Network Working Group                                          T. Ylonen
Internet-Draft                                                T. Kivinen
Expires: January 12, 2004               SSH Communications Security Corp
                                                             M. Saarinen
                                                 University of Jyvaskyla
                                                                T. Rinne
                                                             S. Lehtinen
                                        SSH Communications Security Corp
                                                           July 14, 2003

                       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
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      The list of current Internet-Drafts can be accessed at

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      This Internet-Draft will expire on January 12, 2004.

Copyright Notice

      Copyright (C) The Internet Society (2003).  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

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      protocol consists of three major components: The Transport Layer
      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 . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.    Specification of Requirements  . . . . . . . . . . . . . . .  4
   3.    Architecture . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.1   Host Keys  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.2   Extensibility  . . . . . . . . . . . . . . . . . . . . . . .  6
   3.3   Policy Issues  . . . . . . . . . . . . . . . . . . . . . . .  6
   3.4   Security Properties  . . . . . . . . . . . . . . . . . . . .  7
   3.5   Packet Size and Overhead . . . . . . . . . . . . . . . . . .  7
   3.6   Localization and Character Set Support . . . . . . . . . . .  8
   4.    Data Type Representations Used in the SSH Protocols  . . . .  9
   5.    Algorithm Naming . . . . . . . . . . . . . . . . . . . . . . 11
   6.    Message Numbers  . . . . . . . . . . . . . . . . . . . . . . 12
   7.    IANA Considerations  . . . . . . . . . . . . . . . . . . . . 12
   8.    Security Considerations  . . . . . . . . . . . . . . . . . . 13
   8.1   Pseudo-Random Number Generation  . . . . . . . . . . . . . . 13
   8.2   Transport  . . . . . . . . . . . . . . . . . . . . . . . . . 14
   8.2.1 Confidentiality  . . . . . . . . . . . . . . . . . . . . . . 14
   8.2.2 Data Integrity . . . . . . . . . . . . . . . . . . . . . . . 17
   8.2.3 Replay . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
   8.2.4 Man-in-the-middle  . . . . . . . . . . . . . . . . . . . . . 18
   8.2.5 Denial-of-service  . . . . . . . . . . . . . . . . . . . . . 20
   8.2.6 Covert Channels  . . . . . . . . . . . . . . . . . . . . . . 21
   8.2.7 Forward Secrecy  . . . . . . . . . . . . . . . . . . . . . . 21
   8.3   Authentication Protocol  . . . . . . . . . . . . . . . . . . 21
   8.3.1 Weak Transport . . . . . . . . . . . . . . . . . . . . . . . 22
   8.3.2 Debug messages . . . . . . . . . . . . . . . . . . . . . . . 22
   8.3.3 Local security policy  . . . . . . . . . . . . . . . . . . . 23
   8.3.4 Public key authentication  . . . . . . . . . . . . . . . . . 23
   8.3.5 Password authentication  . . . . . . . . . . . . . . . . . . 24
   8.3.6 Host based authentication  . . . . . . . . . . . . . . . . . 24
   8.4   Connection protocol  . . . . . . . . . . . . . . . . . . . . 24
   8.4.1 End point security . . . . . . . . . . . . . . . . . . . . . 24
   8.4.2 Proxy forwarding . . . . . . . . . . . . . . . . . . . . . . 24
   8.4.3 X11 forwarding . . . . . . . . . . . . . . . . . . . . . . . 25
   9.    Intellectual Property  . . . . . . . . . . . . . . . . . . . 25
   10.   Additional Information . . . . . . . . . . . . . . . . . . . 26
         References . . . . . . . . . . . . . . . . . . . . . . . . . 26
         Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 29

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         Full Copyright Statement . . . . . . . . . . . . . . . . . . 31

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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 protocol.
      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-

   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 [FIPS-186]).

      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

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      public host key.

      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
      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 CAs.

         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

      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 [RFC-1282]).

      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 channels.

      All implementations SHOULD provide an option to not accept host

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      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 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

      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

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         preferred algorithm (e.g.  the first algorithm listed in each
      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
         authentication for some or all users.  The required algorithms
         MAY depend on the location where the user is trying to log in
      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 policy.

   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

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      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 percent.
      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 type.

      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-

      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

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      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

      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 configurable.

   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


         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).

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         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.

         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

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         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
       ("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

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

      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

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         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)

     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.

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      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
      reserved for private use.

