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Network Working Group T. Ylonen
Internet-Draft T. Kivinen
Expires: March 2, 2003 SSH Communications Security Corp
M. Saarinen
University of Jyvaskyla
T. Rinne
S. Lehtinen
SSH Communications Security Corp
September 2002
SSH Authentication Protocol
draft-ietf-secsh-userauth-17.txt
Status of this Memo
This document is an Internet-Draft and is in full conformance with
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 March 2, 2003.
Copyright Notice
Copyright (C) The Internet Society (2002). All Rights Reserved.
Abstract
SSH is a protocol for secure remote login and other secure network
services over an insecure network. This document describes the
SSH authentication protocol framework and public key, password,
and host-based client authentication methods. Additional
authentication methods are described in separate documents. The
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SSH authentication protocol runs on top of the SSH transport layer
protocol and provides a single authenticated tunnel for the SSH
connection protocol.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. The Authentication Protocol Framework . . . . . . . . . . . . 3
2.1 Authentication Requests . . . . . . . . . . . . . . . . . . . 4
2.2 Responses to Authentication Requests . . . . . . . . . . . . . 5
2.3 The "none" Authentication Request . . . . . . . . . . . . . . 6
2.4 Completion of User Authentication . . . . . . . . . . . . . . 6
2.5 Banner Message . . . . . . . . . . . . . . . . . . . . . . . . 6
3. Authentication Protocol Message Numbers . . . . . . . . . . . 7
4. Public Key Authentication Method: publickey . . . . . . . . . 7
5. Password Authentication Method: password . . . . . . . . . . . 9
6. Host-Based Authentication: hostbased . . . . . . . . . . . . . 11
7. Security Considerations . . . . . . . . . . . . . . . . . . . 12
8. Intellectual Property . . . . . . . . . . . . . . . . . . . . 12
9. Additional Information . . . . . . . . . . . . . . . . . . . . 13
References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 14
Full Copyright Statement . . . . . . . . . . . . . . . . . . . 15
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1. Introduction
The SSH authentication protocol is a general-purpose user
authentication protocol. It is intended to be run over the SSH
transport layer protocol [SSH-TRANS]. This protocol assumes that
the underlying protocols provide integrity and confidentiality
protection.
This document should be read only after reading the SSH
architecture document [SSH-ARCH]. This document freely uses
terminology and notation from the architecture document without
reference or further explanation.
The service name for this protocol is "ssh-userauth".
When this protocol starts, it receives the session identifier from
the lower-level protocol (this is the exchange hash H from the
first key exchange). The session identifier uniquely identifies
this session and is suitable for signing in order to prove
ownership of a private key. This protocol also needs to know
whether the lower-level protocol provides confidentiality
protection.
2. The Authentication Protocol Framework
The server drives the authentication by telling the client which
authentication methods can be used to continue the exchange at any
given time. The client has the freedom to try the methods listed
by the server in any order. This gives the server complete
control over the authentication process if desired, but also gives
enough flexibility for the client to use the methods it supports
or that are most convenient for the user, when multiple methods
are offered by the server.
Authentication methods are identified by their name, as defined in
[SSH-ARCH]. The "none" method is reserved, and MUST NOT be listed
as supported. However, it MAY be sent by the client. The server
MUST always reject this request, unless the client is to be
allowed in without any authentication, in which case the server
MUST accept this request. The main purpose of sending this
request is to get the list of supported methods from the server.
The server SHOULD have a timeout for authentication, and
disconnect if the authentication has not been accepted within the
timeout period. The RECOMMENDED timeout period is 10 minutes.
Additionally, the implementation SHOULD limit the number of failed
authentication attempts a client may perform in a single session
(the RECOMMENDED limit is 20 attempts). If the threshold is
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exceeded, the server SHOULD disconnect.
2.1 Authentication Requests
All authentication requests MUST use the following message format.
Only the first few fields are defined; the remaining fields depend
on the authentication method.
byte SSH_MSG_USERAUTH_REQUEST
string user name (in ISO-10646 UTF-8 encoding [RFC2279])
string service name (in US-ASCII)
string method name (US-ASCII)
The rest of the packet is method-specific.
