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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 Transport Layer Protocol
                   draft-ietf-secsh-transport-15.txt

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-
   Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt.

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

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

Copyright Notice

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

Abstract

   SSH is a protocol for secure remote login and other secure network
   services over an insecure network.

   This document describes the SSH transport layer protocol which
   typically runs on top of TCP/IP.  The protocol can be used as a basis
   for a number of secure network services.  It provides strong



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   encryption, server authentication, and integrity protection.  It may
   also provide compression.

   Key exchange method, public key algorithm, symmetric encryption
   algorithm, message authentication algorithm, and hash algorithm are
   all negotiated.

   This document also describes the Diffie-Hellman key exchange method
   and the minimal set of algorithms that are needed to implement the
   SSH transport layer protocol.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Conventions Used in This Document  . . . . . . . . . . . . . .  3
   3.  Connection Setup . . . . . . . . . . . . . . . . . . . . . . .  3
   3.1 Use over TCP/IP  . . . . . . . . . . . . . . . . . . . . . . .  3
   3.2 Protocol Version Exchange  . . . . . . . . . . . . . . . . . .  3
   3.3 Compatibility With Old SSH Versions  . . . . . . . . . . . . .  4
   3.4 Old Client, New Server . . . . . . . . . . . . . . . . . . . .  4
   3.5 New Client, Old Server . . . . . . . . . . . . . . . . . . . .  5
   4.  Binary Packet Protocol . . . . . . . . . . . . . . . . . . . .  5
   4.1 Maximum Packet Length  . . . . . . . . . . . . . . . . . . . .  6
   4.2 Compression  . . . . . . . . . . . . . . . . . . . . . . . . .  6
   4.3 Encryption . . . . . . . . . . . . . . . . . . . . . . . . . .  7
   4.4 Data Integrity . . . . . . . . . . . . . . . . . . . . . . . .  9
   4.5 Key Exchange Methods . . . . . . . . . . . . . . . . . . . . . 10
   4.6 Public Key Algorithms  . . . . . . . . . . . . . . . . . . . . 10
   5.  Key Exchange . . . . . . . . . . . . . . . . . . . . . . . . . 13
   5.1 Algorithm Negotiation  . . . . . . . . . . . . . . . . . . . . 13
   5.2 Output from Key Exchange . . . . . . . . . . . . . . . . . . . 16
   5.3 Taking Keys Into Use . . . . . . . . . . . . . . . . . . . . . 17
   6.  Diffie-Hellman Key Exchange  . . . . . . . . . . . . . . . . . 17
   6.1 diffie-hellman-group1-sha1 . . . . . . . . . . . . . . . . . . 19
   7.  Key Re-Exchange  . . . . . . . . . . . . . . . . . . . . . . . 19
   8.  Service Request  . . . . . . . . . . . . . . . . . . . . . . . 20
   9.  Additional Messages  . . . . . . . . . . . . . . . . . . . . . 21
   9.1 Disconnection Message  . . . . . . . . . . . . . . . . . . . . 21
   9.2 Ignored Data Message . . . . . . . . . . . . . . . . . . . . . 22
   9.3 Debug Message  . . . . . . . . . . . . . . . . . . . . . . . . 22
   9.4 Reserved Messages  . . . . . . . . . . . . . . . . . . . . . . 23
   10. Summary of Message Numbers . . . . . . . . . . . . . . . . . . 23
   11. Security Considerations  . . . . . . . . . . . . . . . . . . . 23
   12. Intellectual Property  . . . . . . . . . . . . . . . . . . . . 25
   13. Additional Information . . . . . . . . . . . . . . . . . . . . 25
       References . . . . . . . . . . . . . . . . . . . . . . . . . . 25
       Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 27
       Full Copyright Statement . . . . . . . . . . . . . . . . . . . 28



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1. Introduction

   The SSH transport layer is a secure low level transport protocol.  It
   provides strong encryption, cryptographic host authentication, and
   integrity protection.

   Authentication in this protocol level is host-based; this protocol
   does not perform user authentication.  A higher level protocol for
   user authentication can be designed on top of this protocol.

   The protocol has been designed to be simple, flexible, to allow
   parameter negotiation, and to minimize the number of round-trips.
   Key exchange method, public key algorithm, symmetric encryption
   algorithm, message authentication algorithm, and hash algorithm are
   all negotiated.  It is expected that in most environments, only 2
   round-trips will be needed for full key exchange, server
   authentication, service request, and acceptance notification of
   service request.  The worst case is 3 round-trips.

2. Conventions Used in This Document

   The keywords "MUST", "MUST NOT", "REQUIRED", "SHOULD", "SHOULD NOT",
   and "MAY" that appear in this document are to be interpreted as
   described in [RFC2119]

   The used data types and terminology are specified in the architecture
   document [SSH-ARCH]

   The architecture document also discusses the algorithm naming
   conventions that MUST be used with the SSH protocols.

3. Connection Setup

   SSH works over any 8-bit clean, binary-transparent transport.  The
   underlying transport SHOULD protect against transmission errors as
   such errors cause the SSH connection to terminate.

   The client initiates the connection.

3.1 Use over TCP/IP

   When used over TCP/IP, the server normally listens for connections on
   port 22.  This port number has been registered with the IANA, and has
   been officially assigned for SSH.

3.2 Protocol Version Exchange

   When the connection has been established, both sides MUST send an



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   identification string of the form "SSH-protoversion-softwareversion
   comments", followed by carriage return and newline characters (ASCII
   13 and 10, respectively).  Both sides MUST be able to process
   identification strings without carriage return character.  No null
   character is sent.  The maximum length of the string is 255
   characters, including the carriage return and newline.

   The part of the identification string preceding carriage return and
   newline is used in the Diffie-Hellman key exchange (see Section
   Section 6).