   8. Security Considerations

      In order to make the entire body of Security Considerations more
      accessible, Security Considerations for the transport,
      authentication, and connection documents have been gathered here.

      The transport protocol [1] provides a confidential channel over an
      insecure network.  It performs server host authentication, key
      exchange, encryption, and integrity protection.  It also derives a
      unique session id that may be used by higher-level protocols.

      The authentication protocol [2] provides a suite of mechanisms
      which can be used to authenticate the client user to the server.
      Individual mechanisms specified in the in authentication protocol
      use the session id provided by the transport protocol and/or
      depend on the security and integrity guarantees of the transport

      The connection protocol [3] specifies a mechanism to multiplex
      multiple streams [channels] of data over the confidential and
      authenticated transport.  It also specifies channels for accessing
      an interactive shell, for 'proxy-forwarding' various external
      protocols over the secure transport (including arbitrary TCP/IP
      protocols), and for accessing secure 'subsystems' on the server

   8.1 Pseudo-Random Number Generation

      This protocol binds each session key to the session by including
      random, session specific data in the hash used to produce session

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      keys.  Special care should be taken to ensure that all of the
      random numbers are of good quality.  If the random data here
      (e.g., DH parameters) are pseudo-random then the pseudo-random
      number generator should be cryptographically secure (i.e., its
      next output not easily guessed even when knowing all previous
      outputs) and, furthermore, proper entropy needs to be added to the
      pseudo-random number generator.  RFC 1750 [1750] offers
      suggestions for sources of random numbers and entropy.
      Implementors should note the importance of entropy and the well-
      meant, anecdotal warning about the difficulty in properly
      implementing pseudo-random number generating functions.

      The amount of entropy available to a given client or server may
      sometimes be less than what is required.  In this case one must
      either resort to pseudo-random number generation regardless of
      insufficient entropy or refuse to run the protocol.  The latter is

   8.2 Transport

   8.2.1 Confidentiality

      It is beyond the scope of this document and the Secure Shell
      Working Group to analyze or recommend specific ciphers other than
      the ones which have been established and accepted within the
      industry.  At the time of this writing, ciphers commonly in use
      include 3DES, ARCFOUR, twofish, serpent and blowfish.  AES has
      been accepted by The published as a US Federal Information
      Processing Standards [FIPS-197] and the cryptographic community as
      being acceptable for this purpose as well has accepted AES.  As
      always, implementors and users should check current literature to
      ensure that no recent vulnerabilities have been found in ciphers
      used within products.  Implementors should also check to see which
      ciphers are considered to be relatively stronger than others and
      should recommend their use to users over relatively weaker
      ciphers.  It would be considered good form for an implementation
      to politely and unobtrusively notify a user that a stronger cipher
      is available and should be used when a weaker one is actively

      The "none" cipher is provided for debugging and SHOULD NOT be used
      except for that purpose.  It's cryptographic properties are
      sufficiently described in RFC 2410, which will show that its use
      does not meet the intent of this protocol.

      The relative merits of these and other ciphers may also be found
      in current literature.  Two references that may provide
      information on the subject are [SCHNEIER] and

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      [KAUFMAN,PERLMAN,SPECINER].  Both of these describe the CBC mode
      of operation of certain ciphers and the weakness of this scheme.
      Essentially, this mode is theoretically vulnerable to chosen
      cipher-text attacks because of the high predictability of the
      start of packet sequence.  However, this attack is still deemed
      difficult and not considered fully practicable especially if
      relatively longer block sizes are used.

      Additionally, another CBC mode attack may be mitigated through the
      insertion of packets containing SSH_MSG_IGNORE.  Without this
      technique, a specific attack may be successful.  For this attack
      (commonly known as the Rogaway attack
      [ROGAWAY],[DAI],[BELLARE,KOHNO,NAMPREMPRE]) to work, the attacker
      would need to know the IV of the next block that is going to be
      encrypted.  In CBC mode that is the output of the encryption of
      the previous block.  If the attacker does not have any way to see
      the packet yet (i.e it is in the internal buffers of the ssh
      implementation or even in the kernel) then this attack will not
      work.  If the last packet has been sent out to the network (i.e
      the attacker has access to it) then he can use the attack.

