Network Working Group                                          T. Ylonen
Internet-Draft                                                T. Kivinen
Expires: March 21, 2003                 SSH Communications Security Corp
                                                             M. Saarinen
                                                 University of Jyvaskyla
                                                                T. Rinne
                                                             S. Lehtinen
                                        SSH Communications Security Corp
                                                      September 20, 2002

                       SSH Protocol Architecture

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-

   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

   The list of Internet-Draft Shadow Directories can be accessed at

   This Internet-Draft will expire on March 21, 2003.

Copyright Notice

   Copyright (C) The Internet Society (2002).  All Rights Reserved.


   SSH is a protocol for secure remote login and other secure network
   services over an insecure network.  This document describes the
   architecture of the SSH protocol, as well as the notation and
   terminology used in SSH protocol documents.  It also discusses the
   SSH algorithm naming system that allows local extensions.  The SSH
   protocol consists of three major components: The Transport Layer

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   Protocol provides server authentication, confidentiality, and
   integrity with perfect forward secrecy.  The User Authentication
   Protocol authenticates the client to the server.  The Connection
   Protocol multiplexes the encrypted tunnel into several logical
   channels.  Details of these protocols are described in separate

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Specification of Requirements  . . . . . . . . . . . . . . . .  3
   3.  Architecture . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.1 Host Keys  . . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.2 Extensibility  . . . . . . . . . . . . . . . . . . . . . . . .  5
   3.3 Policy Issues  . . . . . . . . . . . . . . . . . . . . . . . .  5
   3.4 Security Properties  . . . . . . . . . . . . . . . . . . . . .  6
   3.5 Packet Size and Overhead . . . . . . . . . . . . . . . . . . .  6
   3.6 Localization and Character Set Support . . . . . . . . . . . .  7
   4.  Data Type Representations Used in the SSH Protocols  . . . . .  8
   5.  Algorithm Naming . . . . . . . . . . . . . . . . . . . . . . . 10
   6.  Message Numbers  . . . . . . . . . . . . . . . . . . . . . . . 10
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 11
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 12
   9.  Intellectual Property  . . . . . . . . . . . . . . . . . . . . 12
   10. Additional Information . . . . . . . . . . . . . . . . . . . . 12
       References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
       Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 14
       Full Copyright Statement . . . . . . . . . . . . . . . . . . . 15

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1. Introduction

   SSH is a protocol for secure remote login and other secure network
   services over an insecure network.  It consists of three major
   o  The Transport Layer Protocol [SSH-TRANS] provides server
      authentication, confidentiality, and integrity.  It may optionally
      also provide compression.  The transport layer will typically be
      run over a TCP/IP connection, but might also be used on top of any
      other reliable data stream.
   o  The User Authentication Protocol [SSH-USERAUTH] authenticates the
      client-side user to the server.  It runs over the transport layer
   o  The Connection Protocol [SSH-CONNECT] multiplexes the encrypted
      tunnel into several logical channels.  It runs over the user
      authentication protocol.

   The client sends a service request once a secure transport layer
   connection has been established.  A second service request is sent
   after user authentication is complete.  This allows new protocols to
   be defined and coexist with the protocols listed above.

   The connection protocol provides channels that can be used for a wide
   range of purposes.  Standard methods are provided for setting up
   secure interactive shell sessions and for forwarding ("tunneling")
   arbitrary TCP/IP ports and X11 connections.

2. Specification of Requirements

   All documents related to the SSH protocols shall use the keywords
   "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" to describe
   requirements.  They are to be interpreted as described in [RFC-2119].

3. Architecture

3.1 Host Keys

   Each server host SHOULD have a host key.  Hosts MAY have multiple
   host keys using multiple different algorithms.  Multiple hosts MAY
   share the same host key.  If a host has keys at all, it MUST have at
   least one key using each REQUIRED public key algorithm (currently DSS

   The server host key is used during key exchange to verify that the
   client is really talking to the correct server.  For this to be
   possible, the client must have a priori knowledge of the server's
   public host key.

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   Two different trust models can be used:
   o  The client has a local database that associates each host name (as
      typed by the user) with the corresponding public host key.  This
      method requires no centrally administered infrastructure, and no
      third-party coordination.  The downside is that the database of
      name-to-key associations may become burdensome to maintain.
   o  The host name-to-key association is certified by some trusted
      certification authority.  The client only knows the CA root key,
      and can verify the validity of all host keys certified by accepted

      The second alternative eases the maintenance problem, since
      ideally only a single CA key needs to be securely stored on the
      client.  On the other hand, each host key must be appropriately
      certified by a central authority before authorization is possible.
      Also, a lot of trust is placed on the central infrastructure.

