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RFC 4253 - The Secure Shell (SSH) Transport Layer Protocol


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Network Working Group                                          T. Ylonen
Request for Comments: 4253              SSH Communications Security Corp
Category: Standards Track                                C. Lonvick, Ed.
                                                     Cisco Systems, Inc.
                                                            January 2006

            The Secure Shell (SSH) Transport Layer Protocol

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2006).

Abstract

   The Secure Shell (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
   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. Contributors ....................................................3
   3. Conventions Used in This Document ...............................3
   4. Connection Setup ................................................4
      4.1. Use over TCP/IP ............................................4
      4.2. Protocol Version Exchange ..................................4
   5. Compatibility With Old SSH Versions .............................5
      5.1. Old Client, New Server .....................................6
      5.2. New Client, Old Server .....................................6
      5.3. Packet Size and Overhead ...................................6
   6. Binary Packet Protocol ..........................................7
      6.1. Maximum Packet Length ......................................8
      6.2. Compression ................................................8
      6.3. Encryption .................................................9
      6.4. Data Integrity ............................................12
      6.5. Key Exchange Methods ......................................13
      6.6. Public Key Algorithms .....................................13
   7. Key Exchange ...................................................15
      7.1. Algorithm Negotiation .....................................17
      7.2. Output from Key Exchange ..................................20
      7.3. Taking Keys Into Use ......................................21
   8. Diffie-Hellman Key Exchange ....................................21
      8.1. diffie-hellman-group1-sha1 ................................23
      8.2. diffie-hellman-group14-sha1 ...............................23
   9. Key Re-Exchange ................................................23
   10. Service Request ...............................................24
   11. Additional Messages ...........................................25
      11.1. Disconnection Message ....................................25
      11.2. Ignored Data Message .....................................26
      11.3. Debug Message ............................................26
      11.4. Reserved Messages ........................................27
   12. Summary of Message Numbers ....................................27
   13. IANA Considerations ...........................................27
   14. Security Considerations .......................................28
   15. References ....................................................29
      15.1. Normative References .....................................29
      15.2. Informative References ...................................30
   Authors' Addresses ................................................31
   Trademark Notice ..................................................31

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 and flexible to allow
   parameter negotiation, and to minimize the number of round-trips.
   The 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.  Contributors

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

   Many people contributed to the development of this document over the
   years.  People who should be acknowledged include Mats Andersson, Ben
   Harris, Bill Sommerfeld, Brent McClure, Niels Moller, Damien Miller,
   Derek Fawcus, Frank Cusack, Heikki Nousiainen, Jakob Schlyter, Jeff
   Van Dyke, Jeffrey Altman, Jeffrey Hutzelman, Jon Bright, Joseph
   Galbraith, Ken Hornstein, Markus Friedl, Martin Forssen, Nicolas
   Williams, Niels Provos, Perry Metzger, Peter Gutmann, Simon
   Josefsson, Simon Tatham, Wei Dai, Denis Bider, der Mouse, and
   Tadayoshi Kohno.  Listing their names here does not mean that they
   endorse this document, but that they have contributed to it.

3.  Conventions Used in This Document

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

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

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

      byte      SSH_MSG_CHANNEL_DATA
      uint32    recipient channel
      string    data

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

4.  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.

4.1.  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.

4.2.  Protocol Version Exchange

   When the connection has been established, both sides MUST send an
   identification string.  This identification string MUST be

      SSH-protoversion-softwareversion SP comments CR LF

   Since the protocol being defined in this set of documents is version
   2.0, the 'protoversion' MUST be "2.0".  The 'comments' string is
   OPTIONAL.  If the 'comments' string is included, a 'space' character
   (denoted above as SP, ASCII 32) MUST separate the 'softwareversion'
   and 'comments' strings.  The identification MUST be terminated by a
   single Carriage Return (CR) and a single Line Feed (LF) character
   (ASCII 13 and 10, respectively).  Implementers who wish to maintain

   compatibility with older, undocumented versions of this protocol may
   want to process the identification string without expecting the
   presence of the carriage return character for reasons described in
   Section 5 of this document.  The null character MUST NOT be sent.
   The maximum length of the string is 255 characters, including the
   Carriage Return and Line Feed.

