Network Working Group F. Cusack

Network Working Group F. Cusack
Request for Comments: 4256 savecore.net
Category: Standards Track M. Forssen
AppGate Network Security AB
January 2006

Generic Message Exchange Authentication for
the Secure Shell Protocol (SSH)

Status of This Memo

This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.

Copyright Notice

Copyright (C) The Internet Society (2006).

Abstract

The Secure Shell Protocol (SSH) is a protocol for secure remote login
and other secure network services over an insecure network. This
document describes a general purpose authentication method for the
SSH protocol, suitable for interactive authentications where the
authentication data should be entered via a keyboard (or equivalent
alphanumeric input device). The major goal of this method is to
allow the SSH client to support a whole class of authentication
mechanism(s) without knowing the specifics of the actual
authentication mechanism(s).

1. Introduction

The SSH authentication protocol [SSH-USERAUTH] is a general-purpose
user authentication protocol. It is intended to be run over the SSH
transport layer protocol [SSH-TRANS]. The authentication protocol
assumes that the underlying protocols provide integrity and
confidentiality protection.

This document describes a general purpose authentication method for
the SSH authentication protocol. This method is suitable for
interactive authentication methods that do not need any special
software support on the client side. Instead, all authentication
data should be entered via the keyboard. The major goal of this
method is to allow the SSH client to have little or no knowledge of

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RFC 4256 SSH Generic Interactive Authentication January 2006

the specifics of the underlying authentication mechanism(s) used by
the SSH server. This will allow the server to arbitrarily select or
change the underlying authentication mechanism(s) without having to
update client code.

The name for this authentication method is "keyboard-interactive".

This document should be read only after reading the SSH architecture
document [SSH-ARCH] and the SSH authentication document
[SSH-USERAUTH]. This document freely uses terminology and notation
from both documents without reference or further explanation.

This document also describes some of the client interaction with the
user in obtaining the authentication information. While this is
somewhat out of the scope of a protocol specification, it is
described here anyway because some aspects of the protocol are
specifically designed based on user interface issues, and omitting
this information may lead to incompatible or awkward implementations.

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC-2119].

2. Rationale

Currently defined authentication methods for SSH are tightly coupled
with the underlying authentication mechanism. This makes it
difficult to add new mechanisms for authentication as all clients
must be updated to support the new mechanism. With the generic
method defined here, clients will not require code changes to support
new authentication mechanisms, and if a separate authentication layer
is used, such as [PAM], then the server may not need any code changes
either.

This presents a significant advantage to other methods, such as the
"password" method (defined in [SSH-USERAUTH]), as new (presumably
stronger) methods may be added "at will" and system security can be
transparently enhanced.

Challenge-response and One Time Password mechanisms are also easily
supported with this authentication method.

However, this authentication method is limited to authentication
mechanisms that do not require any special code, such as hardware
drivers or password mangling, on the client.

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RFC 4256 SSH Generic Interactive Authentication January 2006

3. Protocol Exchanges

The client initiates the authentication with an
SSH_MSG_USERAUTH_REQUEST message. The server then requests
authentication information from the client with an
SSH_MSG_USERAUTH_INFO_REQUEST message. The client obtains the
information from the user and then responds with an
SSM_MSG_USERAUTH_INFO_RESPONSE message. The server MUST NOT send
another SSH_MSG_USERAUTH_INFO_REQUEST before it has received the
answer from the client.

3.1. Initial Exchange

The authentication starts with the client sending the following
packet:

byte SSH_MSG_USERAUTH_REQUEST
string user name (ISO-10646 UTF-8, as defined in [RFC-3629])
string service name (US-ASCII)
string "keyboard-interactive" (US-ASCII)
string language tag (as defined in [RFC-3066])
string submethods (ISO-10646 UTF-8)

The language tag is deprecated and SHOULD be the empty string. It
may be removed in a future revision of this specification. Instead,
the server SHOULD select the language to be used based on the tags
communicated during key exchange [SSH-TRANS].

If the language tag is not the empty string, the server SHOULD use
the specified language for any messages sent to the client as part of
this protocol. The language tag SHOULD NOT be used for language
selection for messages outside of this protocol. If the server does
not support the requested language, the language to be used is
implementation-dependent.