   The protocol provides the option that the server name - host key
   association is not checked when connecting to the host for the first
   time.  This allows communication without prior communication of host
   keys or certification.  The connection still provides protection
   against passive listening; however, it becomes vulnerable to active
   man-in-the-middle attacks.  Implementations SHOULD NOT normally allow
   such connections by default, as they pose a potential security
   problem.  However, as there is no widely deployed key infrastructure
   available on the Internet yet, this option makes the protocol much
   more usable during the transition time until such an infrastructure
   emerges, while still providing a much higher level of security than
   that offered by older solutions (e.g.  telnet [RFC-854] and rlogin

   Implementations SHOULD try to make the best effort to check host
   keys.  An example of a possible strategy is to only accept a host key
   without checking the first time a host is connected, save the key in
   a local database, and compare against that key on all future
   connections to that host.

   Implementations MAY provide additional methods for verifying the
   correctness of host keys, e.g.  a hexadecimal fingerprint derived
   from the SHA-1 hash of the public key.  Such fingerprints can easily
   be verified by using telephone or other external communication

   All implementations SHOULD provide an option to not accept host keys
   that cannot be verified.

   We believe that ease of use is critical to end-user acceptance of
   security solutions, and no improvement in security is gained if the

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   new solutions are not used.  Thus, providing the option not to check
   the server host key is believed to improve the overall security of
   the Internet, even though it reduces the security of the protocol in
   configurations where it is allowed.

3.2 Extensibility

   We believe that the protocol will evolve over time, and some
   organizations will want to use their own encryption, authentication
   and/or key exchange methods.  Central registration of all extensions
   is cumbersome, especially for experimental or classified features.
   On the other hand, having no central registration leads to conflicts
   in method identifiers, making interoperability difficult.

   We have chosen to identify algorithms, methods, formats, and
   extension protocols with textual names that are of a specific format.
   DNS names are used to create local namespaces where experimental or
   classified extensions can be defined without fear of conflicts with
   other implementations.

   One design goal has been to keep the base protocol as simple as
   possible, and to require as few algorithms as possible.  However, all
   implementations MUST support a minimal set of algorithms to ensure
   interoperability (this does not imply that the local policy on all
   hosts would necessary allow these algorithms).  The mandatory
   algorithms are specified in the relevant protocol documents.

   Additional algorithms, methods, formats, and extension protocols can
   be defined in separate drafts.  See Section Algorithm Naming (Section
   5) for more information.

3.3 Policy Issues

   The protocol allows full negotiation of encryption, integrity, key
   exchange, compression, and public key algorithms and formats.
   Encryption, integrity, public key, and compression algorithms can be
   different for each direction.

   The following policy issues SHOULD be addressed in the configuration
   mechanisms of each implementation:
   o  Encryption, integrity, and compression algorithms, separately for
      each direction.  The policy MUST specify which is the preferred
      algorithm (e.g.  the first algorithm listed in each category).
   o  Public key algorithms and key exchange method to be used for host
      authentication.  The existence of trusted host keys for different
      public key algorithms also affects this choice.
   o  The authentication methods that are to be required by the server
      for each user.  The server's policy MAY require multiple

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      authentication for some or all users.  The required algorithms MAY
      depend on the location where the user is trying to log in from.
   o  The operations that the user is allowed to perform using the
      connection protocol.  Some issues are related to security; for
      example, the policy SHOULD NOT allow the server to start sessions
      or run commands on the client machine, and MUST NOT allow
      connections to the authentication agent unless forwarding such
      connections has been requested.  Other issues, such as which
      TCP/IP ports can be forwarded and by whom, are clearly issues of
      local policy.  Many of these issues may involve traversing or
      bypassing firewalls, and are interrelated with the local security

3.4 Security Properties

   The primary goal of the SSH protocol is improved security on the
   Internet.  It attempts to do this in a way that is easy to deploy,
   even at the cost of absolute security.
   o  All encryption, integrity, and public key algorithms used are
      well-known, well-established algorithms.
   o  All algorithms are used with cryptographically sound key sizes
      that are believed to provide protection against even the strongest
      cryptanalytic attacks for decades.
   o  All algorithms are negotiated, and in case some algorithm is
      broken, it is easy to switch to some other algorithm without
      modifying the base protocol.