   The part of the identification string preceding the Carriage Return
   and Line Feed is used in the Diffie-Hellman key exchange (see Section
   8).

   The server MAY send other lines of data before sending the version
   string.  Each line SHOULD be terminated by a Carriage Return and Line
   Feed.  Such lines MUST NOT begin with "SSH-", and SHOULD be encoded
   in ISO-10646 UTF-8 [RFC3629] (language is not specified).  Clients
   MUST be able to process such lines.  Such lines MAY be silently
   ignored, or MAY be displayed to the client user.  If they are
   displayed, control character filtering, as 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.

   Both the 'protoversion' and 'softwareversion' strings MUST consist of
   printable US-ASCII characters, with the exception of whitespace
   characters and the minus sign (-).  The 'softwareversion' string is
   primarily used to trigger compatibility extensions and to indicate
   the capabilities of an implementation.  The 'comments' string SHOULD
   contain additional information that might be useful in solving user
   problems.  As such, an example of a valid identification string is

      SSH-2.0-billsSSH_3.6.3q3<CR><LF>

   This identification string does not contain the optional 'comments'
   string and is thus terminated by a CR and LF immediately after the
   'softwareversion' string.

   Key exchange will begin immediately after sending this identifier.
   All packets following the identification string SHALL use the binary
   packet protocol, which is described in Section 6.

5.  Compatibility With Old SSH Versions

   As stated earlier, the 'protoversion' specified for this protocol is
   "2.0".  Earlier versions of this protocol have not been formally
   documented, but it is widely known that they use 'protoversion' of
   "1.x" (e.g., "1.5" or "1.3").  At the time of this writing, many
   implementations of SSH are utilizing protocol version 2.0, but it is
   known that there are still devices using the previous 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 the older version of the protocol.  Information in this
   section is only relevant for implementations supporting compatibility
   with SSH versions 1.x.  For those interested, the only known
   documentation of the 1.x protocol is contained in README files that
   are shipped along with the source code [ssh-1.2.30].

5.1.  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 'protoversion' as "1.99".  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 identification string.

   In the compatibility mode, the server SHOULD NOT send any further
   data after sending its identification 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 identification string.

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

5.2.  New Client, Old Server

   Since the new client MAY immediately send additional data after its
   identification string (before receiving the server's identification
   string), the old protocol may already be corrupt 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.

5.3.  Packet Size and Overhead

   Some readers will worry about the increase in packet size due to new
   headers, padding, and the Message Authentication Code (MAC).  The
   minimum packet size is in the order of 28 bytes (depending on
   negotiated algorithms).  The increase is negligible for large
   packets, but very significant for one-byte packets (telnet-type
   sessions).  There are, however, several factors that make this a
   non-issue in almost all cases:

   o  The minimum size of a TCP/IP header is 32 bytes.  Thus, the
      increase is actually from 33 to 51 bytes (roughly).

   o  The minimum size of the data field of an Ethernet packet is 46
      bytes [RFC0894].  Thus, the increase is no more than 5 bytes.
      When Ethernet headers are considered, the increase is less than 10
      percent.

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

   The only environment where the packet size increase is likely to have
   a significant effect is PPP [RFC1661] over slow modem lines (PPP
   compresses the TCP/IP headers, emphasizing the increase in packet
   size).  However, with modern modems, the time needed to transfer is
   in the order of 2 milliseconds, which is a lot faster than people can
   type.

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

6.  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 - MAC); m = mac_length

      packet_length
         The length of the packet in bytes, not including 'mac' or the
         'packet_length' field itself.

      padding_length
         Length of 'random 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 || random 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.

      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 the length of the concatenation of 'packet_length',
   'padding_length', 'payload', and 'random 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.  Also note that the
   insertion of variable amounts of 'random padding' may help thwart
   traffic analysis.

   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.

6.1.  Maximum Packet Length

   All implementations MUST be able to process packets with an
   uncompressed payload length of 32768 bytes or less and a total packet
   size of 35000 bytes or less (including 'packet_length',
   'padding_length', 'payload', 'random padding', and 'mac').  The
   maximum of 35000 bytes is an arbitrarily chosen value that is larger
   than the uncompressed length noted above.  Implementations SHOULD
   support longer packets, where they might be needed.  For example, if
   an implementation wants to send a very large number of certificates,
   the larger packets MAY be sent if the identification string indicates
   that the other party is able to process them.  However,
   implementations SHOULD check that the packet length is reasonable in
   order for the implementation to avoid denial of service and/or buffer
   overflow attacks.