   The server MAY send other lines of data before sending the version
   string.  Each line SHOULD be terminated by a carriage return and
   newline.  Such lines MUST NOT begin with "SSH-", and SHOULD be
   encoded in ISO-10646 UTF-8 [RFC2279] (language is not specified).
   Clients MUST be able to process such lines; they MAY be silently
   ignored, or MAY be displayed to the client user; if they are
   displayed, control character filtering discussed in [SSH-ARCH] SHOULD
   be used.  The primary use of this feature is to allow TCP-wrappers to
   display an error message before disconnecting.

   Version strings MUST consist of printable US-ASCII characters, not
   including whitespaces or a minus sign (-).  The version string is
   primarily used to trigger compatibility extensions and to indicate
   the capabilities of an implementation.  The comment string should
   contain additional information that might be useful in solving user
   problems.

   The protocol version described in this document is 2.0.

   Key exchange will begin immediately after sending this identifier.
   All packets following the identification string SHALL use the binary
   packet protocol, to be described below.

3.3 Compatibility With Old SSH Versions

   During the transition period, it is important to be able to work in a
   way that is compatible with the installed SSH clients and servers
   that use an older version of the protocol.  Information in this
   section is only relevant for implementations supporting compatibility
   with SSH versions 1.x.

3.4 Old Client, New Server

   Server implementations MAY support a configurable "compatibility"
   flag that enables compatibility with old versions.  When this flag is
   on, the server SHOULD identify its protocol version as "1.99".
   Clients using protocol 2.0 MUST be able to identify this as identical



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   to "2.0".  In this mode the server SHOULD NOT send the carriage
   return character (ASCII 13) after the version identification string.

   In the compatibility mode the server SHOULD NOT send any further data
   after its initialization string until it has received an
   identification string from the client.  The server can then determine
   whether the client is using an old protocol, and can revert to the
   old protocol if required.  In the compatibility mode, the server MUST
   NOT send additional data before the version string.

   When compatibility with old clients is not needed, the server MAY
   send its initial key exchange data immediately after the
   identification string.

3.5 New Client, Old Server

   Since the new client MAY immediately send additional data after its
   identification string (before receiving server's identification), the
   old protocol may already have been corrupted when the client learns
   that the server is old.  When this happens, the client SHOULD close
   the connection to the server, and reconnect using the old protocol.

4. Binary Packet Protocol

   Each packet is in the following format:

     uint32    packet_length
     byte      padding_length
     byte[n1]  payload; n1 = packet_length - padding_length - 1
     byte[n2]  random padding; n2 = padding_length
     byte[m]   mac (message authentication code); m = mac_length

      packet_length
         The length of the packet (bytes), not including MAC or the
         packet_length field itself.

      padding_length
         Length of padding (bytes).

      payload
         The useful contents of the packet.  If compression has been
         negotiated, this field is compressed.  Initially, compression
         MUST be "none".

      random padding
         Arbitrary-length padding, such that the total length of
         (packet_length || padding_length || payload || padding) is a
         multiple of the cipher block size or 8, whichever is larger.



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         There MUST be at least four bytes of padding.  The padding
         SHOULD consist of random bytes.  The maximum amount of padding
         is 255 bytes.

      mac
         Message authentication code.  If message authentication has
         been negotiated, this field contains the MAC bytes.  Initially,
         the MAC algorithm MUST be "none".


   Note that length of the concatenation of packet length, padding
   length, payload, and padding MUST be a multiple of the cipher block
   size or 8, whichever is larger.  This constraint MUST be enforced
   even when using stream ciphers.  Note that the packet length field is
   also encrypted, and processing it requires special care when sending
   or receiving packets.

   The minimum size of a packet is 16 (or the cipher block size,
   whichever is larger) bytes (plus MAC); implementations SHOULD decrypt
   the length after receiving the first 8 (or cipher block size,
   whichever is larger) bytes of a packet.

4.1 Maximum Packet Length

   All implementations MUST be able to process packets with uncompressed
   payload length of 32768 bytes or less and total packet size of 35000
   bytes or less (including length, padding length, payload, padding,
   and MAC.).  The maximum of 35000 bytes is an arbitrary chosen value
   larger than uncompressed size.  Implementations SHOULD support longer
   packets, where they might be needed, e.g.  if an implementation wants
   to send a very large number of certificates.  Such packets MAY be
   sent if the version string indicates that the other party is able to
   process them.  However, implementations SHOULD check that the packet
   length is reasonable for the implementation to avoid denial-of-
   service and/or buffer overflow attacks.

4.2 Compression

   If compression has been negotiated, the payload field (and only it)
   will be compressed using the negotiated algorithm.  The length field
   and MAC will be computed from the compressed payload.  Encryption
   will be done after compression.

   Compression MAY be stateful, depending on the method.  Compression
   MUST be independent for each direction, and implementations MUST
   allow independently choosing the algorithm for each direction.

   The following compression methods are currently defined:



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     none     REQUIRED        no compression
     zlib     OPTIONAL        ZLIB (LZ77) compression

   The "zlib" compression is described in [RFC1950] and in [RFC1951].
   The compression context is initialized after each key exchange, and
   is passed from one packet to the next with only a partial flush being
   performed at the end of each packet.  A partial flush means that the
   current compressed block is ended and all data will be output.  If
   the current block is not a stored block, one or more empty blocks are
   added after the current block to ensure that there are at least 8
   bits counting from the start of the end-of-block code of the current
   block to the end of the packet payload.

   Additional methods may be defined as specified in [SSH-ARCH].

4.3 Encryption

   An encryption algorithm and a key will be negotiated during the key
   exchange.  When encryption is in effect, the packet length, padding
   length, payload and padding fields of each packet MUST be encrypted
   with the given algorithm.

