Subject: Re: drafts appear to have been published; please review.

From: Darren Moffat <[email-masked]>
Date: Mon, 30 Jul 2001 16:35:26 -0700 (PDT)

>i haven't yet seen a draft announcement go out, but it looks like the
>core drafts have been published.
>
>My understanding is that the transport-layer draft missed the
>publication deadline by a couple minutes, but the other three made it.
>
>I'm going to look into putting the document which should have been
>published up on another site for now since they four should be a
>matched set.

Since it isn't too large I've attached it below for now.

--
Darren J Moffat

Network Working Group                                          T. Ylonen
Internet-Draft                                                T. Kivinen
Expires: January 18, 2002               SSH Communications Security Corp
                                                             M. Saarinen
                                                 University of Jyvaskyla
                                                                T. Rinne
                                                             S. Lehtinen
                                        SSH Communications Security Corp
                                                           July 20, 2001

                      SSH Transport Layer Protocol
                   draft-ietf-secsh-transport-11.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 January 18, 2002.

Copyright Notice

      Copyright (C) The Internet Society (2001).  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

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      basis for a number of secure network services.  It provides strong
      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 . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Conventions Used in This Document  . . . . . . . . . . . . . .  4
   3.  Connection Setup . . . . . . . . . . . . . . . . . . . . . . .  4
   3.1 Use over TCP/IP  . . . . . . . . . . . . . . . . . . . . . . .  4
   3.2 Protocol Version Exchange  . . . . . . . . . . . . . . . . . .  4
   3.3 Compatibility With Old SSH Versions  . . . . . . . . . . . . .  5
   3.4 Old Client, New Server . . . . . . . . . . . . . . . . . . . .  5
   3.5 New Client, Old Server . . . . . . . . . . . . . . . . . . . .  6
   4.  Binary Packet Protocol . . . . . . . . . . . . . . . . . . . .  6
   4.1 Maximum Packet Length  . . . . . . . . . . . . . . . . . . . .  7
   4.2 Compression  . . . . . . . . . . . . . . . . . . . . . . . . .  7
   4.3 Encryption . . . . . . . . . . . . . . . . . . . . . . . . . .  8
   4.4 Data Integrity . . . . . . . . . . . . . . . . . . . . . . . . 10
   4.5 Key Exchange Methods . . . . . . . . . . . . . . . . . . . . . 11
   4.6 Public Key Algorithms  . . . . . . . . . . . . . . . . . . . . 11
   5.  Key Exchange . . . . . . . . . . . . . . . . . . . . . . . . . 14
   5.1 Algorithm Negotiation  . . . . . . . . . . . . . . . . . . . . 14
   5.2 Output from Key Exchange . . . . . . . . . . . . . . . . . . . 17
   5.3 Taking Keys Into Use . . . . . . . . . . . . . . . . . . . . . 18
   6.  Diffie-Hellman Key Exchange  . . . . . . . . . . . . . . . . . 18
   6.1 diffie-hellman-group1-sha1 . . . . . . . . . . . . . . . . . . 20
   7.  Key Re-Exchange  . . . . . . . . . . . . . . . . . . . . . . . 21
   8.  Service Request  . . . . . . . . . . . . . . . . . . . . . . . 21
   9.  Additional Messages  . . . . . . . . . . . . . . . . . . . . . 22
   9.1 Disconnection Message  . . . . . . . . . . . . . . . . . . . . 22
   9.2 Ignored Data Message . . . . . . . . . . . . . . . . . . . . . 23
   9.3 Debug Message  . . . . . . . . . . . . . . . . . . . . . . . . 23
   9.4 Reserved Messages  . . . . . . . . . . . . . . . . . . . . . . 24
   10. Summary of Message Numbers . . . . . . . . . . . . . . . . . . 24
   11. Security Considerations  . . . . . . . . . . . . . . . . . . . 24
   12. Trademark Issues . . . . . . . . . . . . . . . . . . . . . . . 25
   13. Additional Information . . . . . . . . . . . . . . . . . . . . 25
       References . . . . . . . . . . . . . . . . . . . . . . . . . . 25
       Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 27

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       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".