6.2.  Compression

   If compression has been negotiated, the 'payload' field (and only it)
   will be compressed using the negotiated algorithm.  The
   'packet_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 independent choosing of the algorithm for each direction.  In
   practice however, it is RECOMMENDED that the compression method be
   the same in both directions.

   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].
   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] and
   [SSH-NUMBERS].

6.3.  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.
   Implementations MUST allow the algorithm for each direction to be
   independently selected, if multiple algorithms are allowed by local
   policy.  In practice however, it is RECOMMENDED that the same
   algorithm be used in both directions.

   The following ciphers are currently defined:

      3des-cbc         REQUIRED          three-key 3DES in CBC mode
      blowfish-cbc     OPTIONAL          Blowfish in CBC mode
      twofish256-cbc   OPTIONAL          Twofish in CBC mode,
                                         with a 256-bit key
      twofish-cbc      OPTIONAL          alias for "twofish256-cbc"
                                         (this is being retained
                                         for historical reasons)
      twofish192-cbc   OPTIONAL          Twofish with a 192-bit key
      twofish128-cbc   OPTIONAL          Twofish with a 128-bit key
      aes256-cbc       OPTIONAL          AES in CBC mode,
                                         with a 256-bit key
      aes192-cbc       OPTIONAL          AES with a 192-bit key
      aes128-cbc       RECOMMENDED       AES with a 128-bit key
      serpent256-cbc   OPTIONAL          Serpent in CBC mode, with
                                         a 256-bit key
      serpent192-cbc   OPTIONAL          Serpent with a 192-bit key
      serpent128-cbc   OPTIONAL          Serpent with a 128-bit key
      arcfour          OPTIONAL          the ARCFOUR stream cipher
                                         with a 128-bit key
      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 [FIPS-46-3].  Note that since this algorithm only has
   an effective key length of 112 bits ([SCHNEIER]), it does not meet
   the specifications that SSH encryption algorithms should use keys of
   128 bits or more.  However, this algorithm is still REQUIRED for
   historical reasons; essentially, all known implementations at the
   time of this writing support this algorithm, and it is commonly used
   because it is the fundamental interoperable algorithm.  At some
   future time, it is expected that another algorithm, one with better
   strength, will become so prevalent and ubiquitous that the use of
   "3des-cbc" will be deprecated by another STANDARDS ACTION.

   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 is the same as above, but with a 192-bit
   key.

   The "twofish128-cbc" cipher is the same as above, but with a 128-bit
   key.

   The "aes256-cbc" cipher is AES (Advanced Encryption Standard)
   [FIPS-197], in CBC mode.  This version uses a 256-bit key.

   The "aes192-cbc" cipher is the same as above, but with a 192-bit key.

   The "aes128-cbc" cipher is the same as above, but with a 128-bit key.

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

   The "serpent192-cbc" cipher is the same as above, but with a 192-bit
   key.

   The "serpent128-cbc" cipher is the same as above, but with a 128-bit
   key.

   The "arcfour" cipher is the Arcfour stream cipher with 128-bit keys.
   The Arcfour cipher is believed to be compatible with the RC4 cipher
   [SCHNEIER].  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].

   The "cast128-cbc" cipher is the CAST-128 cipher in CBC mode with a
   128-bit key [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] and in
   [SSH-NUMBERS].

6.4.  Data Integrity

   Data integrity is protected by including with each packet a 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 'mac' for the selected MAC
   algorithm 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 'random 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.  In practice however, it is RECOMMENDED that the
   same algorithm be used in both directions.

   The value of 'mac' 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.

   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.

   SHA-1 is described in [FIPS-180-2] and MD5 is described in [RFC1321].

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

6.5.  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.

   Two REQUIRED key exchange methods have been defined:

      diffie-hellman-group1-sha1 REQUIRED
      diffie-hellman-group14-sha1 REQUIRED

   These methods are described in Section 8.