   The encrypted data in all packets sent in one direction SHOULD be
   considered a single data stream.  For example, initialization vectors
   SHOULD be passed from the end of one packet to the beginning of the
   next packet.  All ciphers SHOULD use keys with an effective key
   length of 128 bits or more.

   The ciphers in each direction MUST run independently of each other,
   and implementations MUST allow independently choosing the algorithm
   for each direction (if multiple algorithms are allowed by local
   policy).

   The following ciphers are currently defined:

     3des-cbc         REQUIRED          three-key 3DES in CBC mode
     blowfish-cbc     RECOMMENDED       Blowfish in CBC mode
     twofish256-cbc   OPTIONAL          Twofish in CBC mode,
                                        with 256-bit key
     twofish-cbc      OPTIONAL          alias for "twofish256-cbc" (this
                                        is being retained for
                                        historical reasons)
     twofish192-cbc   OPTIONAL          Twofish with 192-bit key
     twofish128-cbc   RECOMMENDED       Twofish with 128-bit key
     aes256-cbc       OPTIONAL          AES (Rijndael) in CBC mode,
                                        with 256-bit key
     aes192-cbc       OPTIONAL          AES with 192-bit key
     aes128-cbc       RECOMMENDED       AES with 128-bit key



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     serpent256-cbc   OPTIONAL          Serpent in CBC mode, with
                                        256-bit key
     serpent192-cbc   OPTIONAL          Serpent with 192-bit key
     serpent128-cbc   OPTIONAL          Serpent with 128-bit key
     arcfour          OPTIONAL          the ARCFOUR stream cipher
     idea-cbc         OPTIONAL          IDEA in CBC mode
     cast128-cbc      OPTIONAL          CAST-128 in CBC mode
     none             OPTIONAL          no encryption; NOT RECOMMENDED

   The "3des-cbc" cipher is three-key triple-DES (encrypt-decrypt-
   encrypt), where the first 8 bytes of the key are used for the first
   encryption, the next 8 bytes for the decryption, and the following 8
   bytes for the final encryption.  This requires 24 bytes of key data
   (of which 168 bits are actually used).  To implement CBC mode, outer
   chaining MUST be used (i.e., there is only one initialization
   vector).  This is a block cipher with 8 byte blocks.  This algorithm
   is defined in [SCHNEIER]

   The "blowfish-cbc" cipher is Blowfish in CBC mode, with 128 bit keys
   [SCHNEIER].  This is a block cipher with 8 byte blocks.

   The "twofish-cbc" or "twofish256-cbc" cipher is Twofish in CBC mode,
   with 256 bit keys as described [TWOFISH].  This is a block cipher
   with 16 byte blocks.

   The "twofish192-cbc" cipher.  Same as above but with 192-bit key.

   The "twofish128-cbc" cipher.  Same as above but with 128-bit key.

   The "aes256-cbc" cipher is AES (Advanced Encryption Standard),
   formerly Rijndael, in CBC mode.  This version uses 256-bit key.

   The "aes192-cbc" cipher.  Same as above but with 192-bit key.

   The "aes128-cbc" cipher.  Same as above but with 128-bit key.

   The "serpent256-cbc" cipher in CBC mode, with 256-bit key as
   described in the Serpent AES submission.

   The "serpent192-cbc" cipher.  Same as above but with 192-bit key.

   The "serpent128-cbc" cipher.  Same as above but with 128-bit key.

   The "arcfour" is the Arcfour stream cipher with 128 bit keys.  The
   Arcfour cipher is believed to be compatible with the RC4 cipher
   [SCHNEIER].  RC4 is a registered trademark of RSA Data Security Inc.
   Arcfour (and RC4) has problems with weak keys, and should be used
   with caution.



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   The "idea-cbc" cipher is the IDEA cipher in CBC mode [SCHNEIER].
   IDEA is patented by Ascom AG.

   The "cast128-cbc" cipher is the CAST-128 cipher in CBC mode
   [RFC2144].

   The "none" algorithm specifies that no encryption is to be done.
   Note that this method provides no confidentiality protection, and it
   is not recommended.  Some functionality (e.g.  password
   authentication) may be disabled for security reasons if this cipher
   is chosen.

   Additional methods may be defined as specified in [SSH-ARCH].

4.4 Data Integrity

   Data integrity is protected by including with each packet a message
   authentication code (MAC) that is computed from a shared secret,
   packet sequence number, and the contents of the packet.

   The message authentication algorithm and key are negotiated during
   key exchange.  Initially, no MAC will be in effect, and its length
   MUST be zero.  After key exchange, the selected MAC will be computed
   before encryption from the concatenation of packet data:

     mac = MAC(key, sequence_number || unencrypted_packet)

   where unencrypted_packet is the entire packet without MAC (the length
   fields, payload and padding), and sequence_number is an implicit
   packet sequence number represented as uint32.  The sequence number is
   initialized to zero for the first packet, and is incremented after
   every packet (regardless of whether encryption or MAC is in use).  It
   is never reset, even if keys/algorithms are renegotiated later.  It
   wraps around to zero after every 2^32 packets.  The packet sequence
   number itself is not included in the packet sent over the wire.

   The MAC algorithms for each direction MUST run independently, and
   implementations MUST allow choosing the algorithm independently for
   both directions.

   The MAC bytes resulting from the MAC algorithm MUST be transmitted
   without encryption as the last part of the packet.  The number of MAC
   bytes depends on the algorithm chosen.