The user name and service are repeated in every new authentication
attempt, and MAY change. The server implementation MUST carefully
check them in every message, and MUST flush any accumulated
authentication states if they change. If it is unable to flush
some authentication state, it MUST disconnect if the user or
service name changes.
The service name specifies the service to start after
authentication. There may be several different authenticated
services provided. If the requested service is not available, the
server MAY disconnect immediately or at any later time. Sending a
proper disconnect message is RECOMMENDED. In any case, if the
service does not exist, authentication MUST NOT be accepted.
If the requested user does not exist, the server MAY disconnect,
or MAY send a bogus list of acceptable authentication methods, but
never accept any. This makes it possible for the server to avoid
disclosing information on which accounts exist. In any case, if
the user does not exist, the authentication request MUST NOT be
accepted.
While there is usually little point for clients to send requests
that the server does not list as acceptable, sending such requests
is not an error, and the server SHOULD simply reject requests that
it does not recognize.
An authentication request MAY result in a further exchange of
messages. All such messages depend on the authentication method
used, and the client MAY at any time continue with a new
SSH_MSG_USERAUTH_REQUEST message, in which case the server MUST
abandon the previous authentication attempt and continue with the
new one.
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2.2 Responses to Authentication Requests
If the server rejects the authentication request, it MUST respond
with the following:
byte SSH_MSG_USERAUTH_FAILURE
string authentications that can continue
boolean partial success
"Authentications that can continue" is a comma-separated list of
authentication method names that may productively continue the
authentication dialog.
It is RECOMMENDED that servers only include those methods in the
list that are actually useful. However, it is not illegal to
include methods that cannot be used to authenticate the user.
Already successfully completed authentications SHOULD NOT be
included in the list, unless they really should be performed again
for some reason.
"Partial success" MUST be TRUE if the authentication request to
which this is a response was successful. It MUST be FALSE if the
request was not successfully processed.
When the server accepts authentication, it MUST respond with the
following:
byte SSH_MSG_USERAUTH_SUCCESS
Note that this is not sent after each step in a multi-method
authentication sequence, but only when the authentication is
complete.
The client MAY send several authentication requests without
waiting for responses from previous requests. The server MUST
process each request completely and acknowledge any failed
requests with a SSH_MSG_USERAUTH_FAILURE message before processing
the next request.
A request that results in further exchange of messages will be
aborted by a second request. It is not possible to send a second
request without waiting for a response from the server, if the
first request will result in further exchange of messages. No
SSH_MSG_USERAUTH_FAILURE message will be sent for the aborted
method.
SSH_MSG_USERAUTH_SUCCESS MUST be sent only once. When
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SSH_MSG_USERAUTH_SUCCESS has been sent, any further authentication
requests received after that SHOULD be silently ignored.
Any non-authentication messages sent by the client after the
request that resulted in SSH_MSG_USERAUTH_SUCCESS being sent MUST
be passed to the service being run on top of this protocol. Such
messages can be identified by their message numbers (see Section
Message Numbers (Section 3)).
2.3 The "none" Authentication Request
A client may request a list of authentication methods that may
continue by using the "none" authentication method.
If no authentication at all is needed for the user, the server
MUST return SSH_MSG_USERAUTH_SUCCESS. Otherwise, the server MUST
return SSH_MSG_USERAUTH_FAILURE and MAY return with it a list of
authentication methods that can continue.
This method MUST NOT be listed as supported by the server.
2.4 Completion of User Authentication
Authentication is complete when the server has responded with
SSH_MSG_USERAUTH_SUCCESS; all authentication related messages
received after sending this message SHOULD be silently ignored.
After sending SSH_MSG_USERAUTH_SUCCESS, the server starts the
requested service.
2.5 Banner Message
In some jurisdictions, sending a warning message before
authentication may be relevant for getting legal protection. Many
UNIX machines, for example, normally display text from
`/etc/issue', or use "tcp wrappers" or similar software to display
a banner before issuing a login prompt.
The SSH server may send a SSH_MSG_USERAUTH_BANNER message at any
time before authentication is successful. This message contains
text to be displayed to the client user before authentication is
attempted. The format is as follows:
byte SSH_MSG_USERAUTH_BANNER
string message (ISO-10646 UTF-8)
string language tag (as defined in [RFC1766])
The client SHOULD by default display the message on the screen.