      In the optimal case an implementor would need to add an extra
      packet only if the packet has been sent out onto the network and
      there are no other packets waiting for transmission.  Implementors
      may wish to check to see if there are any unsent packets awaiting
      transmission, but unfortunately it is not normally easy to obtain
      this information from the kernel or buffers.  If there are not,
      then a packet containing SSH_MSG_IGNORE SHOULD be sent.  If a new
      packet is added to the stream every time the attacker knows the IV
      that is supposed to be used for the next packet, then the attacker
      will not be able to guess the correct IV, thus the attack will
      never be successfull.

      As an example, consider the following case:

         Client                                                  Server
         ------                                                  ------
         TCP(seq=x, len=500)            ->
         contains Record 1

                             [500 ms passes, no ACK]

        TCP(seq=x, len=1000)            ->
         contains Records 1,2


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      1.  The Nagle algorithm + TCP retransmits mean that the two
          records get coalesced into a single TCP segment
      2.  Record 2 is *not* at the beginning of the TCP segment and
          never will be, since it gets ACKed.
      3.  Yet, the attack is possible because Record 1 has already been

      As this example indicates, it's totally unsafe to use the
      existence of unflushed data in the TCP buffers proper as a guide
      to whether you need an empty packet, since when you do the second
      write(), the buffers will contain the un-ACKed Record 1.

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      On the other hand, it's perfectly safe to have the following

         Client                                                  Server
         ------                                                  ------
         TCP(seq=x, len=500)           ->
            contains SSH_MSG_IGNORE

         TCP(seq=y, len=500)           ->
            contains Data

      Provided that the IV for second SSH Record is fixed after the data for
      the Data packet is determined -i.e. you do:
           read from user
           encrypt null packet
           encrypt data packet

   8.2.2 Data Integrity

      This protocol does allow the Data Integrity mechanism to be
      disabled.  Implementors SHOULD be wary of exposing this feature
      for any purpose other than debugging.  Users and administrators
      SHOULD be explicitly warned anytime the "none" MAC is enabled.

      So long as the "none" MAC is not used, this protocol provides data

      Because MACs use a 32 bit sequence number, they might start to
      leak information after 2**32 packets have been sent.  However,
      following the rekeying recommendations should prevent this attack.
      The transport protocol [1] recommends rekeying after one gigabyte
      of data, and the smallest possible packet is 16 bytes.  Therefore,
      rekeying SHOULD happen after 2**28 packets at the very most.

   8.2.3 Replay

      The use of a MAC other than 'none' provides integrity and
      authentication.  In addition, the transport protocol provides a
      unique session identifier (bound in part to pseudo-random data
      that is part of the algorithm and key exchange process) that can
      be used by higher level protocols to bind data to a given session
      and prevent replay of data from prior sessions.  For example, the
      authentication protocol uses this to prevent replay of signatures
      from previous sessions.  Because public key authentication
      exchanges are cryptographically bound to the session (i.e., to the
      initial key exchange) they cannot be successfully replayed in

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      other sessions.  Note that the session ID can be made public
      without harming the security of the protocol.

      If two session happen to have the same session ID [hash of key
      exchanges] then packets from one can be replayed against the
      other.  It must be stressed that the chances of such an occurrence
      are, needless to say, minimal when using modern cryptographic
      methods.  This is all the more so true when specifying larger hash
      function outputs and DH parameters.

      Replay detection using monotonically increasing sequence numbers
      as input to the MAC, or HMAC in some cases, is described in RFC
      2085 [2085], RFC 2246 [2246], RFC 2743 [2743], RFC 1964 [1964],
      RFC 2025 [2025], and RFC 1510 [1510].  The underlying construct is
      discussed in RFC 2104 [2104].  Essentially a different sequence
      number in each packet ensures that at least this one input to the
      MAC function will be unique and will provide a nonrecurring MAC
      output that is not predictable to an attacker.  If the session
      stays active long enough, however, this sequence number will wrap.
      This event may provide an attacker an opportunity to replay a
      previously recorded packet with an identical sequence number but
      only if the peers have not rekeyed since the transmission of the
      first packet with that sequence number.  If the peers have
      rekeyed, then the replay will be detected as the MAC check will
      fail.  For this reason, it must be emphasized that peers MUST
      rekey before a wrap of the sequence numbers.  Naturally, if an
      attacker does attempt to replay a captured packet before the peers
      have rekeyed, then the receiver of the duplicate packet will not
      be able to validate the MAC and it will be discarded.  The reason
      that the MAC will fail is because the receiver will formulate a
      MAC based upon the packet contents, the shared secret, and the
      expected sequence number.  Since the replayed packet will not be
      using that expected sequence number (the sequence number of the
      replayed packet will have already been passed by the receiver)
      then the calculated MAC will not match the MAC received with the

   8.2.4 Man-in-the-middle

      This protocol makes no assumptions nor provisions for an
      infrastructure or means for distributing the public keys of hosts.
      It is expected that this protocol will sometimes be used without
      first verifying the association between the server host key and
      the server host name.  Such usage is vulnerable to man-in-the-
      middle attacks.  This section describes this and encourages
      administrators and users to understand the importance of verifying
      this association before any session is initiated.