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      Clients using protocol 2.0 MUST be able to identify this as
      identical 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".
         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.
         There MUST be at least four bytes of padding.  The padding
         SHOULD consist of random bytes.  The maximum amount of padding
         is 255 bytes.

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         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:

     none     REQUIRED        no compression
     zlib     OPTIONAL        ZLIB (LZ77) compression

      The "zlib" compression is described in [RFC1950] and in [RFC1951].

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      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 all data will be output, but the next packet will
      continue using compression tables from the end of the previous
      packet.

      Additional methods may be defined as specified in .

   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
     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

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     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.

      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].

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      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 .

   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 .

   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 .

   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      RECOMMENDED  sign    X.509 certificates (RSA key)
   x509v3-sign-dss      RECOMMENDED  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 .

      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.

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

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

   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

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      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 "x509v3-sign-rsa" method indicates that the certificates, the
      public key, and the resulting signature are in X.509v3 compatible
      DER-encoded format.  The formats used in X.509v3 is described in
      [RFC2459].  This method indicates that the key (or one of the keys
      in the certificate) is an RSA-key.

      The "x509v3-sign-dss".  As above, but indicates that the key (or
      one of the keys in the certificate) is a DSS-key.

      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-key.

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   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 ).  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

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            exchange 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.
            After receiving the SSH_MSG_KEXINIT packet from the other
            side, each party will know whether their guess was right.

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            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 , 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

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      available; the key is taken from the beginning of this value.  In
      other words:

     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):

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

      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

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     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

      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].

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   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 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 .

      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

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      If the server rejects the service request, it SHOULD send an
      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 Section 10).  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
     #define SSH_DISCONNECT_MAC_ERROR                        5
     #define SSH_DISCONNECT_COMPRESSION_ERROR                6

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     #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  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  should be used to avoid attacks by sending
      terminal control characters.

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   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 possibility in mind.

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      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.

   12. Trademark Issues

      As of this writing, SSH Communications Security Oy claims ssh as
      its trademark.  As with all IPR claims the IETF takes no position
      regarding the validity or scope of this trademark claim.

   13. Additional Information

      The current document editor is: [email-masked].  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
                      Languages", RFC 1766, March 1995.

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

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      [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-09.txt, July 2001.

      [SSH-TRANS]     Ylonen, T., "SSH Transport Layer Protocol", I-D
                      draft-ietf-transport-11.txt, July 2001.

      [SSH-USERAUTH]  Ylonen, T., "SSH Authentication Protocol", I-D
                      draft-ietf-userauth-11.txt, July 2001.

      [SSH-CONNECT]   Ylonen, T., "SSH Connection Protocol", I-D draft-
                      ietf-connect-11.txt, July 2001.

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Authors' Addresses

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

   EMail: [email-masked]

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

   EMail: [email-masked]

   Markku-Juhani O. Saarinen
   University of Jyvaskyla

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

   EMail: [email-masked]

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

   EMail: [email-masked]

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Full Copyright Statement

      Copyright (C) The Internet Society (2001).  All Rights Reserved.

      This document and translations of it may be copied and furnished
      to others, and derivative works that comment on or otherwise
      explain it or assist in its implementation may be prepared,
      copied, published and distributed, in whole or in part, without
      restriction of any kind, provided that the above copyright notice
      and this paragraph are included on all such copies and derivative
      works.  However, this document itself may not be modified in any
      way, such as by removing the copyright notice or references to the
      Internet Society or other Internet organizations, except as needed
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      languages other than English.

      The limited permissions granted above are perpetual and will not
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      This document and the information contained herein is provided on
      an "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET
      ENGINEERING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR
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Acknowledgement

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

Ylonen, et. al.         Expires January 18, 2002               [Page 28]