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However, since the message is likely to be sent for every login
attempt, and since some client software will need to open a
separate window for this warning, the client software may allow
the user to explicitly disable the display of banners from the
server. The message may consist of multiple lines.
If the message string is displayed, control character filtering
discussed in [SSH-ARCH] SHOULD be used to avoid attacks by sending
terminal control characters.
3. Authentication Protocol Message Numbers
All message numbers used by this authentication protocol are in
the range from 50 to 79, which is part of the range reserved for
protocols running on top of the SSH transport layer protocol.
Message numbers of 80 and higher are reserved for protocols
running after this authentication protocol, so receiving one of
them before authentication is complete is an error, to which the
server MUST respond by disconnecting (preferably with a proper
disconnect message sent first to ease troubleshooting).
After successful authentication, such messages are passed to the
higher-level service.
These are the general authentication message codes:
#define SSH_MSG_USERAUTH_REQUEST 50
#define SSH_MSG_USERAUTH_FAILURE 51
#define SSH_MSG_USERAUTH_SUCCESS 52
#define SSH_MSG_USERAUTH_BANNER 53
In addition to the above, there is a range of message numbers
(60..79) reserved for method-specific messages. These messages
are only sent by the server (client sends only
SSH_MSG_USERAUTH_REQUEST messages). Different authentication
methods reuse the same message numbers.
4. Public Key Authentication Method: publickey
The only REQUIRED authentication method is public key
authentication. All implementations MUST support this method;
however, not all users need to have public keys, and most local
policies are not likely to require public key authentication for
all users in the near future.
With this method, the possession of a private key serves as
authentication. This method works by sending a signature created
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with a private key of the user. The server MUST check that the
key is a valid authenticator for the user, and MUST check that the
signature is valid. If both hold, the authentication request MUST
be accepted; otherwise it MUST be rejected. (Note that the server
MAY require additional authentications after successful
authentication.)
Private keys are often stored in an encrypted form at the client
host, and the user must supply a passphrase before the signature
can be generated. Even if they are not, the signing operation
involves some expensive computation. To avoid unnecessary
processing and user interaction, the following message is provided
for querying whether authentication using the key would be
acceptable.
byte SSH_MSG_USERAUTH_REQUEST
string user name
string service
string "publickey"
boolean FALSE
string public key algorithm name
string public key blob
Public key algorithms are defined in the transport layer
specification [SSH-TRANS]. The public key blob may contain
certificates.
Any public key algorithm may be offered for use in authentication.
In particular, the list is not constrained by what was negotiated
during key exchange. If the server does not support some
algorithm, it MUST simply reject the request.
The server MUST respond to this message with either
SSH_MSG_USERAUTH_FAILURE or with the following:
byte SSH_MSG_USERAUTH_PK_OK
string public key algorithm name from the request
string public key blob from the request
To perform actual authentication, the client MAY then send a
signature generated using the private key. The client MAY send
the signature directly without first verifying whether the key is
acceptable. The signature is sent using the following packet:
byte SSH_MSG_USERAUTH_REQUEST
string user name
string service
string "publickey"
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boolean TRUE
string public key algorithm name
string public key to be used for authentication
string signature
Signature is a signature by the corresponding private key over the
following data, in the following order:
string session identifier
byte SSH_MSG_USERAUTH_REQUEST
string user name
string service
string "publickey"
boolean TRUE
string public key algorithm name
string public key to be used for authentication
When the server receives this message, it MUST check whether the
supplied key is acceptable for authentication, and if so, it MUST
check whether the signature is correct.
If both checks succeed, this method is successful. Note that the
server may require additional authentications. The server MUST
respond with SSH_MSG_USERAUTH_SUCCESS (if no more authentications
are needed), or SSH_MSG_USERAUTH_FAILURE (if the request failed,
or more authentications are needed).
The following method-specific message numbers are used by the
publickey authentication method.