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      There are three cases of man-in-the-middle attacks to consider.
      The first is where an attacker places a device between the client
      and the server before the session is initiated.  In this case, the
      attack device is trying to mimic the legitimate server and will
      offer its public key to the client when the client initiates a
      session.  If it were to offer the public key of the server, then
      it would not be able to decrypt or sign the transmissions between
      the legitimate server and the client unless it also had access to
      the private-key of the host.  The attack device will also,
      simultaneously to this, initiate a session to the legitimate
      server masquerading itself as the client.  If the public key of
      the server had been securely distributed to the client prior to
      that session initiation, the key offered to the client by the
      attack device will not match the key stored on the client.  In
      that case, the user SHOULD be given a warning that the offered
      host key does not match the host key cached on the client.  As
      described in Section 3.1 of [ARCH], the user may be free to accept
      the new key and continue the session.  It is RECOMMENDED that the
      warning provide sufficient information to the user of the client
      device so they may make an informed decision.  If the user chooses
      to continue the session with the stored public-key of the server
      (not the public-key offered at the start of the session), then the
      session specific data between the attacker and server will be
      different between the client-to-attacker session and the attacker-
      to-server sessions due to the randomness discussed above.  From
      this, the attacker will not be able to make this attack work since
      the attacker will not be able to correctly sign packets containing
      this session specific data from the server since he does not have
      the private key of that server.

      The second case that should be considered is similar to the first
      case in that it also happens at the time of connection but this
      case points out the need for the secure distribution of server
      public keys.  If the server public keys are not securely
      distributed then the client cannot know if it is talking to the
      intended server.  An attacker may use social engineering
      techniques to pass off server keys to unsuspecting users and may
      then place a man-in-the-middle attack device between the
      legitimate server and the clients.  If this is allowed to happen
      then the clients will form client-to-attacker sessions and the
      attacker will form attacker-to-server sessions and will be able to
      monitor and manipulate all of the traffic between the clients and
      the legitimate servers.  Server administrators are encouraged to
      make host key fingerprints available for checking by some means
      whose security does not rely on the integrity of the actual host
      keys.  Possible mechanisms are discussed in Section 3.1 of [SSH-
      ARCH] and may also include secured Web pages, physical pieces of
      paper, etc.  Implementors SHOULD provide recommendations on how

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      best to do this with their implementation.  Because the protocol
      is extensible, future extensions to the protocol may provide
      better mechanisms for dealing with the need to know the server's
      host key before connecting.  For example, making the host key
      fingerprint available through a secure DNS lookup, or using
      kerberos over gssapi during key exchange to authenticate the
      server are possibilities.

      In the third man-in-the-middle case, attackers may attempt to
      manipulate packets in transit between peers after the session has
      been established.  As described in the Replay part of this
      section, a successful attack of this nature is very improbable.
      As in the Replay section, this reasoning does assume that the MAC
      is secure and that it is infeasible to construct inputs to a MAC
      algorithm to give a known output.  This is discussed in much
      greater detail in Section 6 of RFC 2104.  If the MAC algorithm has
      a vulnerability or is weak enough, then the attacker may be able
      to specify certain inputs to yield a known MAC.  With that they
      may be able to alter the contents of a packet in transit.
      Alternatively the attacker may be able to exploit the algorithm
      vulnerability or weakness to find the shared secret by reviewing
      the MACs from captured packets.  In either of those cases, an
      attacker could construct a packet or packets that could be
      inserted into an SSH stream.  To prevent that, implementors are
      encouraged to utilize commonly accepted MAC algorithms and
      administrators are encouraged to watch current literature and
      discussions of cryptography to ensure that they are not using a
      MAC algorithm that has a recently found vulnerability or weakness.

