            []byte `ssh:"rest"`
+	Name        string `sshtype:"60"`
+	Instruction string
+	Language    string
+	NumPrompts  uint32
+	Prompts     []byte `ssh:"rest"`
 }
 
 // See RFC 4254, section 5.1.

ssh/server.go

diff --git a/ssh/server.go b/ssh/server.go
index d28e1ad..9bb714c 100644
--- a/ssh/server.go
+++ b/ssh/server.go
@@ -695,7 +695,7 @@
 	*connection
 }
 
-func (c *sshClientKeyboardInteractive) Challenge(user, instruction string, questions []string, echos []bool) (answers []string, err error) {
+func (c *sshClientKeyboardInteractive) Challenge(name, instruction string, questions []string, echos []bool) (answers []string, err error) {
 	if len(questions) != len(echos) {
 		return nil, errors.New("ssh: echos and questions must have equal length")
 	}
@@ -707,6 +707,7 @@
 	}
 
 	if err := c.transport.writePacket(Marshal(&userAuthInfoRequestMsg{
+		Name:        name,
 		Instruction: instruction,
 		NumPrompts:  uint32(len(questions)),
 		Prompts:     prompts,