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   The following MAC algorithms are currently defined:

     hmac-sha1    REQUIRED        HMAC-SHA1 (digest length = key
                                  length = 20)
     hmac-sha1-96 RECOMMENDED     first 96 bits of HMAC-SHA1 (digest
                                  length = 12, key length = 20)
     hmac-md5     OPTIONAL        HMAC-MD5 (digest length = key
                                  length = 16)
     hmac-md5-96  OPTIONAL        first 96 bits of HMAC-MD5 (digest
                                  length = 12, key length = 16)
     none         OPTIONAL        no MAC; NOT RECOMMENDED

   The "hmac-*" algorithms are described in [RFC2104] The "*-n" MACs use
   only the first n bits of the resulting value.

   The hash algorithms are described in [SCHNEIER].

   Additional methods may be defined as specified in [SSH-ARCH].

4.5 Key Exchange Methods

   The key exchange method specifies how one-time session keys are
   generated for encryption and for authentication, and how the server
   authentication is done.

   Only one REQUIRED key exchange method has been defined:

     diffie-hellman-group1-sha1       REQUIRED

   This method is described later in this document.

   Additional methods may be defined as specified in [SSH-ARCH].

4.6 Public Key Algorithms

   This protocol has been designed to be able to operate with almost any
   public key format, encoding, and algorithm (signature and/or
   encryption).

   There are several aspects that define a public key type:
   o  Key format: how is the key encoded and how are certificates
      represented.  The key blobs in this protocol MAY contain
      certificates in addition to keys.
   o  Signature and/or encryption algorithms.  Some key types may not
      support both signing and encryption.  Key usage may also be
      restricted by policy statements in e.g.  certificates.  In this
      case, different key types SHOULD be defined for the different
      policy alternatives.



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   o  Encoding of signatures and/or encrypted data.  This includes but
      is not limited to padding, byte order, and data formats.

   The following public key and/or certificate formats are currently defined:

   ssh-dss              REQUIRED     sign    Simple DSS
   ssh-rsa              RECOMMENDED  sign    Simple RSA
   x509v3-sign-rsa      OPTIONAL     sign    X.509 certificates (RSA key)
   x509v3-sign-dss      OPTIONAL     sign    X.509 certificates (DSS key)
   spki-sign-rsa        OPTIONAL     sign    SPKI certificates (RSA key)
   spki-sign-dss        OPTIONAL     sign    SPKI certificates (DSS key)
   pgp-sign-rsa         OPTIONAL     sign    OpenPGP certificates (RSA key)
   pgp-sign-dss         OPTIONAL     sign    OpenPGP certificates (DSS key)

   Additional key types may be defined as specified in [SSH-ARCH].

   The key type MUST always be explicitly known (from algorithm
   negotiation or some other source).  It is not normally included in
   the key blob.

   Certificates and public keys are encoded as follows:

     string   certificate or public key format identifier
     byte[n]  key/certificate data

   The certificate part may have be a zero length string, but a public
   key is required.  This is the public key that will be used for
   authentication; the certificate sequence contained in the certificate
   blob can be used to provide authorization.

   Public key / certifcate formats that do not explicitly specify a
   signature format identifier MUST use the public key / certificate
   format identifier as the signature identifier.

   Signatures are encoded as follows:
     string    signature format identifier (as specified by the
               public key / cert format)
     byte[n]   signature blob in format specific encoding.


   The "ssh-dss" key format has the following specific encoding:

     string    "ssh-dss"
     mpint     p
     mpint     q
     mpint     g
     mpint     y




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   Here the p, q, g, and y parameters form the signature key blob.

   Signing and verifying using this key format is done according to the
   Digital Signature Standard [FIPS-186] using the SHA-1 hash.  A
   description can also be found in [SCHNEIER].

   The resulting signature is encoded as follows:

     string    "ssh-dss"
     string    dss_signature_blob

   dss_signature_blob is encoded as a string containing r followed by s
   (which are 160 bits long integers, without lengths or padding,
   unsigned and in network byte order).

   The "ssh-rsa" key format has the following specific encoding:

     string    "ssh-rsa"
     mpint     e
     mpint     n

   Here the e and n parameters form the signature key blob.

   Signing and verifying using this key format is done according to
   [SCHNEIER] and [PKCS1] using the SHA-1 hash.

   The resulting signature is encoded as follows:

     string    "ssh-rsa"
     string    rsa_signature_blob

   rsa_signature_blob is encoded as a string containing s (which is an
   integer, without lengths or padding, unsigned and in network byte
   order).

   The "spki-sign-rsa" method indicates that the certificate blob
   contains a sequence of SPKI certificates.  The format of SPKI
   certificates is described in [RFC2693].  This method indicates that
   the key (or one of the keys in the certificate) is an RSA-key.

   The "spki-sign-dss".  As above, but indicates that the key (or one of
   the keys in the certificate) is a DSS-key.

   The "pgp-sign-rsa" method indicates the certificates, the public key,
   and the signature are in OpenPGP compatible binary format
   ([RFC2440]).  This method indicates that the key is an RSA-key.

   The "pgp-sign-dss".  As above, but indicates that the key is a DSS-



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   key.

5. Key Exchange

   Key exchange begins by each side sending lists of supported
   algorithms.  Each side has a preferred algorithm in each category,
   and it is assumed that most implementations at any given time will
   use the same preferred algorithm.  Each side MAY guess which
   algorithm the other side is using, and MAY send an initial key
   exchange packet according to the algorithm if appropriate for the
   preferred method.

   Guess is considered wrong, if:
   o  the kex algorithm and/or the host key algorithm is guessed wrong
      (server and client have different preferred algorithm), or
   o  if any of the other algorithms cannot be agreed upon (the
      procedure is defined below in Section Section 5.1).

   Otherwise, the guess is considered to be right and the optimistically
   sent packet MUST be handled as the first key exchange packet.

   However, if the guess was wrong, and a packet was optimistically sent
   by one or both parties, such packets MUST be ignored (even if the
   error in the guess would not affect the contents of the initial
   packet(s)), and the appropriate side MUST send the correct initial
   packet.