      In summary, the use of this protocol without a reliable
      association of the binding between a host and its host keys is
      inherently insecure and is NOT RECOMMENDED.  It may however be
      necessary in non-security critical environments, and will still
      provide protection against passive attacks.  Implementors of
      protocols and applications running on top of this protocol should
      keep this possibility in mind.

   8.2.5 Denial-of-service

      This protocol is designed to be used over a reliable transport.
      If transmission errors or message manipulation occur, the
      connection is closed.  The connection SHOULD be re-established if
      this occurs.  Denial of service attacks of this type ("wire
      cutter") are almost impossible to avoid.

      In addition, this protocol is vulnerable to Denial of Service
      attacks because an attacker can force the server to go through the
      CPU and memory intensive tasks of connection setup and key

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      exchange without authenticating.  Implementors SHOULD provide
      features that make this more difficult.  For example, only
      allowing connections from a subset of IPs known to have valid

   8.2.6 Covert Channels

      The protocol was not designed to eliminate covert channels.  For
      example, the padding, SSH_MSG_IGNORE messages, and several other
      places in the protocol can be used to pass covert information, and
      the recipient has no reliable way to verify whether such
      information is being sent.

   8.2.7 Forward Secrecy

      It should be noted that the Diffie-Hellman key exchanges may
      provide perfect forward secrecy (PFS).  PFS is essentially defined
      as the cryptographic property of a key-establishment protocol in
      which the compromise of a session key or long-term private key
      after a given session does not cause the compromise of any earlier
      session.  [ANSI T1.523-2001]  SSHv2 sessions resulting from a key
      exchange using diffie-hellman-group1-sha1 are secure even if
      private keying/authentication material is later revealed, but not
      if the session keys are revealed.  So, given this definition of
      PFS, SSHv2 does have PFS.  It is hoped that all other key exchange
      mechanisms proposed and used in the future will also provide PFS.
      This property is not commuted to any of the applications or
      protocols using SSH as a transport however.  The transport layer
      of SSH provides confidentiality for password authentication and
      other methods that rely on secret data.

      Of course, if the DH private parameters for the client and server
      are revealed then the session key is revealed, but these items can
      be thrown away after the key exchange completes.  It's worth
      pointing out that these items should not be allowed to end up on
      swap space and that they should be erased from memory as soon as
      the key exchange completes.

   8.3 Authentication Protocol

      The purpose of this protocol is to perform client user
      authentication.  It assumes that this run 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.

      Several authentication methods with different security
      characteristics are allowed.  It is up to the server's local

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      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 allowed.

      The server may go into a "sleep" period after repeated
      unsuccessful authentication attempts to make key search more
      difficult for attackers.  Care should be taken so that this
      doesn't become a self-denial of service vector.

   8.3.1 Weak Transport

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

      The assumption as stated above that the Authentication Protocol
      only run over a secure transport that has previously authenticated
      the server is very important to note.  People deploying SSH are
      reminded of the consequences of man-in-the-middle attacks if the
      client does not have a very strong a priori association of the
      server with the host key of that server.  Specifically for the
      case of the Authentication Protocol the client may form a session
      to a man-in-the-middle attack device and divulge user credentials
      such as their username and password.  Even in the cases of
      authentication where no user credentials are divulged, an attacker
      may still gain information they shouldn't have by capturing key-
      strokes in much the same way that a honeypot works.

   8.3.2 Debug messages

      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 required.
      Administrators of host machines should make all attempts to
      compartmentalize all event notification messages and protect them
      from unwarranted observation.  Developers should be aware of the
      sensitive nature of some of the normal event messages and debug
      messages and may want to provide guidance to administrators on
      ways to keep this information away from unauthorized people.
      Developers should consider minimizing the amount of sensitive
      information obtainable by users during the authentication phase in
      accordance with the local policies.  For this reason, it is
      RECOMMENDED that debug messages be initially disabled at the time

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      of deployment and require an active decision by an administrator
      to allow them to be enabled.  It is also RECOMMENDED that a
      message expressing this concern be presented to the administrator
      of a system when the action is taken to enable debugging messages.

   8.3.3 Local security policy

      Implementer MUST ensure that the credentials provided validate the
      professed user and also MUST ensure that the local policy of the
      server permits the user the access requested.  In particular,
      because of the flexible nature of the SSH connection protocol, it
      may not be possible to determine the local security policy, if
      any, that should apply at the time of authentication because the
      kind of service being requested is not clear at that instant.  For
      example, local policy might allow a user to access files on the
      server, but not start an interactive shell.  However, during the
      authentication protocol, it is not known whether the user will be
      accessing files or attempting to use an interactive shell, or even
      both.  In any event, where local security policy for the server
      host exists, it MUST be applied and enforced correctly.