   Additional methods may be defined as specified in [SSH-NUMBERS].  The
   name "diffie-hellman-group1-sha1" is used for a key exchange method
   using an Oakley group, as defined in [RFC2409].  SSH maintains its
   own group identifier space that is logically distinct from Oakley
   [RFC2412] and IKE; however, for one additional group, the Working
   Group adopted the number assigned by [RFC3526], using diffie-
   hellman-group14-sha1 for the name of the second defined group.
   Implementations should treat these names as opaque identifiers and
   should not assume any relationship between the groups used by SSH and
   the groups defined for IKE.

6.6.  Public Key Algorithms

   This protocol has been designed 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 (e.g., in certificates).  In this
      case, different key types SHOULD be defined for the different
      policy alternatives.

   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   Raw DSS Key
   ssh-rsa           RECOMMENDED  sign   Raw RSA 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] and
   in [SSH-NUMBERS].

   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 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/certificate 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/certificate 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

   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-186-2] using the SHA-1 hash
   [FIPS-180-2].

   The resulting signature is encoded as follows:

      string    "ssh-dss"
      string    dss_signature_blob

   The value for 'dss_signature_blob' is encoded as a string containing
   r, followed by s (which are 160-bit 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 performed according to
   the RSASSA-PKCS1-v1_5 scheme in [RFC3447] using the SHA-1 hash.

   The resulting signature is encoded as follows:

      string    "ssh-rsa"
      string    rsa_signature_blob

   The value for '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 "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" is as above, but indicates that the key is a
   DSS-key.

7.  Key Exchange

   Key exchange (kex) begins by each side sending name-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.

   The 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 7.1).

   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.

   A key exchange method uses explicit server authentication if the key
   exchange messages include a signature or other proof of the server's
   authenticity.  A key exchange method uses implicit server
   authentication if, in order to prove its authenticity, the server
   also has to prove that it knows the shared secret, K, by sending a
   message and a corresponding MAC that the client can verify.

   The key exchange method defined by this document uses explicit server
   authentication.  However, key exchange methods with implicit server
   authentication MAY be used with this protocol.  After a key exchange
   with implicit server authentication, the client MUST wait for a
   response to its service request message before sending any further
   data.

7.1.  Algorithm Negotiation

   Key exchange begins by each side sending the following packet:

      byte         SSH_MSG_KEXINIT
      byte[16]     cookie (random bytes)
      name-list    kex_algorithms
      name-list    server_host_key_algorithms
      name-list    encryption_algorithms_client_to_server
      name-list    encryption_algorithms_server_to_client
      name-list    mac_algorithms_client_to_server
      name-list    mac_algorithms_server_to_client
      name-list    compression_algorithms_client_to_server
      name-list    compression_algorithms_server_to_client
      name-list    languages_client_to_server
      name-list    languages_server_to_client
      boolean      first_kex_packet_follows
      uint32       0 (reserved for future extension)

   Each of the algorithm name-lists MUST be a comma-separated list of
   algorithm names (see Algorithm Naming in [SSH-ARCH] and additional
   information in [SSH-NUMBERS]).  Each supported (allowed) algorithm
   MUST be listed in order of preference, from most to least.

   The first algorithm in each name-list MUST be the preferred (guessed)
   algorithm.  Each name-list 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
         A name-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
         algorithm requires a signature or an encryption-capable host
         key.  It MUST be possible to determine this from the public key
         algorithm name.  The first algorithm on the client's name-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
         A name-list of acceptable symmetric encryption algorithms (also
         known as ciphers) in order of preference.  The chosen
         encryption algorithm to each direction MUST be the first
         algorithm on the client's name-list that is also on the
         server's name-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
         6.3.

      mac_algorithms
         A name-list of acceptable MAC algorithms in order of
         preference.  The chosen MAC algorithm MUST be the first
         algorithm on the client's name-list that is also on the
         server's name-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 6.4.

      compression_algorithms
         A name-list of acceptable compression algorithms in order of
         preference.  The chosen compression algorithm MUST be the first
         algorithm on the client's name-list that is also on the
         server's name-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 6.2.

      languages
         This is a name-list of language tags in order of preference
         [RFC3066].  Both parties MAY ignore this name-list.  If there
         are no language preferences, this name-list SHOULD be empty as
         defined in Section 5 of [SSH-ARCH].  Language tags SHOULD NOT
         be present unless they are known to be needed by the sending
         party.

