Network Working Group T. Ylonen
Internet-Draft SSH Communications Security Corp
Expires: March 2, 2003 D. Moffat, Ed.
Sun Microsystems, Inc
September 2002
SSH Authentication Protocol
draft-ietf-secsh-userauth-18.txt
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
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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 SSH authentication
protocol runs on top of the SSH transport layer protocol and provides
a single authenticated tunnel for the SSH connection protocol.
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Table of Contents
1. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Conventions Used in This Document . . . . . . . . . . . . . 3
3.1 The Authentication Protocol Framework . . . . . . . . . . . 3
3.1.1 Authentication Requests . . . . . . . . . . . . . . . . . . 4
3.1.2 Responses to Authentication Requests . . . . . . . . . . . . 5
3.1.3 The "none" Authentication Request . . . . . . . . . . . . . 6
3.1.4 Completion of User Authentication . . . . . . . . . . . . . 6
3.1.5 Banner Message . . . . . . . . . . . . . . . . . . . . . . . 7
3.2 Authentication Protocol Message Numbers . . . . . . . . . . 7
3.3 Public Key Authentication Method: publickey . . . . . . . . 8
3.4 Password Authentication Method: password . . . . . . . . . . 10
3.5 Host-Based Authentication: hostbased . . . . . . . . . . . . 11
4. Security Considerations . . . . . . . . . . . . . . . . . . 12
Normative . . . . . . . . . . . . . . . . . . . . . . . . . 13
Informative . . . . . . . . . . . . . . . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 14
Intellectual Property and Copyright Statements . . . . . . . 15
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1. Contributors
The major original contributors of this document were: Tatu Ylonen,
Tero Kivinen, Timo J. Rinne, Sami Lehtinen (all of SSH Communications
Security Corp), and Markku-Juhani O. Saarinen (University of
Jyvaskyla)
The document editor is: [email protected]. Comments on this
internet draft should be sent to the IETF SECSH working group,
details at: http://ietf.org/html.charters/secsh-charter.html
2. 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.
3. Conventions Used in This Document
The keywords "MUST", "MUST NOT", "REQUIRED", "SHOULD", "SHOULD NOT",
and "MAY" that appear in this document are to be interpreted as
described in [RFC2119]
The used data types and terminology are specified in the architecture
document [SSH-ARCH]
The architecture document also discusses the algorithm naming
conventions that MUST be used with the SSH protocols.
3.1 The Authentication Protocol Framework
The server drives the authentication by telling the client which
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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 exceeded, the server
SHOULD disconnect.
3.1.1 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,
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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.
3.1.2 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:
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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
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)).
3.1.3 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.
3.1.4 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.
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3.1.5 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 [RFC3066])
The client SHOULD by default display the message on the screen.
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.
3.2 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
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#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.
3.3 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
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
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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"
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
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3.4 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
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 [RFC3066])
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
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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
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
3.5 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,
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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
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.
4. Security Considerations
The purpose of this protocol is to perform client user
authentication. It assumed that this runs over a secure transport
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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]
Normative
[SSH-ARCH]
Ylonen, T., "SSH Protocol Architecture", I-D
draft-ietf-architecture-15.txt, Oct 2003.
[SSH-TRANS]
Ylonen, T., "SSH Transport Layer Protocol", I-D
draft-ietf-transport-17.txt, Oct 2003.
[SSH-USERAUTH]
Ylonen, T., "SSH Authentication Protocol", I-D
draft-ietf-userauth-18.txt, Oct 2003.
[SSH-CONNECT]
Ylonen, T., "SSH Connection Protocol", I-D
draft-ietf-connect-18.txt, Oct 2003.
[SSH-NUMBERS]
Lehtinen, S. and D. Moffat, "SSH Protocol Assigned
Numbers", I-D draft-ietf-secsh-assignednumbers-05.txt, Oct
2003.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
Informative
[RFC3066] Alvestrand, H., "Tags for the Identification of
Languages", BCP 47, RFC 3066, January 2001.
[RFC2279] Yergeau, F., "UTF-8, a transformation format of ISO
10646", RFC 2279, January 1998.
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Authors' Addresses
Tatu Ylonen
SSH Communications Security Corp
Fredrikinkatu 42
HELSINKI FIN-00100
Finland
EMail: [email protected]
Darren J. Moffat (editor)
Sun Microsystems, Inc
17 Network Circle
Menlo Park 95025
USA
EMail: [email protected]
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