   Specific concessions were made to make wide-spread fast deployment
   easier.  The particular case where this comes up is verifying that
   the server host key really belongs to the desired host; the protocol
   allows the verification to be left out (but this is NOT RECOMMENDED).
   This is believed to significantly improve usability in the short
   term, until widespread Internet public key infrastructures emerge.

3.5 Packet Size and Overhead

   Some readers will worry about the increase in packet size due to new
   headers, padding, and MAC.  The minimum packet size is in the order
   of 28 bytes (depending on negotiated algorithms).  The increase is
   negligible for large packets, but very significant for one-byte
   packets (telnet-type sessions).  There are, however, several factors
   that make this a non-issue in almost all cases:
   o  The minimum size of a TCP/IP header is 32 bytes.  Thus, the
      increase is actually from 33 to 51 bytes (roughly).
   o  The minimum size of the data field of an Ethernet packet is 46
      bytes [RFC-894].  Thus, the increase is no more than 5 bytes.
      When Ethernet headers are considered, the increase is less than 10

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   o  The total fraction of telnet-type data in the Internet is
      negligible, even with increased packet sizes.

   The only environment where the packet size increase is likely to have
   a significant effect is PPP [RFC-1134] over slow modem lines (PPP
   compresses the TCP/IP headers, emphasizing the increase in packet
   size).  However, with modern modems, the time needed to transfer is
   in the order of 2 milliseconds, which is a lot faster than people can

   There are also issues related to the maximum packet size.  To
   minimize delays in screen updates, one does not want excessively
   large packets for interactive sessions.  The maximum packet size is
   negotiated separately for each channel.

3.6 Localization and Character Set Support

   For the most part, the SSH protocols do not directly pass text that
   would be displayed to the user.  However, there are some places where
   such data might be passed.  When applicable, the character set for
   the data MUST be explicitly specified.  In most places, ISO 10646
   with UTF-8 encoding is used [RFC-2279].  When applicable, a field is
   also provided for a language tag [RFC-1766].

   One big issue is the character set of the interactive session.  There
   is no clear solution, as different applications may display data in
   different formats.  Different types of terminal emulation may also be
   employed in the client, and the character set to be used is
   effectively determined by the terminal emulation.  Thus, no place is
   provided for directly specifying the character set or encoding for
   terminal session data.  However, the terminal emulation type (e.g.
   "vt100") is transmitted to the remote site, and it implicitly
   specifies the character set and encoding.  Applications typically use
   the terminal type to determine what character set they use, or the
   character set is determined using some external means.  The terminal
   emulation may also allow configuring the default character set.  In
   any case, the character set for the terminal session is considered
   primarily a client local issue.

   Internal names used to identify algorithms or protocols are normally
   never displayed to users, and must be in US-ASCII.

   The client and server user names are inherently constrained by what
   the server is prepared to accept.  They might, however, occasionally
   be displayed in logs, reports, etc.  They MUST be encoded using ISO
   10646 UTF-8, but other encodings may be required in some cases.  It
   is up to the server to decide how to map user names to accepted user
   names.  Straight bit-wise binary comparison is RECOMMENDED.

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   For localization purposes, the protocol attempts to minimize the
   number of textual messages transmitted.  When present, such messages
   typically relate to errors, debugging information, or some externally
   configured data.  For data that is normally displayed, it SHOULD be
   possible to fetch a localized message instead of the transmitted
   message by using a numerical code.  The remaining messages SHOULD be

4. Data Type Representations Used in the SSH Protocols

      A byte represents an arbitrary 8-bit value (octet) [RFC-1700].
      Fixed length data is sometimes represented as an array of bytes,
      written byte[n], where n is the number of bytes in the array.


      A boolean value is stored as a single byte.  The value 0
      represents FALSE, and the value 1 represents TRUE.  All non-zero
      values MUST be interpreted as TRUE; however, applications MUST NOT
      store values other than 0 and 1.


      Represents a 32-bit unsigned integer.  Stored as four bytes in the
      order of decreasing significance (network byte order).  For
      example, the value 699921578 (0x29b7f4aa) is stored as 29 b7 f4


      Represents a 64-bit unsigned integer.  Stored as eight bytes in
      the order of decreasing significance (network byte order).


      Arbitrary length binary string.  Strings are allowed to contain
      arbitrary binary data, including null characters and 8-bit
      characters.  They are stored as a uint32 containing its length
      (number of bytes that follow) and zero (= empty string) or more
      bytes that are the value of the string.  Terminating null
      characters are not used.