      Implementors are encouraged to provide a default local policy and
      make its parameters known to administrators and users.  At the
      discretion of the implementors, this default policy may be along
      the lines of 'anything goes' where there are no restrictions
      placed upon users, or it may be along the lines of 'excessively
      restrictive' in which case the administrators will have to
      actively make changes to this policy to meet their needs.
      Alternatively, it may be some attempt at providing something
      practical and immediately useful to the administrators of the
      system so they don't have to put in much effort to get SSH
      working.  Whatever choice is made MUST be applied and enforced as
      required above.

   8.3.4 Public key authentication

      The use of public-key authentication assumes that the client host
      has not been compromised.

      This risk can be mitigated by the use of passphrases on private
      keys; however, this is not an enforceable policy.  The use of
      smartcards, or other technology to make passphrases an enforceable
      policy is suggested.

      The server could require both password and public-key
      authentication, however, this requires the client to expose its
      password to the server (see section on password authentication

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   8.3.5 Password authentication

      The password mechanism as specified in the authentication protocol
      assumes that the server has not been compromised.  If the server
      has been compromised, using password authentication will reveal a
      valid username / password combination to the attacker, which may
      lead to further compromises.

      This vulnerability can be mitigated by using an alternative form
      of authentication.  For example, public-key authentication makes
      no assumptions about security on the server.

   8.3.6 Host based authentication

      Host based authentication assumes that the client has not been
      compromised.  There are no mitigating strategies, other than to
      use host based authentication in combination with another
      authentication method.

   8.4 Connection protocol

   8.4.1 End point security

      End point security is assumed by the connection protocol.  If the
      server has been compromised, any terminal sessions, port
      forwarding, or systems accessed on the host are compromised.
      There are no mitigating factors for this.

      If the client end point has been compromised, and the server fails
      to stop the attacker at the authentication protocol, all services
      exposed (either as subsystems or through forwarding) will be
      vulnerable to attack.  Implementors SHOULD provide mechanisms for
      administrators to control which services are exposed to limit the
      vulnerability of other services.

      These controls might include controlling which machines and ports
      can be target in 'port-forwarding' operations, which users are
      allowed to use interactive shell facilities, or which users are
      allowed to use exposed subsystems.

   8.4.2 Proxy forwarding

      The SSH connection protocol allows for proxy forwarding of other
      protocols such as SNMP, POP3, and HTTP.  This may be a concern for
      network administrators who wish to control the access of certain
      applications by users located outside of their physical location.
      Essentially, the forwarding of these protocols may violate site
      specific security policies as they may be undetectably tunneled

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      through a firewall.  Implementors SHOULD provide an administrative
      mechanism to control the proxy forwarding functionality so that
      site specific security policies may be upheld.

      In addition, a reverse proxy forwarding functionality is
      available, which again can be used to bypass firewall controls.

      As indicated above, end-point security is assumed during proxy
      forwarding operations.  Failure of end-point security will
      compromise all data passed over proxy forwarding.

   8.4.3 X11 forwarding

      Another form of proxy forwarding provided by the ssh connection
      protocol is the forwarding of the X11 protocol.  If end-point
      security has been compromised, X11 forwarding may allow attacks
      against the X11 server.  Users and administrators should, as a
      matter of course, use appropriate X11 security mechanisms to
      prevent unauthorized use of the X11 server.  Implementors,
      administrators and users who wish to further explore the security
      mechanisms of X11 are invited to read [SCHEIFLER] and analyze
      previously reported problems with the interactions between SSH
      forwarding and X11 in CERT vulnerabilities VU#363181 and VU#118892

      X11 display forwarding with SSH, by itself, is not sufficient to
      correct well known problems with X11 security [VENEMA].  However,
      X11 display forwarding in SSHv2 (or other, secure protocols),
      combined with actual and pseudo-displays which accept connections
      only over local IPC mechanisms authorized by permissions or ACLs,
      does correct many X11 security problems as long as the "none" MAC
      is not used.  It is RECOMMENDED that X11 display implementations
      default to allowing display opens only over local IPC.  It is
      RECOMMENDED that SSHv2 server implementations that support X11
      forwarding default to allowing display opens only over local IPC.
      On single-user systems it might be reasonable to default to
      allowing local display opens over TCP/IP.