The submethods field is included so the user can give a hint of which
actual methods he wants to use. It is a comma-separated list of
authentication submethods (software or hardware) that the user
prefers. If the client has knowledge of the submethods preferred by
the user, presumably through a configuration setting, it MAY use the
submethods field to pass this information to the server. Otherwise,
it MUST send the empty string.

The actual names of the submethods is something the user and the
server need to agree upon.

Server interpretation of the submethods field is implementation-
dependent.

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RFC 4256 SSH Generic Interactive Authentication January 2006

One possible implementation strategy of the submethods field on the
server is that, unless the user may use multiple different
submethods, the server ignores this field. If the user may
authenticate using one of several different submethods, the server
should treat the submethods field as a hint on which submethod the
user wants to use this time.

Note that when this message is sent to the server, the client has not
yet prompted the user for a password, and so that information is NOT
included with this initial message (unlike the "password" method).

The server MUST reply with an SSH_MSG_USERAUTH_SUCCESS,
SSH_MSG_USERAUTH_FAILURE, or SSH_MSG_USERAUTH_INFO_REQUEST message.

The server SHOULD NOT reply with the SSH_MSG_USERAUTH_FAILURE message
if the failure is based on the user name or service name; instead, it
SHOULD send SSH_MSG_USERAUTH_INFO_REQUEST message(s), which look just
like the one(s) that would have been sent in cases where
authentication should proceed, and then send the failure message
(after a suitable delay, as described below). The goal is to make it
impossible to find valid usernames by comparing the results when
authenticating as different users.

The server MAY reply with an SSH_MSG_USERAUTH_SUCCESS message if no
authentication is required for the user in question. However, a
better approach, for reasons discussed above, might be to reply with
an SSH_MSG_USERAUTH_INFO_REQUEST message and ignore (don't validate)
the response.

3.2. Information Requests

Requests are generated from the server using the
SSH_MSG_USERAUTH_INFO_REQUEST message.

The server may send as many requests as are necessary to authenticate
the client; the client MUST be prepared to handle multiple exchanges.
However, the server MUST NOT ever have more than one
SSH_MSG_USERAUTH_INFO_REQUEST message outstanding. That is, it may
not send another request before the client has answered.

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RFC 4256 SSH Generic Interactive Authentication January 2006

The SSH_MSG_USERAUTH_INFO_REQUEST message is defined as follows:

byte SSH_MSG_USERAUTH_INFO_REQUEST
string name (ISO-10646 UTF-8)
string instruction (ISO-10646 UTF-8)
string language tag (as defined in [RFC-3066])
int num-prompts
string prompt[1] (ISO-10646 UTF-8)
boolean echo[1]
...
string prompt[num-prompts] (ISO-10646 UTF-8)
boolean echo[num-prompts]

The language tag is deprecated and SHOULD be the empty string. It
may be removed in a future revision of this specification. Instead,
the server SHOULD select the language used based on the tags
communicated during key exchange [SSH-TRANS].

If the language tag is not the empty string, the server SHOULD use
the specified language for any messages sent to the client as part of
this protocol. The language tag SHOULD NOT be used for language
selection for messages outside of this protocol. If the server does
not support the requested language, the language to be used is
implementation-dependent.

The server SHOULD take into consideration that some clients may not
be able to properly display a long name or prompt field (see next
section), and limit the lengths of those fields if possible. For
example, instead of an instruction field of "Enter Password" and a
prompt field of "Password for [email protected]: ", a better choice
might be an instruction field of "Password authentication for
[email protected]" and a prompt field of "Password: ". It is
expected that this authentication method would typically be backended
by [PAM] and so such choices would not be possible.

The name and instruction fields MAY be empty strings; the client MUST
be prepared to handle this correctly. The prompt field(s) MUST NOT
be empty strings.

The num-prompts field may be `0', in which case there will be no
prompt/echo fields in the message, but the client SHOULD still
display the name and instruction fields (as described below).

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RFC 4256 SSH Generic Interactive Authentication January 2006

3.3. User Interface

Upon receiving a request message, the client SHOULD prompt the user
as follows:

A command line interface (CLI) client SHOULD print the name and
instruction (if non-empty), adding newlines. Then, for each prompt
in turn, the client SHOULD display the prompt and read the user
input.