      Strings are also used to store text.  In that case, US-ASCII is
      used for internal names, and ISO-10646 UTF-8 for text that might
      be displayed to the user.  The terminating null character SHOULD
      NOT normally be stored in the string.

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      For example, the US-ASCII string "testing" is represented as 00 00
      00 07 t e s t i n g.  The UTF8 mapping does not alter the encoding
      of US-ASCII characters.


      Represents multiple precision integers in two's complement format,
      stored as a string, 8 bits per byte, MSB first.  Negative numbers
      have the value 1 as the most significant bit of the first byte of
      the data partition.  If the most significant bit would be set for
      a positive number, the number MUST be preceded by a zero byte.
      Unnecessary leading bytes with the value 0 or 255 MUST NOT be
      included.  The value zero MUST be stored as a string with zero
      bytes of data.

      By convention, a number that is used in modular computations in
      Z_n SHOULD be represented in the range 0 <= x < n.

       value (hex)        representation (hex)
       0                  00 00 00 00
       9a378f9b2e332a7    00 00 00 08 09 a3 78 f9 b2 e3 32 a7
       80                 00 00 00 02 00 80
       -1234              00 00 00 02 ed cc
       -deadbeef          00 00 00 05 ff 21 52 41 11


      A string containing a comma separated list of names.  A name list
      is represented as a uint32 containing its length (number of bytes
      that follow) followed by a comma-separated list of zero or more
      names.  A name MUST be non-zero length, and it MUST NOT contain a
      comma (',').  Context may impose additional restrictions on the
      names; for example, the names in a list may have to be valid
      algorithm identifier (see Algorithm Naming below), or [RFC-1766]
      language tags.  The order of the names in a list may or may not be
      significant, also depending on the context where the list is is
      used.  Terminating NUL characters are not used, neither for the
      individual names, nor for the list as a whole.

       value              representation (hex)
       (), the empty list 00 00 00 00
       ("zlib")           00 00 00 04 7a 6c 69 62

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       ("zlib", "none")   00 00 00 09 7a 6c 69 62 2c 6e 6f 6e 65

5. Algorithm Naming

   The SSH protocols refer to particular hash, encryption, integrity,
   compression, and key exchange algorithms or protocols by names.
   There are some standard algorithms that all implementations MUST
   support.  There are also algorithms that are defined in the protocol
   specification but are OPTIONAL.  Furthermore, it is expected that
   some organizations will want to use their own algorithms.

   In this protocol, all algorithm identifiers MUST be printable US-
   ASCII non-empty strings no longer than 64 characters.  Names MUST be

   There are two formats for algorithm names:
   o  Names that do not contain an at-sign (@) are reserved to be
      assigned by IETF consensus (RFCs).  Examples include `3des-cbc',
      `sha-1', `hmac-sha1', and `zlib' (the quotes are not part of the
      name).  Names of this format MUST NOT be used without first
      registering them.  Registered names MUST NOT contain an at-sign
      (@) or a comma (,).
   o  Anyone can define additional algorithms by using names in the
      format name@domainname, e.g.  "".  The format
      of the part preceding the at sign is not specified; it MUST
      consist of US-ASCII characters except at-sign and comma.  The part
      following the at-sign MUST be a valid fully qualified internet
      domain name [RFC-1034] controlled by the person or organization
      defining the name.  It is up to each domain how it manages its
      local namespace.

6. Message Numbers

   SSH packets have message numbers in the range 1 to 255.  These
   numbers have been allocated as follows:

     Transport layer protocol:

       1 to 19    Transport layer generic (e.g. disconnect, ignore, debug,
       20 to 29   Algorithm negotiation
       30 to 49   Key exchange method specific (numbers can be reused for
                  different authentication methods)

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     User authentication protocol:

       50 to 59   User authentication generic
       60 to 79   User authentication method specific (numbers can be
                  reused for different authentication methods)

     Connection protocol:

       80 to 89   Connection protocol generic
       90 to 127  Channel related messages

     Reserved for client protocols:

       128 to 191 Reserved

     Local extensions:

       192 to 255 Local extensions

7. IANA Considerations

   Allocation of the following types of names in the SSH protocols is
   assigned by IETF consensus:
   o  encryption algorithm names,
   o  MAC algorithm names,
   o  public key algorithm names (public key algorithm also implies
      encoding and signature/encryption capability),
   o  key exchange method names, and
   o  protocol (service) names.