      Implementors of the X11 forwarding protocol SHOULD implement the
      magic cookie access checking spoofing mechanism as described in
      [ssh-connect] as an additional mechanism to prevent unauthorized
      use of the proxy.

   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

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      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

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

      [FIPS-197]                  National Institue of Standards and
                                  Technology, ., "FIPS 197,
                                  Specification for the Advanced
                                  Encryption Standard", November 2001.

      [ANSI T1.523-2001]          American National Standards Insitute,
                                  Inc., "Telecom Glossary 2000",
                                  February 2001.

      [SCHEIFLER]                 Scheifler, R., "X Window System : The
                                  Complete Reference to Xlib, X
                                  Protocol, Icccm, Xlfd, 3rd edition.",
                                  Digital Press ISBN 1555580882,
                                  Feburary 1992.

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

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Internet-Draft          SSH Protocol Architecture              July 2003

                                  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.

      [RFC1510]                   Kohl, J. and C. Neuman, "The Kerberos
                                  Network Authentication Service (V5)",
                                  RFC 1510, September 1993.

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

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

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

      [RFC1964]                   Linn, J., "The Kerberos Version 5 GSS-
                                  API Mechanism", RFC 1964, June 1996.

      [RFC2025]                   Adams, C., "The Simple Public-Key GSS-
                                  API Mechanism (SPKM)", RFC 2025,
                                  October 1996.

      [RFC2085]                   Oehler, M. and R. Glenn, "HMAC-MD5 IP
                                  Authentication with Replay
                                  Prevention", RFC 2085, February 1997.

      [RFC2104]                   Krawczyk, H., Bellare, M. and R.

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                                  Canetti, "HMAC: Keyed-Hashing for
                                  Message Authentication", RFC 2104,
                                  February 1997.

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

      [RFC2246]                   Dierks, T. and C. Allen, "The TLS
                                  Protocol Version 1.0", RFC 2246,
                                  January 1999.

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

      [RFC2410]                   Glenn, R. and S. Kent, "The NULL
                                  Encryption Algorithm and Its Use With
                                  IPsec", RFC 2410, November 1998.

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

      [RFC2743]                   Linn, J., "Generic Security Service
                                  Application Program Interface Version
                                  2, Update 1", RFC 2743, January 2000.

      [SSH-ARCH]                  Ylonen, T., "SSH Protocol
                                  Architecture", I-D draft-ietf-
                                  architecture-14.txt, July 2003.

      [SSH-TRANS]                 Ylonen, T., "SSH Transport Layer
                                  Protocol", I-D draft-ietf-transport-
                                  16.txt, July 2003.

      [SSH-USERAUTH]              Ylonen, T., "SSH Authentication
                                  Protocol", I-D draft-ietf-userauth-
                                  17.txt, July 2003.

      [SSH-CONNECT]               Ylonen, T., "SSH Connection Protocol",
                                  I-D draft-ietf-connect-17.txt, July

      [SSH-NUMBERS]               Lehtinen, S. and D. Moffat, "SSH
                                  Protocol Assigned Numbers", I-D draft-

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                                  July 2003.

      [SCHNEIER]                  Schneier, B., "Applied Cryptography
                                  Second Edition: protocols algorithms
                                  and source in code in C", 1996.

      [KAUFMAN,PERLMAN,SPECINER]  Kaufman, C., Perlman, R. and M.
                                  Speciner, "Network Security: PRIVATE
                                  Communication in a PUBLIC World",

      [CERT]                      CERT Coordination Center, The.,

      [VENEMA]                    Venema, W., "Murphy's Law and Computer
                                  Security", Proceedings of 6th USENIX
                                  Security Symposium, San Jose CA
                                  , July 1996.

      [ROGAWAY]                   Rogaway, P., "Problems with Proposed
                                  IP Cryptography", Unpublished paper
                                  , 1996.

      [DAI]                       Dai, W., "An attack against SSH2
                                  protocol", Email to the SECSH Working
                                  archive/secsh/2002-02.mail, Feb 2002.

      [BELLARE,KOHNO,NAMPREMPRE]  Bellaire, M., Kohno, T. and C.
                                  Namprempre, "Authenticated Encryption
                                  in SSH: Fixing the SSH Binary Packet
                                  Protocol", , Sept 2002.

Authors' Addresses

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


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Internet-Draft          SSH Protocol Architecture              July 2003

   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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