   Server authentication in the key exchange MAY be implicit.  After a
   key exchange with implicit server authentication, the client MUST
   wait for response to its service request message before sending any
   further data.

5.1 Algorithm Negotiation

   Key exchange begins by each side sending the following packet:

     byte      SSH_MSG_KEXINIT
     byte[16]  cookie (random bytes)
     string    kex_algorithms
     string    server_host_key_algorithms
     string    encryption_algorithms_client_to_server
     string    encryption_algorithms_server_to_client
     string    mac_algorithms_client_to_server
     string    mac_algorithms_server_to_client
     string    compression_algorithms_client_to_server
     string    compression_algorithms_server_to_client
     string    languages_client_to_server
     string    languages_server_to_client



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     boolean   first_kex_packet_follows
     uint32    0 (reserved for future extension)

   Each of the algorithm strings MUST be a comma-separated list of
   algorithm names (see ''Algorithm Naming'' in [SSH-ARCH]).  Each
   supported (allowed) algorithm MUST be listed in order of preference.

   The first algorithm in each list MUST be the preferred (guessed)
   algorithm.  Each string MUST contain at least one algorithm name.


      cookie
         The cookie MUST be a random value generated by the sender.  Its
         purpose is to make it impossible for either side to fully
         determine the keys and the session identifier.

      kex_algorithms
         Key exchange algorithms were defined above.  The first
         algorithm MUST be the preferred (and guessed) algorithm.  If
         both sides make the same guess, that algorithm MUST be used.
         Otherwise, the following algorithm MUST be used to choose a key
         exchange method: iterate over client's kex algorithms, one at a
         time.  Choose the first algorithm that satisfies the following
         conditions:
         +  the server also supports the algorithm,
         +  if the algorithm requires an encryption-capable host key,
            there is an encryption-capable algorithm on the server's
            server_host_key_algorithms that is also supported by the
            client, and
         +  if the algorithm requires a signature-capable host key,
            there is a signature-capable algorithm on the server's
            server_host_key_algorithms that is also supported by the
            client.
         +  If no algorithm satisfying all these conditions can be
            found, the connection fails, and both sides MUST disconnect.

      server_host_key_algorithms
         List of the algorithms supported for the server host key.  The
         server lists the algorithms for which it has host keys; the
         client lists the algorithms that it is willing to accept.
         (There MAY be multiple host keys for a host, possibly with
         different algorithms.)

         Some host keys may not support both signatures and encryption
         (this can be determined from the algorithm), and thus not all
         host keys are valid for all key exchange methods.

         Algorithm selection depends on whether the chosen key exchange



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         algorithm requires a signature or encryption capable host key.
         It MUST be possible to determine this from the public key
         algorithm name.  The first algorithm on the client's list that
         satisfies the requirements and is also supported by the server
         MUST be chosen.  If there is no such algorithm, both sides MUST
         disconnect.

      encryption_algorithms
         Lists the acceptable symmetric encryption algorithms in order
         of preference.  The chosen encryption algorithm to each
         direction MUST be the first algorithm  on the client's list
         that is also on the server's list.  If there is no such
         algorithm, both sides MUST disconnect.

         Note that "none" must be explicitly listed if it is to be
         acceptable.  The defined algorithm names are listed in Section
         Section 4.3.

      mac_algorithms
         Lists the acceptable MAC algorithms in order of preference.
         The chosen MAC algorithm MUST be the first algorithm on the
         client's list that is also on the server's list.  If there is
         no such algorithm, both sides MUST disconnect.

         Note that "none" must be explicitly listed if it is to be
         acceptable.  The MAC algorithm names are listed in Section
         Figure 1.

      compression_algorithms
         Lists the acceptable compression algorithms in order of
         preference.  The chosen compression algorithm MUST be the first
         algorithm on the client's list that is also on the server's
         list.  If there is no such algorithm, both sides MUST
         disconnect.

         Note that "none" must be explicitly listed if it is to be
         acceptable.  The compression algorithm names are listed in
         Section Section 4.2.

      languages
         This is a comma-separated list of language tags in order of
         preference [RFC1766].  Both parties MAY ignore this list.  If
         there are no language preferences, this list SHOULD be empty.

      first_kex_packet_follows
         Indicates whether a guessed key exchange packet follows.  If a
         guessed packet will be sent, this MUST be TRUE.  If no guessed
         packet will be sent, this MUST be FALSE.



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         After receiving the SSH_MSG_KEXINIT packet from the other side,
         each party will know whether their guess was right.  If the
         other party's guess was wrong, and this field was TRUE, the
         next packet MUST be silently ignored, and both sides MUST then
         act as determined by the negotiated key exchange method.  If
         the guess was right, key exchange MUST continue using the
         guessed packet.

   After the KEXINIT packet exchange, the key exchange algorithm is run.
   It may involve several packet exchanges, as specified by the key
   exchange method.

5.2 Output from Key Exchange

   The key exchange produces two values: a shared secret K, and an
   exchange hash H.  Encryption and authentication keys are derived from
   these.  The exchange hash H from the first key exchange is
   additionally used as the session identifier, which is a unique
   identifier for this connection.  It is used by authentication methods
   as a part of the data that is signed as a proof of possession of a
   private key.  Once computed, the session identifier is not changed,
   even if keys are later re-exchanged.


   Each key exchange method specifies a hash function that is used in
   the key exchange.  The same hash algorithm MUST be used in key
   derivation.  Here, we'll call it HASH.