A graphical user interface (GUI) client has many choices on how to
prompt the user. 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. In that dialog window, the
instruction field would be a text message, and the prompts would be
labels for text entry fields. All fields SHOULD be presented to the
user. For example, an implementation SHOULD NOT discard the name
field because its windows lack titles; instead, it SHOULD find
another way to display this information. If prompts are presented in
a dialog window, then the client SHOULD NOT present each prompt in a
separate window.

All clients MUST properly handle an instruction field with embedded
newlines. They SHOULD also be able to display at least 30 characters
for the name and prompts. If the server presents names or prompts
longer than 30 characters, the client MAY truncate these fields to
the length it can display. If the client does truncate any fields,
there MUST be an obvious indication that such truncation has
occurred. The instruction field SHOULD NOT be truncated.

Clients SHOULD use control character filtering, as discussed in
[SSH-ARCH], to avoid attacks by including terminal control characters
in the fields to be displayed.

For each prompt, the corresponding echo field indicates whether the
user input should be echoed as characters are typed. Clients SHOULD
correctly echo/mask user input for each prompt independently of other
prompts in the request message. If a client does not honor the echo
field for whatever reason, then the client MUST err on the side of
masking input. A GUI client might like to have a checkbox toggling
echo/mask. Clients SHOULD NOT add any additional characters to the
prompt, such as ": " (colon-space); the server is responsible for
supplying all text to be displayed to the user. Clients MUST also
accept empty responses from the user and pass them on as empty
strings.

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RFC 4256 SSH Generic Interactive Authentication January 2006

3.4. Information Responses

After obtaining the requested information from the user, the client
MUST respond with an SSH_MSG_USERAUTH_INFO_RESPONSE message.

The format of the SSH_MSG_USERAUTH_INFO_RESPONSE message is as
follows:

byte SSH_MSG_USERAUTH_INFO_RESPONSE
int num-responses
string response[1] (ISO-10646 UTF-8)
...
string response[num-responses] (ISO-10646 UTF-8)

Note that the responses are encoded in ISO-10646 UTF-8. It is up to
the server how it interprets the responses and validates them.
However, if the client reads the responses in some other encoding
(e.g., ISO 8859-1), it MUST convert the responses to ISO-10646 UTF-8
before transmitting.

From an internationalization standpoint, it is desired that if a user
enters responses, the authentication process will work regardless of
what OS and client software they are using. Doing so requires
normalization. Systems supporting non-ASCII passwords SHOULD always
normalize passwords and usernames whenever they are added to the
database, or compare them (with or without hashing) to existing
entries in the database. SSH implementations that both store the
passwords and compare them SHOULD use [SASLPREP] for normalization.

If the num-responses field does not match the num-prompts field in
the request message, the server MUST send a failure message.

In the case that the server sends a `0' num-prompts field in the
request message, the client MUST send a response message with a `0'
num-responses field to complete the exchange.

The responses MUST be ordered as the prompts were ordered. That is,
response[n] MUST be the answer to prompt[n].

After receiving the response, the server MUST send either an
SSH_MSG_USERAUTH_SUCCESS, SSH_MSG_USERAUTH_FAILURE, or another
SSH_MSG_USERAUTH_INFO_REQUEST message.

If the server fails to authenticate the user (through the underlying
authentication mechanism(s)), it SHOULD NOT send another request
message(s) in an attempt to obtain new authentication data; instead,
it SHOULD send a failure message. The only time the server should
send multiple request messages is if additional authentication data

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RFC 4256 SSH Generic Interactive Authentication January 2006

is needed (i.e., because there are multiple underlying authentication
mechanisms that must be used to authenticate the user).

If the server intends to respond with a failure message, it MAY delay
for an implementation-dependent time before sending it to the client.
It is suspected that implementations are likely to make the time
delay configurable; a suggested default is 2 seconds.

4. Authentication Examples

Here are two example exchanges between a client and server. The
first is an example of challenge/response with a handheld token.
This is an authentication that is not otherwise possible with other
authentication methods.