/* Key-based */
#define SSH_MSG_USERAUTH_PK_OK 60
5. Password Authentication Method: password
Password authentication uses the following packets. Note that a
server MAY request the user to change the password. All
implementations SHOULD support password authentication.
byte SSH_MSG_USERAUTH_REQUEST
string user name
string service
string "password"
boolean FALSE
string plaintext password (ISO-10646 UTF-8)
Note that the password is encoded in ISO-10646 UTF-8. It is up to
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the server how it interprets the password and validates it against
the password database. However, if the client reads the password
in some other encoding (e.g., ISO 8859-1 (ISO Latin1)), it MUST
convert the password to ISO-10646 UTF-8 before transmitting, and
the server MUST convert the password to the encoding used on that
system for passwords.
Note that even though the cleartext password is transmitted in the
packet, the entire packet is encrypted by the transport layer.
Both the server and the client should check whether the underlying
transport layer provides confidentiality (i.e., if encryption is
being used). If no confidentiality is provided (none cipher),
password authentication SHOULD be disabled. If there is no
confidentiality or no MAC, password change SHOULD be disabled.
Normally, the server responds to this message with success or
failure. However, if the password has expired the server SHOULD
indicate this by responding with
SSH_MSG_USERAUTH_PASSWD_CHANGEREQ. In anycase the server MUST NOT
allow an expired password to be used for authentication.
byte SSH_MSG_USERAUTH_PASSWD_CHANGEREQ
string prompt (ISO-10646 UTF-8)
string language tag (as defined in [RFC1766])
In this case, the client MAY continue with a different
authentication method, or request a new password from the user and
retry password authentication using the following message. The
client MAY also send this message instead of the normal password
authentication request without the server asking for it.
byte SSH_MSG_USERAUTH_REQUEST
string user name
string service
string "password"
boolean TRUE
string plaintext old password (ISO-10646 UTF-8)
string plaintext new password (ISO-10646 UTF-8)
The server must reply to request message with
SSH_MSG_USERAUTH_SUCCESS, SSH_MSG_USERAUTH_FAILURE, or another
SSH_MSG_USERAUTH_PASSWD_CHANGEREQ. The meaning of these is as
follows:
SSH_MSG_USERAUTH_SUCCESS The password has been changed, and
authentication has been successfully completed.
SSH_MSG_USERAUTH_FAILURE with partial success The password has
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been changed, but more authentications are needed.
SSH_MSG_USERAUTH_FAILURE without partial success The password
has not been changed. Either password changing was not
supported, or the old password was bad. Note that if the
server has already sent SSH_MSG_USERAUTH_PASSWD_CHANGEREQ, we
know that it supports changing the password.
SSH_MSG_USERAUTH_CHANGEREQ The password was not changed because
the new password was not acceptable (e.g. too easy to guess).
The following method-specific message numbers are used by the
password authentication method.
#define SSH_MSG_USERAUTH_PASSWD_CHANGEREQ 60
6. Host-Based Authentication: hostbased
Some sites wish to allow authentication based on the host where
the user is coming from, and the user name on the remote host.
While this form of authentication is not suitable for high-
security sites, it can be very convenient in many environments.
This form of authentication is OPTIONAL. When used, special care
SHOULD be taken to prevent a regular user from obtaining the
private host key.
The client requests this form of authentication by sending the
following message. It is similar to the UNIX "rhosts" and
"hosts.equiv" styles of authentication, except that the identity
of the client host is checked more rigorously.
This method works by having the client send a signature created
with the private key of the client host, which the server checks
with that host's public key. Once the client host's identity is
established, authorization (but no further authentication) is
performed based on the user names on the server and the client,
and the client host name.
byte SSH_MSG_USERAUTH_REQUEST
string user name
string service
string "hostbased"
string public key algorithm for host key
string public host key and certificates for client host
string client host name (FQDN; US-ASCII)
string user name on the client host (ISO-10646 UTF-8)
string signature
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Public key algorithm names for use in "public key algorithm for
host key" are defined in the transport layer specification. The
"public host key for client host" may include certificates.
Signature is a signature with the private host key of the
following data, in this order:
string session identifier
byte SSH_MSG_USERAUTH_REQUEST
string user name
string service
string "hostbased"
string public key algorithm for host key
string public host key and certificates for client host
string client host name (FQDN; US-ASCII)
string user name on the client host(ISO-10646 UTF-8)
The server MUST verify that the host key actually belongs to the
client host named in the message, that the given user on that host
is allowed to log in, and that the signature is a valid signature
on the appropriate value by the given host key. The server MAY
ignore the client user name, if it wants to authenticate only the
client host.
