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Network Working Group F. Cusack
INTERNET-DRAFT Google, Inc.
Expires November 1, 2003 M. Forssen
Appgate AB
May 1, 2003
Generic Message Exchange Authentication For SSH
<draft-ietf-secsh-auth-kbdinteract-05.txt>
Status of this Memo
This document is an Internet-Draft and is subject to all provisions
of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as
Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
<http://www.ietf.org/ietf/1id-abstracts.txt>.
The list of Internet-Draft Shadow Directories can be accessed at
<http://www.ietf.org/shadow.html>.
This Internet-Draft will expire on November 1, 2003.
Abstract
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. 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).
F. Cusack, M. Forssen Expires November 1, 2003 [Page 1]
Internet Draft SSH Generic Interactive Authentication May 1, 2003
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 which 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 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 since 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.
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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.
This authentication method is however limited to authentication
mechanisms which do not require any special code, such as hardware
drivers or password mangling, on the client.
3. Protocol Exchanges
The client initiates the authentication with a
SSH_MSG_USERAUTH_REQUEST message. The server then requests
authentication information from the client with a
SSH_MSG_USERAUTH_INFO_REQUEST message. The client obtains the
information from the user and then responds with a
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-2279])
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. The
server SHOULD instead 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. The language to be
used if the server does not support the requested language is
implementation-dependent.
The submethods field is included so the user can give a hint of which
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actual methods he wants to use. It is a a comma-separated list of
authentication submethods (software or hardware) which 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 which the user and
the server needs to agree upon.
Server interpretation of the submethods field is implementation-
dependent.
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 either a 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) which 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 just comparing the results when
authenticating as different users.
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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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 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 user23@host.domain: ", a better choice
might be an instruction field of
"Password authentication for user23@host.domain" 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 language tag SHOULD describe the language used in the textual
fields. If the server does not know the language used, or if
multiple languages are used, the language tag MUST be the empty
string.
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).
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
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Internet Draft SSH Generic Interactive Authentication May 1, 2003
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; it SHOULD instead 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 occured.
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 or
not 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.
3.4 Information Responses
After obtaining the requested information from the user, the client
MUST respond with a 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)
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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.
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.
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 a
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
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 to the client.
It is suspected that implementations are likely to make the time
delay a 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 ""
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Internet Draft SSH Generic Interactive Authentication May 1, 2003
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
The second example is of 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"
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Internet Draft SSH Generic Interactive Authentication May 1, 2003
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
[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, depends
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.
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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 frequency of 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 Level", BCP 14, RFC 2119, March 1997.
[RFC-2279] Yergeau, F., "UTF-8, a transformation format of
Unicode and ISO 10646", RFC 2279, October 1996.
[RFC-3066] Alvestrand, H., "Tags for the Identification of
Languages", BCP 47, RFC 3066, January 2001.
[SSH-ARCH] Ylonen, T., Kivinen, T, Saarinen, M., Rinne, T., and
Lehtinen, S., "SSH Protocol Architecture", work in
progress, draft-ietf-secsh-architecture-13.txt,
September, 2002.
[SSH-CONNECT] Ylonen, T., Kivinen, T, Saarinen, M., Rinne, T., and
Lehtinen, S., "SSH Connection Protocol", work in
progress, draft-ietf-secsh-connect-16.txt, September,
2002.
[SSH-TRANS] Ylonen, T., Kivinen, T, Saarinen, M., Rinne, T., and
Lehtinen, S., "SSH Transport Layer Protocol", work in
progress, draft-ietf-secsh-transport-15.txt,
September, 2002.
[SSH-USERAUTH] Ylonen, T., Kivinen, T, Saarinen, M., Rinne, T., and
Lehtinen, S., "SSH Authentication Protocol", work in
progress, draft-ietf-secsh-userauth-16.txt,
September, 2002.
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7.2 Informative References
[PAM] Samar, V., Schemers, R., "Unified Login With
Pluggable Authentication Modules (PAM)", OSF RFC
86.0, October 1995
8. Author's Addresses
Frank Cusack
Google, Inc.
2400 Bayshore Parkway
Mountain View, CA 94043
Email: frank@google.com
Martin Forssen
Appgate AB
Stora Badhusgatan 18-20
SE-411 21 Gothenburg
SWEDEN
Email: maf@appgate.com
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