   These names MUST be printable US-ASCII strings, and MUST NOT contain
   the characters at-sign ('@'), comma (','), or whitespace or control
   characters (ASCII codes 32 or less).  Names are case-sensitive, and
   MUST NOT be longer than 64 characters.

   Names with the at-sign ('@') in them are allocated by the owner of
   DNS name after the at-sign (hierarchical allocation in [RFC-2343]),
   otherwise the same restrictions as above.

   Each category of names listed above has a separate namespace.
   However, using the same name in multiple categories SHOULD be avoided
   to minimize confusion.

   Message numbers (see Section Message Numbers (Section 6)) in the
   range of 0..191 should be allocated via IETF consensus; message
   numbers in the 192..255 range (the "Local extensions" set) are

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   reserved for private use.

8. Security Considerations

   Special care should be taken to ensure that all of the random numbers
   are of good quality.  The random numbers SHOULD be produced with safe
   mechanisms discussed in [RFC-1750].

   When displaying text, such as error or debug messages to the user,
   the client software SHOULD replace any control characters (except
   tab, carriage return and newline) with safe sequences to avoid
   attacks by sending terminal control characters.

   Not using MAC or encryption SHOULD be avoided.  The user
   authentication protocol is subject to man-in-the-middle attacks if
   the encryption is disabled.  The SSH protocol does not protect
   against message alteration if no MAC is used.

9. Intellectual Property

   The IETF takes no position regarding the validity or scope of any
   intellectual property or other rights that might be claimed to
   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

10. 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:


   [FIPS-186]      Federal Information Processing Standards Publication,

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                   ., "FIPS PUB 186, Digital Signature Standard", May

   [RFC0854]       Postel, J. and J. Reynolds, "Telnet Protocol
                   Specification", STD 8, RFC 854, May 1983.

   [RFC0894]       Hornig, C., "Standard for the transmission of IP
                   datagrams over Ethernet networks", STD 41, RFC 894,
                   Apr 1984.

   [RFC1034]       Mockapetris, P., "Domain names - concepts and
                   facilities", STD 13, RFC 1034, Nov 1987.

   [RFC1134]       Perkins, D., "Point-to-Point Protocol: A proposal for
                   multi-protocol transmission of datagrams over Point-
                   to-Point links", RFC 1134, Nov 1989.

   [RFC1282]       Kantor, B., "BSD Rlogin", RFC 1282, December 1991.

   [RFC1700]       Reynolds, J. and J. Postel, "Assigned Numbers", STD
                   2, RFC 1700, October 1994.

   [RFC1750]       Eastlake, D., Crocker, S. and J. Schiller,
                   "Randomness Recommendations for Security", RFC 1750,
                   December 1994.

   [RFC1766]       Alvestrand, H., "Tags for the Identification of
                   Languages", RFC 1766, March 1995.

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

   [RFC2279]       Yergeau, F., "UTF-8, a transformation format of ISO
                   10646", RFC 2279, January 1998.

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

   [SSH-ARCH]      Ylonen, T., "SSH Protocol Architecture", I-D draft-
                   ietf-architecture-13.txt, September 2002.

   [SSH-TRANS]     Ylonen, T., "SSH Transport Layer Protocol", I-D
                   draft-ietf-transport-15.txt, September 2002.

   [SSH-USERAUTH]  Ylonen, T., "SSH Authentication Protocol", I-D draft-
                   ietf-userauth-16.txt, September 2002.

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   [SSH-CONNECT]   Ylonen, T., "SSH Connection Protocol", I-D draft-
                   ietf-connect-16.txt, September 2002.

Authors' Addresses

   Tatu Ylonen
   SSH Communications Security Corp
   Fredrikinkatu 42
   HELSINKI  FIN-00100


   Tero Kivinen
   SSH Communications Security Corp
   Fredrikinkatu 42
   HELSINKI  FIN-00100


   Markku-Juhani O. Saarinen
   University of Jyvaskyla

   Timo J. Rinne
   SSH Communications Security Corp
   Fredrikinkatu 42
   HELSINKI  FIN-00100


   Sami Lehtinen
   SSH Communications Security Corp
   Fredrikinkatu 42
   HELSINKI  FIN-00100


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Full Copyright Statement

   Copyright (C) The Internet Society (2002).  All Rights Reserved.

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   The limited permissions granted above are perpetual and will not be
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   This document and the information contained herein is provided on an


   Funding for the RFC Editor function is currently provided by the
   Internet Society.

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