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Network Working Group                                          T. Ylonen
INTERNET-DRAFT                                                T. Kivinen
draft-ietf-secsh-userauth-03.txt                             M. Saarinen
Expires in six months                                                SSH
                                                         7 November 1997

                      SSH Authentication Protocol

Status of This memo

This document is an Internet-Draft. 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
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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 deferred to separate documents.

The SSH authentication protocol runs on top the SSH transport layer
protocol and provides a single authenticated tunnel for the SSH
connection protocol.

T. Ylonen, T. Kivinen, and M. Saarinen                          [page 1]

INTERNET-DRAFT                                           7 November 1997

Table of Contents

1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . .  2
2.  The Authentication Protocol Framework   . . . . . . . . . . . . .  2
  2.1.  Authentication Requests   . . . . . . . . . . . . . . . . . .  3
  2.2.  Responses to Authentication Requests  . . . . . . . . . . . .  3
  2.3.  The none Authentication Request   . . . . . . . . . . . . . .  4
  2.4.  Completion of User Authentication   . . . . . . . . . . . . .  5
  2.5.  Banner Message  . . . . . . . . . . . . . . . . . . . . . . .  5
3.  Authentication Protocol Message Numbers   . . . . . . . . . . . .  5
4.  Public Key Authentication Method: publickey   . . . . . . . . . .  6
5.  Password Authentication Method: password  . . . . . . . . . . . .  7
6.  Host-Based Authentication: hostbased  . . . . . . . . . . . . . .  9
7.  Security Considerations   . . . . . . . . . . . . . . . . . . . . 10
8.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
9.  Author's Address  . . . . . . . . . . . . . . . . . . . . . . . . 11

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.  The session identifier uniquely identifies this
session and is suitable for signing 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
authentications can usefully continue the dialog 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 it so 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 names, 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

T. Ylonen, T. Kivinen, and M. Saarinen                          [page 2]

INTERNET-DRAFT                                           7 November 1997

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.

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.

  string    user name (in ISO-10646 UTF-8 encoding)
  string    service name (in US-ASCII)
  string    method name (US-ASCII)
  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
state 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 any time later.  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 authentications but never accept any.
This makes it possible for the server to avoid disclosing information
about 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

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.

T. Ylonen, T. Kivinen, and M. Saarinen                          [page 3]

INTERNET-DRAFT                                           7 November 1997

2.2.  Responses to Authentication Requests

If the server rejects the authentication request, it MUST respond with

  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


Note that this is not sent after each step in a multi-method authentica-
tion sequence, but only when authentication is complete.

The client MAY send several authentication requests without waiting for
responses from previous requests.  The server MUST acknowledge any
failed requests with a SSH_MSG_USERAUTH_FAILURE message.  However,
SSH_MSG_USERAUTH_SUCCESS MUST sent only once, and once
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'').

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.

T. Ylonen, T. Kivinen, and M. Saarinen                          [page 4]

INTERNET-DRAFT                                           7 November 1997

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

2.5.  Banner Message

In some jurisdictions, sending a warning message before authentication
may be relevant to 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.

  string    message (ISO-10646 UTF-8)
  string    language tag (as defined in RFC 1766)

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.  Authentication Protocol Message Numbers

All message numbers used by this authentication protocol are in the
range 50..79, which is part of the range reserved for protocols running
on top of the SSH transport layer protocol.

Message numbers 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:

T. Ylonen, T. Kivinen, and M. Saarinen                          [page 5]

INTERNET-DRAFT                                           7 November 1997

  #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 (25..29)
reserved for method-specific messages.  These messages are only sent by
the server (client only sends 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 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 encrypted 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.

  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 (as that was affected by which algorithms the server had a
host key).  If the server does not support some algorithm, it MUST
simply reject the request.

The server MUST respond to this message with either


T. Ylonen, T. Kivinen, and M. Saarinen                          [page 6]

INTERNET-DRAFT                                           7 November 1997

  string    public key algorithm name from the request
  string    public key blob from the request

To do actual authentication, the client MAY then send a signature
generated using the private key.  Client MAY send the signature directly
without first verifying whether the key is acceptable.  The signature is
sent using the following packet

  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 this order:

o  session identifier, and

o  packet payload without the signature.

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 password.  All implementations SHOULD
support password authentication.

  string    user name
  string    service
  string    "password"
  boolean   FALSE
  string    plaintext password (ISO-10646 UTF-8)

T. Ylonen, T. Kivinen, and M. Saarinen                          [page 7]

INTERNET-DRAFT                                           7 November 1997

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., 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, the server MAY also respond with

  string    prompt (ISO-10646 UTF-8)
  string    language tag (as defined in RFC 1766)

In this case, the software client SHOULD request a new password from the
user, and send a new request using the following message.  The client
may also send this message instead of the normal password authentication
request without the server asking for it.

  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,
The meaning of these is as follows:

      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

    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.

T. Ylonen, T. Kivinen, and M. Saarinen                          [page 8]

INTERNET-DRAFT                                           7 November 1997

      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.


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 client, and the client host name.

  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    client user name on the remote 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:

o  session identifier, and

o  packet payload without the signature.

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

T. Ylonen, T. Kivinen, and M. Saarinen                          [page 9]

INTERNET-DRAFT                                           7 November 1997

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.

The server may go into a "sleep" period after repeated unsuccesful
authentications to make key search harder.

If the transport layer does not provide encryption, authentication
methods that rely on secret data SHOULD be disabled.  If it does not
provide MAC protection, requests to change authentication data (e.g.
password change) SHOULD be disabled to avoid an attacker from modifying
the ciphertext without being noticed, rendering the new authentication
data unusable (denial of service).

Several authentication methods with different security characteristics
are allowed.  It is up to the server's local policy to decide which
methods (or combinations of methods) it is willing to accept for each
user.  Authentication is no stronger than the weakest combination

Special care should be taken when designing debug messages. These
messages may reveal surprising amounts of information about the host if
not properly designed. Debug messages can be disabled (during user
authentication phase) if high security is sought after.

8.  References

[RFC-1766] Alvestrand, H., "Tags for the Identification of Languages",
March 1995.

[RFC-2044] Yergeau, F., "UTF-8, a Transformation Format of Unicode and
ISO 10646", October 1996.

[SSH-ARCH] Ylonen, T., Kivinen, T, and Saarinen, M., "SSH Protocol
Architecture", Internet Draft, draft-secsh-architecture-00.txt

[SSH-TRANS] Ylonen, T., Kivinen, T, and Saarinen, M., "SSH Transport
Layer Protocol", Internet Draft, draft-secsh-transport-02.txt

[SSH-CONNECT] Ylonen, T., Kivinen, T, and Saarinen, M., "SSH Connection

T. Ylonen, T. Kivinen, and M. Saarinen                         [page 10]

INTERNET-DRAFT                                           7 November 1997

Protocol", Internet Draft, draft-secsh-connect-02.txt

9.  Author's Address

    Tatu Ylonen
    SSH Communications Security Ltd.
    Tekniikantie 12
    FIN-02150 ESPOO
    E-mail: ylo@ssh.fi

    Tero Kivinen
    SSH Communications Security Ltd.
    Tekniikantie 12
    FIN-02150 ESPOO
    E-mail: kivinen@ssh.fi

    Markku-Juhani O. Saarinen
    SSH Communications Security Ltd.
    Tekniikantie 12
    FIN-02150 ESPOO
    E-mail: mjos@ssh.fi

T. Ylonen, T. Kivinen, and M. Saarinen                         [page 11]

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