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

   Once a party has sent a SSH_MSG_KEXINIT message for key exchange or
   re-exchange, until it has sent a SSH_MSG_NEWKEYS message (Section
   7.3), it MUST NOT send any messages other than:

   o  Transport layer generic messages (1 to 19) (but
      SSH_MSG_SERVICE_REQUEST and SSH_MSG_SERVICE_ACCEPT MUST NOT be
      sent);

   o  Algorithm negotiation messages (20 to 29) (but further
      SSH_MSG_KEXINIT messages MUST NOT be sent);

   o  Specific key exchange method messages (30 to 49).

   The provisions of Section 11 apply to unrecognized messages.

   Note, however, that during a key re-exchange, after sending a
   SSH_MSG_KEXINIT message, each party MUST be prepared to process an
   arbitrary number of messages that may be in-flight before receiving a
   SSH_MSG_KEXINIT message from the other party.

7.2.  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.  As
   many bytes as needed are taken from the beginning of the hash value.
   If the key length needed is 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:

      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.

7.3.  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
   used for receiving.

   The purpose of this message is to ensure that a party is able to
   respond with an SSH_MSG_DISCONNECT message that the other party can
   understand if something goes wrong with the key exchange.

      byte      SSH_MSG_NEWKEYS

8.  Diffie-Hellman Key Exchange

   The Diffie-Hellman (DH) key exchange provides a shared secret that
   cannot be determined by either party alone.  The key exchange is
   combined with a signature with the host key to provide host
   authentication.  This key exchange method provides explicit server
   authentication as defined in Section 7.

   The following steps are used to exchange a key.  In this, C is the
   client; S is the server; p is a large safe prime; g is a generator
   for a subgroup of GF(p); q is the order of the subgroup; V_S is S's
   identification string; V_C is C's identification string; K_S is S's
   public host key; I_C is C's SSH_MSG_KEXINIT message and I_S is S's
   SSH_MSG_KEXINIT message that 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.

   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.

   Values of 'e' or 'f' that are not in the range [1, p-1] MUST NOT be
   sent or accepted by either side.  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 SSH_MSG_KEXINIT messages.

   First, the client sends the following:

      byte      SSH_MSG_KEXDH_INIT
      mpint     e

   The server then 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 identification string (CR and LF
                excluded)
      string    V_S, the server's identification string (CR and LF
                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

   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 (e.g., "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.

8.1.  diffie-hellman-group1-sha1

   The "diffie-hellman-group1-sha1" method specifies the Diffie-Hellman
   key exchange with SHA-1 as HASH, and Oakley Group 2 [RFC2409] (1024-
   bit MODP Group).  This method MUST be supported for interoperability
   as all of the known implementations currently support it.  Note that
   this method is named using the phrase "group1", even though it
   specifies the use of Oakley Group 2.

8.2.  diffie-hellman-group14-sha1

   The "diffie-hellman-group14-sha1" method specifies a Diffie-Hellman
   key exchange with SHA-1 as HASH and Oakley Group 14 [RFC3526] (2048-
   bit MODP Group), and it MUST also be supported.

9.  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 7.1).  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 be 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.

10.  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] and [SSH-NUMBERS].

   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
   PRIVATE USE syntax of "servicename@domain".

      byte      SSH_MSG_SERVICE_REQUEST
      string    service name

   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 [SSH-ARCH] and [SSH-NUMBERS]).  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 a response to its service request message
   before sending any further data.

11.  Additional Messages

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

11.1.  Disconnection Message

      byte      SSH_MSG_DISCONNECT
      uint32    reason code
      string    description in ISO-10646 UTF-8 encoding [RFC3629]
      string    language tag [RFC3066]

   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 Disconnection Message 'description' string gives a more specific
   explanation in a human-readable form.  The Disconnection Message
   'reason code' gives the reason in a more machine-readable format
   (suitable for localization), and can have the values as displayed in
   the table below.  Note that the decimal representation is displayed
   in this table for readability, but the values are actually uint32
   values.

