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Network Working Group                                    C. Lonvick, Ed.
Internet-Draft                                       Cisco Systems, Inc.
Expires: August 21, 2005                               February 17, 2005


                      SSH Authentication Protocol
                    draft-ietf-secsh-userauth-26.txt

Status of this Memo

   This document is an Internet-Draft and is subject to all provisions
   of Section 3 of RFC 3667.  By submitting this Internet-Draft, each
   author represents that any applicable patent or other IPR claims of
   which he or she is aware have been or will be disclosed, and any of
   which he or she become aware will be disclosed, in accordance with
   RFC 3668.

   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 August 21, 2005.

Copyright Notice

   Copyright (C) The Internet Society (2005).

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
   4.   The Authentication Protocol Framework  . . . . . . . . . . .   4
   5.   Authentication Requests  . . . . . . . . . . . . . . . . . .   5
     5.1  Responses to Authentication Requests . . . . . . . . . . .   6
     5.2  The "none" Authentication Request  . . . . . . . . . . . .   7
     5.3  Completion of User Authentication  . . . . . . . . . . . .   7
     5.4  Banner Message . . . . . . . . . . . . . . . . . . . . . .   7
   6.   Authentication Protocol Message Numbers  . . . . . . . . . .   8
   7.   Public Key Authentication Method: publickey  . . . . . . . .   8
   8.   Password Authentication Method: password . . . . . . . . . .  10
   9.   Host-Based Authentication: hostbased . . . . . . . . . . . .  12
   10.  IANA Considerations  . . . . . . . . . . . . . . . . . . . .  13
   11.  Security Considerations  . . . . . . . . . . . . . . . . . .  14
   12.  References . . . . . . . . . . . . . . . . . . . . . . . . .  14
     12.1   Normative  . . . . . . . . . . . . . . . . . . . . . . .  14
     12.2   Informative  . . . . . . . . . . . . . . . . . . . . . .  15
        Author's Address . . . . . . . . . . . . . . . . . . . . . .  15
        Intellectual Property and Copyright Statements . . . . . . .  16





























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1.  Contributors

   The major original contributors of this set of documents have been:
   Tatu Ylonen, Tero Kivinen, Timo J. Rinne, Sami Lehtinen (all of SSH
   Communications Security Corp), and Markku-Juhani O. Saarinen
   (University of Jyvaskyla).  Darren Moffit was the original editor of
   this set of documents and also made very substantial contributions.

   Additional contributors to this document include [need list].
   Listing their names here does not mean that they endorse this
   document, but that they have contributed to it.

   Comments on this internet draft should be sent to the IETF SECSH
   working group, details at:
   http://ietf.org/html.charters/secsh-charter.html Note: This paragraph
   will be removed before this document progresses to become an RFC.

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

   All documents related to the SSH protocols shall use the keywords
   "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD",
   "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" to describe
   requirements.  These keywords are to be interpreted as described in
   [RFC2119].

   The keywords "PRIVATE USE", "HIERARCHICAL ALLOCATION", "FIRST COME



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   FIRST SERVED", "EXPERT REVIEW", "SPECIFICATION REQUIRED", "IESG
   APPROVAL", "IETF CONSENSUS", and "STANDARDS ACTION" that appear in
   this document when used to describe namespace allocation are to be
   interpreted as described in [RFC2434].

   Protocol fields and possible values to fill them are defined in this
   set of documents.  Protocol fields will be defined in the message
   definitions.  As an example, SSH_MSG_CHANNEL_DATA is defined as
   follows.

        byte      SSH_MSG_CHANNEL_DATA
        uint32    recipient channel
        string    data

   Throughout these documents, when the fields are referenced, they will
   appear within single quotes.  When values to fill those fields are
   referenced, they will appear within double quotes.  Using the above
   example, possible values for 'data' are "foo" and "bar".

4.  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 exceeded, the server
   SHOULD disconnect.

   Additional thoughts about authentication timeouts and retries may be
   found in [ssh-1.2.30].



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5.  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
      string    service name in US-ASCII
      string    method name in US-ASCII
      The rest of the packet is method-specific.

   The 'user name' and 'service name' 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
   name' 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 name' does not exist, the server MAY
   disconnect, or MAY send a bogus list of acceptable authentication
   'method name' values, 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 name' 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
   name' 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.

   The following 'method name' values are defined.

   public key            REQUIRED
   password              OPTIONAL



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   hostbased             OPTIONAL
   none                  NOT RECOMMENDED

   Additional 'method name' values may be defined as specified in
   [SSH-ARCH] and [SSH-NUMBERS].

5.1  Responses to Authentication Requests

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

     byte         SSH_MSG_USERAUTH_FAILURE
     name-list    authentications that can continue
     boolean      partial success

   The 'authentications that can continue' is a comma-separated
   name-list of authentication 'method name' values that may
   productively continue the authentication dialog.