   Encryption keys MUST be computed as HASH of a known value and K as
   follows:
   o  Initial IV client to server: HASH(K || H || "A" || session_id)
      (Here K is encoded as mpint and "A" as byte and session_id as raw
      data."A" means the single character A, ASCII 65).
   o  Initial IV server to client: HASH(K || H || "B" || session_id)
   o  Encryption key client to server: HASH(K || H || "C" || session_id)
   o  Encryption key server to client: HASH(K || H || "D" || session_id)
   o  Integrity key client to server: HASH(K || H || "E" || session_id)
   o  Integrity key server to client: HASH(K || H || "F" || session_id)

   Key data MUST be taken from the beginning of the hash output.  128
   bits (16 bytes) SHOULD be used for algorithms with variable-length
   keys.  For other algorithms, as many bytes as are needed are taken
   from the beginning of the hash value.  If the key length in longer
   than the output of the HASH, the key is extended by computing HASH of
   the concatenation of K and H and the entire key so far, and appending
   the resulting bytes (as many as HASH generates) to the key.  This
   process is repeated until enough key material is available; the key
   is taken from the beginning of this value.  In other words:




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     K1 = HASH(K || H || X || session_id)   (X is e.g. "A")
     K2 = HASH(K || H || K1)
     K3 = HASH(K || H || K1 || K2)
     ...
     key = K1 || K2 || K3 || ...

   This process will lose entropy if the amount of entropy in K is
   larger than the internal state size of HASH.

5.3 Taking Keys Into Use

   Key exchange ends by each side sending an SSH_MSG_NEWKEYS message.
   This message is sent with the old keys and algorithms.  All messages
   sent after this message MUST use the new keys and algorithms.


   When this message is received, the new keys and algorithms MUST be
   taken into use for receiving.


   This message is the only valid message after key exchange, in
   addition to SSH_MSG_DEBUG, SSH_MSG_DISCONNECT and SSH_MSG_IGNORE
   messages.  The purpose of this message is to ensure that a party is
   able to respond with a disconnect message that the other party can
   understand if something goes wrong with the key exchange.
   Implementations MUST NOT accept any other messages after key exchange
   before receiving SSH_MSG_NEWKEYS.

     byte      SSH_MSG_NEWKEYS


6. Diffie-Hellman Key Exchange

   The Diffie-Hellman key exchange provides a shared secret that can not
   be determined by either party alone.  The key exchange is combined
   with a signature with the host key to provide host authentication.


   In the following description (C is the client, S is the server; p is
   a large safe prime, g is a generator for a subgroup of GF(p), and q
   is the order of the subgroup; V_S is S's version string; V_C is C's
   version string; K_S is S's public host key; I_C is C's KEXINIT
   message and I_S S's KEXINIT message which have been exchanged before
   this part begins):


   1.  C generates a random number x (1 < x < q) and computes e = g^x
       mod p.  C sends "e" to S.



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   2.  S generates a random number y (0 < y < q) and computes f = g^y
       mod p.  S receives "e".  It computes K = e^y mod p, H = hash(V_C
       || V_S || I_C || I_S || K_S || e || f || K) (these elements are
       encoded according to their types; see below), and signature s on
       H with its private host key.  S sends "K_S || f || s" to C.  The
       signing operation may involve a second hashing operation.

   3.  C verifies that K_S really is the host key for S (e.g.  using
       certificates or a local database).  C is also allowed to accept
       the key without verification; however, doing so will render the
       protocol insecure against active attacks (but may be desirable
       for practical reasons in the short term in many environments).  C
       then computes K = f^x mod p, H = hash(V_C || V_S || I_C || I_S ||
       K_S || e || f || K), and verifies the signature s on H.

   Either side MUST NOT send or accept e or f values that are not in the
   range [1, p-1].  If this condition is violated, the key exchange
   fails.


   This is implemented with the following messages.  The hash algorithm
   for computing the exchange hash is defined by the method name, and is
   called HASH.  The public key algorithm for signing is negotiated with
   the KEXINIT messages.

   First, the client sends the following:

     byte      SSH_MSG_KEXDH_INIT
     mpint     e


   The server responds with the following:

     byte      SSH_MSG_KEXDH_REPLY
     string    server public host key and certificates (K_S)
     mpint     f
     string    signature of H

   The hash H is computed as the HASH hash of the concatenation of the
   following:

     string    V_C, the client's version string (CR and NL excluded)
     string    V_S, the server's version string (CR and NL excluded)
     string    I_C, the payload of the client's SSH_MSG_KEXINIT
     string    I_S, the payload of the server's SSH_MSG_KEXINIT
     string    K_S, the host key
     mpint     e, exchange value sent by the client
     mpint     f, exchange value sent by the server
     mpint     K, the shared secret




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   This value is called the exchange hash, and it is used to
   authenticate the key exchange.  The exchange hash SHOULD be kept
   secret.


   The signature algorithm MUST be applied over H, not the original
   data.  Most signature algorithms include hashing and additional
   padding.  For example, "ssh-dss" specifies SHA-1 hashing; in that
   case, the data is first hashed with HASH to compute H, and H is then
   hashed with SHA-1 as part of the signing operation.

6.1 diffie-hellman-group1-sha1

   The "diffie-hellman-group1-sha1" method specifies Diffie-Hellman key
   exchange with SHA-1 as HASH, and the following group:

   The prime p is equal to 2^1024 - 2^960 - 1 + 2^64 * floor( 2^894 Pi +
   129093 ).  Its hexadecimal value is:

         FFFFFFFF FFFFFFFF C90FDAA2 2168C234 C4C6628B 80DC1CD1
         29024E08 8A67CC74 020BBEA6 3B139B22 514A0879 8E3404DD
         EF9519B3 CD3A431B 302B0A6D F25F1437 4FE1356D 6D51C245
         E485B576 625E7EC6 F44C42E9 A637ED6B 0BFF5CB6 F406B7ED
         EE386BFB 5A899FA5 AE9F2411 7C4B1FE6 49286651 ECE65381
         FFFFFFFF FFFFFFFF.