C: byte SSH_MSG_USERAUTH_REQUEST
C: string "user23"
C: string "ssh-userauth"
C: string "keyboard-interactive"
C: string ""
C: string ""

S: byte SSH_MSG_USERAUTH_INFO_REQUEST
S: string "CRYPTOCard Authentication"
S: string "The challenge is '14315716'"
S: string "en-US"
S: int 1
S: string "Response: "
S: boolean TRUE

[Client prompts user for password]

C: byte SSH_MSG_USERAUTH_INFO_RESPONSE
C: int 1
C: string "6d757575"

S: byte SSH_MSG_USERAUTH_SUCCESS

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RFC 4256 SSH Generic Interactive Authentication January 2006

The second example is a standard password authentication; in this
case, the user's password is expired.

C: byte SSH_MSG_USERAUTH_REQUEST
C: string "user23"
C: string "ssh-userauth"
C: string "keyboard-interactive"
C: string "en-US"
C: string ""

S: byte SSH_MSG_USERAUTH_INFO_REQUEST
S: string "Password Authentication"
S: string ""
S: string "en-US"
S: int 1
S: string "Password: "
S: boolean FALSE

[Client prompts user for password]

C: byte SSH_MSG_USERAUTH_INFO_RESPONSE
C: int 1
C: string "password"

S: byte SSH_MSG_USERAUTH_INFO_REQUEST
S: string "Password Expired"
S: string "Your password has expired."
S: string "en-US"
S: int 2
S: string "Enter new password: "
S: boolean FALSE
S: string "Enter it again: "
S: boolean FALSE

[Client prompts user for new password]

C: byte SSH_MSG_USERAUTH_INFO_RESPONSE
C: int 2
C: string "newpass"
C: string "newpass"

S: byte SSH_MSG_USERAUTH_INFO_REQUEST
S: string "Password changed"
S: string "Password successfully changed for user23."
S: string "en-US"
S: int 0

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RFC 4256 SSH Generic Interactive Authentication January 2006

[Client displays message to user]

C: byte SSH_MSG_USERAUTH_INFO_RESPONSE
C: int 0

S: byte SSH_MSG_USERAUTH_SUCCESS

5. IANA Considerations

The userauth type "keyboard-interactive" is used for this
authentication method.

The following method-specific constants are used with this
authentication method:

SSH_MSG_USERAUTH_INFO_REQUEST 60
SSH_MSG_USERAUTH_INFO_RESPONSE 61

6. Security Considerations

The authentication protocol and this authentication method depend on
the security of the underlying SSH transport layer. Without the
confidentiality provided therein, any authentication data passed with
this method is subject to interception.

The number of client-server exchanges required to complete an
authentication using this method may be variable. It is possible
that an observer may gain valuable information simply by counting
that number. For example, an observer may guess that a user's
password has expired, and with further observation may be able to
determine the password lifetime imposed by a site's password
expiration policy.

7. References

7.1. Normative References

[RFC-2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.

[RFC-3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, November 2003.

[RFC-3066] Alvestrand, H., "Tags for the Identification of
Languages", BCP 47, RFC 3066, January 2001.

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RFC 4256 SSH Generic Interactive Authentication January 2006

[SSH-ARCH] Ylonen, T. and C. Lonvick, Ed., "The Secure Shell
(SSH) Protocol Architecture", RFC 4251, January 2006.

[SSH-USERAUTH] Ylonen, T. and C. Lonvick, Ed., "The Secure Shell
(SSH) Authentication Protocol", RFC 4252, January
2006.

[SSH-TRANS] Ylonen, T. and C. Lonvick, Ed., "The Secure Shell
(SSH) Transport Layer Protocol", RFC 4253, January
2006.

[SASLPREP] Zeilenga, K., "SASLprep: Stringprep Profile for User
Names and Passwords", RFC 4013, February 2005.

7.2. Informative References

[PAM] Samar, V., Schemers, R., "Unified Login With
Pluggable Authentication Modules (PAM)", OSF RFC
86.0, October 1995.

Authors' Addresses

Frank Cusack
savecore.net

EMail: [email protected]

Martin Forssen
AppGate Network Security AB
Otterhallegatan 2
SE-411 18 Gothenburg
SWEDEN

EMail: [email protected]

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RFC 4256 SSH Generic Interactive Authentication January 2006

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