   It is RECOMMENDED that servers only include those 'method name'
   values in the name-list that are actually useful.  However, it is not
   illegal to include 'method name' values that cannot be used to
   authenticate the user.

   Already successfully completed authentications SHOULD NOT be included
   in the name-list, unless they really should be performed again for
   some reason.

   The value of '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



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   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 6).

5.2  The "none" Authentication Request

   A client may request a list of authentication 'method name' values
   that may continue by using the "none" authentication 'method name'.

   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 'method name' values that can continue.

   This 'method name' MUST NOT be listed as supported by the server.

5.3  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.

5.4  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
   after this authentication protocol starts and before authentication
   is successful.  This message contains text to be displayed to the
   client user before authentication is attempted.  The format is as
   follows:




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      byte      SSH_MSG_USERAUTH_BANNER
      string    message in ISO-10646 UTF-8 encoding
      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.

6.  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 to ease troubleshooting.

   After successful authentication, such messages are passed to the
   higher-level service.

   These are the general authentication message codes:

     SSH_MSG_USERAUTH_REQUEST            50
     SSH_MSG_USERAUTH_FAILURE            51
     SSH_MSG_USERAUTH_SUCCESS            52
     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.

7.  Public Key Authentication Method: publickey

   The only REQUIRED authentication 'method name' is public key
   authentication.  All implementations MUST support this method;
   however, not all users need to have public keys, and most local



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   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 in ISO-10646 UTF-8 encoding
      string    service name in US-ASCII
      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



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     string    user name
     string    service
     string    "publickey"
     boolean   TRUE
     string    public key algorithm name
     string    public key to be used for authentication
     string    signature

   The value of '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.

     SSH_MSG_USERAUTH_PK_OK              60


8.  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 in ISO-10646 UTF-8 encoding



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   Note that the 'plaintext password' value 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.

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

   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 any case the server MUST NOT allow an expired password to be used
   for authentication.
      byte      SSH_MSG_USERAUTH_PASSWD_CHANGEREQ
      string    prompt in ISO-10646 UTF-8 encoding
      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
     string    "password"
     boolean   TRUE
     string    plaintext old password in ISO-10646 UTF-8 encoding
     string    plaintext new password in ISO-10646 UTF-8 encoding



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   The server must reply to each 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.

     SSH_MSG_USERAUTH_PASSWD_CHANGEREQ   60


9.  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



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     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 expressed as the FQDN in US-ASCII
     string    user name on the client host in ISO-10646 UTF-8 encoding
     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 and certificates for client host' may include certificates.

   The value of '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 expressed as the FQDN in US-ASCII
     string    user name on the client host in ISO-10646 UTF-8 encoding

   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' value 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.

10.  IANA Considerations

   This document is part of a set.  The IANA considerations for the SSH
   protocol as defined in [SSH-ARCH], [SSH-TRANS], [SSH-CONNECT], and
   this document, are detailed in [SSH-NUMBERS].






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11.  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
   [SSH-ARCH].

12.  References

12.1  Normative

   [SSH-ARCH]
              Lonvick, C., "SSH Protocol Architecture",
              I-D draft-ietf-secsh-architecture-21.txt, February 2005.

   [SSH-CONNECT]
              Lonvick, C., "SSH Connection Protocol",
              I-D draft-ietf-secsh-connect-24.txt, February 2005.

   [SSH-TRANS]
              Lonvick, C., "SSH Transport Layer Protocol",
              I-D draft-ietf-secsh-transport-23.txt, February 2005.

   [SSH-NUMBERS]
              Lonvick, C., "SSH Protocol Assigned Numbers",
              I-D draft-ietf-secsh-assignednumbers-11.txt, February
              2005.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2434]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 2434,
              October 1998.

   [RFC3066]  Alvestrand, H., "Tags for the Identification of
              Languages", BCP 47, RFC 3066, January 2001.

   [RFC3629]  Yergeau, F., "UTF-8, a transformation format of ISO
              10646", STD 63, RFC 3629, November 2003.

   [I-D.ietf-sasl-saslprep]



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              Zeilenga, K., "SASLprep: Stringprep profile for user names
              and passwords",
              Internet-Draft draft-ietf-sasl-saslprep-10, July 2004.

12.2  Informative

   [ssh-1.2.30]
              Ylonen, T., "ssh-1.2.30/RFC", File within compressed
              tarball ftp://ftp.funet.fi/pub/unix/security/login/ssh/
              ssh-1.2.30.tar.gz, November 1995.


Author's Address

   Chris Lonvick (editor)
   Cisco Systems, Inc.
   12515 Research Blvd.
   Austin  78759
   USA

   Email: clonvick@cisco.com






























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Acknowledgment

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