It is RECOMMENDED that whenever possible, the server perform
additional checks to verify that the network address obtained from
the (untrusted) network matches the given client host name. This
makes exploiting compromised host keys more difficult. Note that
this may require special handling for connections coming through a
firewall.
7. Security Considerations
The purpose of this protocol is to perform client user
authentication. It assumed that this runs over a secure transport
layer protocol, which has already authenticated the server
machine, established an encrypted communications channel, and
computed a unique session identifier for this session. The
transport layer provides forward secrecy for password
authentication and other methods that rely on secret data.
Full security considerations for this protocol are provided in
Section 8 of [SSH-ARCH]
8. Intellectual Property
The IETF takes no position regarding the validity or scope of any
intellectual property or other rights that might be claimed to
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pertain to the implementation or use of the technology described
in this document or the extent to which any license under such
rights might or might not be available; neither does it represent
that it has made any effort to identify any such rights.
Information on the IETF's procedures with respect to rights in
standards-track and standards-related documentation can be found
in BCP-11. Copies of claims of rights made available for
publication and any assurances of licenses to be made available,
or the result of an attempt made to obtain a general license or
permission for the use of such proprietary rights by implementers
or users of this specification can be obtained from the IETF
Secretariat.
The IETF has been notified of intellectual property rights claimed
in regard to some or all of the specification contained in this
document. For more information consult the online list of claimed
rights.
9. Additional Information
The current document editor is: Darren.Moffat@Sun.COM. Comments
on this internet draft should be sent to the IETF SECSH working
group, details at: http://ietf.org/html.charters/secsh-
charter.html
References
[RFC1766] Alvestrand, H., "Tags for the Identification of
Languages", RFC 1766, March 1995.
[RFC2279] Yergeau, F., "UTF-8, a transformation format of
ISO 10646", RFC 2279, January 1998.
[SSH-ARCH] Ylonen, T., "SSH Protocol Architecture", I-D
draft-ietf-architecture-14.txt, July 2003.
[SSH-TRANS] Ylonen, T., "SSH Transport Layer Protocol", I-D
draft-ietf-transport-16.txt, July 2003.
[SSH-USERAUTH] Ylonen, T., "SSH Authentication Protocol", I-D
draft-ietf-userauth-17.txt, July 2003.
[SSH-CONNECT] Ylonen, T., "SSH Connection Protocol", I-D draft-
ietf-connect-17.txt, July 2003.
[SSH-NUMBERS] Lehtinen, S. and D. Moffat, "SSH Protocol Assigned
Numbers", I-D draft-ietf-secsh-assignednumbers-
03.txt, July 2003.
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Authors' Addresses
Tatu Ylonen
SSH Communications Security Corp
Fredrikinkatu 42
HELSINKI FIN-00100
Finland
EMail: ylo@ssh.com
Tero Kivinen
SSH Communications Security Corp
Fredrikinkatu 42
HELSINKI FIN-00100
Finland
EMail: kivinen@ssh.com
Markku-Juhani O. Saarinen
University of Jyvaskyla
Timo J. Rinne
SSH Communications Security Corp
Fredrikinkatu 42
HELSINKI FIN-00100
Finland
EMail: tri@ssh.com
Sami Lehtinen
SSH Communications Security Corp
Fredrikinkatu 42
HELSINKI FIN-00100
Finland
EMail: sjl@ssh.com
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Full Copyright Statement
Copyright (C) The Internet Society (2002). All Rights Reserved.
This document and translations of it may be copied and furnished
to others, and derivative works that comment on or otherwise
explain it or assist in its implementation may be prepared,
copied, published and distributed, in whole or in part, without
restriction of any kind, provided that the above copyright notice
and this paragraph are included on all such copies and derivative
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Internet Society or other Internet organizations, except as needed
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The limited permissions granted above are perpetual and will not
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This document and the information contained herein is provided on
an "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
Ylonen, et. al. Expires March 2, 2003 [Page 15]
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