   In decimal, this value is:

         179769313486231590770839156793787453197860296048756011706444
         423684197180216158519368947833795864925541502180565485980503
         646440548199239100050792877003355816639229553136239076508735
         759914822574862575007425302077447712589550957937778424442426
         617334727629299387668709205606050270810842907692932019128194
         467627007.

   The generator used with this prime is g = 2.  The group order q is (p
   - 1) / 2.

   This group was taken from the ISAKMP/Oakley specification, and was
   originally generated by Richard Schroeppel at the University of
   Arizona.  Properties of this prime are described in [Orm96].

7. Key Re-Exchange

   Key re-exchange is started by sending an SSH_MSG_KEXINIT packet when
   not already doing a key exchange (as described in Section Section
   5.1).  When this message is received, a party MUST respond with its
   own SSH_MSG_KEXINIT message except when the received SSH_MSG_KEXINIT



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   already was a reply.  Either party MAY initiate the re-exchange, but
   roles MUST NOT be changed (i.e., the server remains the server, and
   the client remains the client).


   Key re-exchange is performed using whatever encryption was in effect
   when the exchange was started.  Encryption, compression, and MAC
   methods are not changed before a new SSH_MSG_NEWKEYS is sent after
   the key exchange (as in the initial key exchange).  Re-exchange is
   processed identically to the initial key exchange, except for the
   session identifier that will remain unchanged.  It is permissible to
   change some or all of the algorithms during the re-exchange.  Host
   keys can also change.  All keys and initialization vectors are
   recomputed after the exchange.  Compression and encryption contexts
   are reset.


   It is recommended that the keys are changed after each gigabyte of
   transmitted data or after each hour of connection time, whichever
   comes sooner.  However, since the re-exchange is a public key
   operation, it requires a fair amount of processing power and should
   not be performed too often.


   More application data may be sent after the SSH_MSG_NEWKEYS packet
   has been sent; key exchange does not affect the protocols that lie
   above the SSH transport layer.

8. Service Request

   After the key exchange, the client requests a service.  The service
   is identified by a name.  The format of names and procedures for
   defining new names are defined in [SSH-ARCH].


   Currently, the following names have been reserved:

     ssh-userauth
     ssh-connection

   Similar local naming policy is applied to the service names, as is
   applied to the algorithm names; a local service should use the
   "servicename@domain" syntax.

     byte      SSH_MSG_SERVICE_REQUEST
     string    service name

   If the server rejects the service request, it SHOULD send an



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   appropriate SSH_MSG_DISCONNECT message and MUST disconnect.


   When the service starts, it may have access to the session identifier
   generated during the key exchange.


   If the server supports the service (and permits the client to use
   it), it MUST respond with the following:

     byte      SSH_MSG_SERVICE_ACCEPT
     string    service name

   Message numbers used by services should be in the area reserved for
   them (see Section 6 in [SSH-ARCH]).  The transport level will
   continue to process its own messages.


   Note that after a key exchange with implicit server authentication,
   the client MUST wait for response to its service request message
   before sending any further data.

9. Additional Messages

   Either party may send any of the following messages at any time.

9.1 Disconnection Message

     byte      SSH_MSG_DISCONNECT
     uint32    reason code
     string    description [RFC2279]
     string    language tag [RFC1766]

   This message causes immediate termination of the connection.  All
   implementations MUST be able to process this message; they SHOULD be
   able to send this message.

   The sender MUST NOT send or receive any data after this message, and
   the recipient MUST NOT accept any data after receiving this message.
   The description field gives a more specific explanation in a human-
   readable form.  The error code gives the reason in a more machine-
   readable format (suitable for localization), and can have the
   following values:

     #define SSH_DISCONNECT_HOST_NOT_ALLOWED_TO_CONNECT      1
     #define SSH_DISCONNECT_PROTOCOL_ERROR                   2
     #define SSH_DISCONNECT_KEY_EXCHANGE_FAILED              3
     #define SSH_DISCONNECT_RESERVED                         4



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     #define SSH_DISCONNECT_MAC_ERROR                        5
     #define SSH_DISCONNECT_COMPRESSION_ERROR                6
     #define SSH_DISCONNECT_SERVICE_NOT_AVAILABLE            7
     #define SSH_DISCONNECT_PROTOCOL_VERSION_NOT_SUPPORTED   8
     #define SSH_DISCONNECT_HOST_KEY_NOT_VERIFIABLE          9
     #define SSH_DISCONNECT_CONNECTION_LOST                 10
     #define SSH_DISCONNECT_BY_APPLICATION                  11
     #define SSH_DISCONNECT_TOO_MANY_CONNECTIONS            12
     #define SSH_DISCONNECT_AUTH_CANCELLED_BY_USER          13
     #define SSH_DISCONNECT_NO_MORE_AUTH_METHODS_AVAILABLE  14
     #define SSH_DISCONNECT_ILLEGAL_USER_NAME               15

   If the description string is displayed, control character filtering
   discussed in [SSH-ARCH] should be used to avoid attacks by sending
   terminal control characters.

9.2 Ignored Data Message

     byte      SSH_MSG_IGNORE
     string    data

   All implementations MUST understand (and ignore) this message at any
   time (after receiving the protocol version).  No implementation is
   required to send them.  This message can be used as an additional
   protection measure against advanced traffic analysis techniques.

9.3 Debug Message

     byte      SSH_MSG_DEBUG
     boolean   always_display
     string    message [RFC2279]
     string    language tag [RFC1766]

   All implementations MUST understand this message, but they are
   allowed to ignore it.  This message is used to pass the other side
   information that may help debugging.  If always_display is TRUE, the
   message SHOULD be displayed.  Otherwise, it SHOULD NOT be displayed
   unless debugging information has been explicitly requested by the
   user.


   The message doesn't need to contain a newline.  It is, however,
   allowed to consist of multiple lines separated by CRLF (Carriage
   Return - Line Feed) pairs.


   If the message string is displayed, terminal control character
   filtering discussed in [SSH-ARCH] should be used to avoid attacks by



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   sending terminal control characters.

9.4 Reserved Messages

   An implementation MUST respond to all unrecognized messages with an
   SSH_MSG_UNIMPLEMENTED message in the order in which the messages were
   received.  Such messages MUST be otherwise ignored.  Later protocol
   versions may define other meanings for these message types.

     byte      SSH_MSG_UNIMPLEMENTED
     uint32    packet sequence number of rejected message


10. Summary of Message Numbers

   The following message numbers have been defined in this protocol:

     #define SSH_MSG_DISCONNECT             1
     #define SSH_MSG_IGNORE                 2
     #define SSH_MSG_UNIMPLEMENTED          3
     #define SSH_MSG_DEBUG                  4
     #define SSH_MSG_SERVICE_REQUEST        5
     #define SSH_MSG_SERVICE_ACCEPT         6

     #define SSH_MSG_KEXINIT                20
     #define SSH_MSG_NEWKEYS                21

     /* Numbers 30-49 used for kex packets.
        Different kex methods may reuse message numbers in
        this range. */

     #define SSH_MSG_KEXDH_INIT             30
     #define SSH_MSG_KEXDH_REPLY            31


11. Security Considerations

   This protocol provides a secure encrypted 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.

   It is expected that this protocol will sometimes be used without
   insisting on reliable association between the server host key and the
   server host name.  Such use is inherently insecure, but may be
   necessary in non-security critical environments, and still provides
   protection against passive attacks.  However, implementors of
   protocols running on top of this protocol should keep this



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   possibility in mind.

   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.

   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.

   Nearly all ciphers specified in this document are used in cipher
   block chaining (CBC) mode.  It's been known for some time that CBC
   modes will reveal information about the plaintext if two ciphertext
   blocks encrypted under the same key are equal; this is one of the
   reasons this document strongly recommends rekeying at least once per
   gigabyte of data, to reduce the chance that a "birthday paradox"
   collision will appear.

   Recent research has uncovered a new attack on CBC mode which, under
   certain conditions, allows a chosen plaintext attacker aware of the
   IV for a forthcoming message to have some chance to artificially
   induce a system into generating ciphertext collisions, allowing the
   attacker's guesses at likely prior plaintexts to be confirmed.

   Any protocol which uses CBC in a way which allows advance knowledge
   of a message's IV (e.g., by using the last block of the preceding
   message as the IV) might be vulnerable to this attack.

   Preliminary analysis of this attack as applied to the SSH protocol
   suggests that the protocol as implemented today is actually fairly
   resistant to this attack.  While estimates vary, on average, an
   attacker would need tens or hundreds of millions of opportunities to
   inject chosen plaintexts to be encrypted with a known IV to confirm
   guesses on the value of a few unknown plaintexts.

   While this attack involves less work than a brute-force attack on the
   underlying cipher (and is thus a matter of some concern), it is also
   likely to be significantly more difficult than attacks on other parts
   of a system using the SSH protocol, and so is unlikely to be an
   immediate risk to real-world systems.  Due to this document's
   recommendation that rekeying occur once an hour, an attacker also has
   a limited amount of time to complete any particular attack.

   Nevertheless, work is underway to specify, in a separate document or



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   documents, additional cipher modes for the SSH protocol to address
   this vulnerability.  Implementors should be prepared to add new
   algorithms to their implementations as this work progresses.

12. 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
   rights.

13. 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: http://ietf.org/html.charters/secsh-charter.html

References

   [FIPS-186]      Federal Information Processing Standards Publication,
                   ., "FIPS PUB 186, Digital Signature Standard", May
                   1994.

   [Orm96]         Orman, H., "The Okaley Key Determination Protcol
                   version1, TR97-92", 1996.

   [RFC2459]       Housley, R., Ford, W., Polk, W. and D. Solo,
                   "Internet X.509 Public Key Infrastructure Certificate
                   and CRL Profile", RFC 2459, January 1999.

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

   [RFC1766]       Alvestrand, H., "Tags for the Identification of



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                   Languages", RFC 1766, March 1995.

   [RFC1950]       Deutsch, P. and J-L. Gailly, "ZLIB Compressed Data
                   Format Specification version 3.3", RFC 1950, May
                   1996.

   [RFC1951]       Deutsch, P., "DEFLATE Compressed Data Format
                   Specification version 1.3", RFC 1951, May 1996.

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

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

   [RFC2144]       Adams, C., "The CAST-128 Encryption Algorithm", RFC
                   2144, May 1997.

   [RFC2440]       Callas, J., Donnerhacke, L., Finney, H. and R.
                   Thayer, "OpenPGP Message Format", RFC 2440, November
                   1998.

   [RFC2693]       Ellison, C., Frantz, B., Lampson, B., Rivest, R.,
                   Thomas, B. and T. Ylonen, "SPKI Certificate Theory",
                   RFC 2693, September 1999.

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

   [TWOFISH]       Schneier, B., "The Twofish Encryptions Algorithm: A
                   128-Bit Block Cipher, 1st Edition", March 1999.

   [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.

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




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Authors' Addresses

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

   EMail: ylo@ssh.com


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

   EMail: kivinen@ssh.com


   Markku-Juhani O. Saarinen
   University of Jyvaskyla


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

   EMail: tri@ssh.com


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

   EMail: sjl@ssh.com











Ylonen, et. al.          Expires March 21, 2003                [Page 27]


Internet-Draft        SSH Transport Layer Protocol        September 2002


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Ylonen, et. al.          Expires March 21, 2003                [Page 28]


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