           Symbolic name                                reason code
           -------------                                -----------
      SSH_DISCONNECT_HOST_NOT_ALLOWED_TO_CONNECT             1
      SSH_DISCONNECT_PROTOCOL_ERROR                          2
      SSH_DISCONNECT_KEY_EXCHANGE_FAILED                     3
      SSH_DISCONNECT_RESERVED                                4
      SSH_DISCONNECT_MAC_ERROR                               5
      SSH_DISCONNECT_COMPRESSION_ERROR                       6
      SSH_DISCONNECT_SERVICE_NOT_AVAILABLE                   7
      SSH_DISCONNECT_PROTOCOL_VERSION_NOT_SUPPORTED          8
      SSH_DISCONNECT_HOST_KEY_NOT_VERIFIABLE                 9
      SSH_DISCONNECT_CONNECTION_LOST                        10
      SSH_DISCONNECT_BY_APPLICATION                         11
      SSH_DISCONNECT_TOO_MANY_CONNECTIONS                   12
      SSH_DISCONNECT_AUTH_CANCELLED_BY_USER                 13
      SSH_DISCONNECT_NO_MORE_AUTH_METHODS_AVAILABLE         14
      SSH_DISCONNECT_ILLEGAL_USER_NAME                      15

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

   Requests for assignments of new Disconnection Message 'reason code'
   values (and associated 'description' text) in the range of 0x00000010
   to 0xFDFFFFFF MUST be done through the IETF CONSENSUS method, as
   described in [RFC2434].  The Disconnection Message 'reason code'
   values in the range of 0xFE000000 through 0xFFFFFFFF are reserved for
   PRIVATE USE.  As noted, the actual instructions to the IANA are in
   [SSH-NUMBERS].

11.2.  Ignored Data Message

      byte      SSH_MSG_IGNORE
      string    data

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

11.3.  Debug Message

      byte      SSH_MSG_DEBUG
      boolean   always_display
      string    message in ISO-10646 UTF-8 encoding [RFC3629]
      string    language tag [RFC3066]

   All implementations MUST understand this message, but they are
   allowed to ignore it.  This message is used to transmit 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, the terminal control character
   filtering discussed in [SSH-ARCH] should be used to avoid attacks by
   sending terminal control characters.

11.4.  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

12.  Summary of Message Numbers

   The following is a summary of messages and their associated message
   number.

         SSH_MSG_DISCONNECT             1
         SSH_MSG_IGNORE                 2
         SSH_MSG_UNIMPLEMENTED          3
         SSH_MSG_DEBUG                  4
         SSH_MSG_SERVICE_REQUEST        5
         SSH_MSG_SERVICE_ACCEPT         6
         SSH_MSG_KEXINIT                20
         SSH_MSG_NEWKEYS                21

   Note that numbers 30-49 are used for kex packets.  Different kex
   methods may reuse message numbers in this range.

13.  IANA Considerations

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

14.  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.

   Full security considerations for this protocol are provided in
   [SSH-ARCH].

15.  References

15.1.  Normative References

   [SSH-ARCH]     Ylonen, T. and C. Lonvick, Ed., "The Secure Shell
                  (SSH) Protocol Architecture", RFC 4251, January 2006.

   [SSH-USERAUTH] Ylonen, T. and C. Lonvick, Ed., "The Secure Shell
                  (SSH) Authentication Protocol", RFC 4252, January
                  2006.

   [SSH-CONNECT]  Ylonen, T. and C. Lonvick, Ed., "The Secure Shell
                  (SSH) Connection Protocol", RFC 4254, January 2006.

   [SSH-NUMBERS]  Lehtinen, S. and C. Lonvick, Ed., "The Secure Shell
                  (SSH) Protocol Assigned Numbers", RFC 4250, January
                  2006.

   [RFC1321]      Rivest, R., "The MD5 Message-Digest Algorithm ", RFC
                  1321, April 1992.

   [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.

   [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.

   [RFC2409]      Harkins, D. and D. Carrel, "The Internet Key Exchange
                  (IKE)", RFC 2409, November 1998.

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

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

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

   [RFC3447]      Jonsson, J. and B. Kaliski, "Public-Key Cryptography
                  Standards (PKCS) #1: RSA Cryptography Specifications
                  Version 2.1", RFC 3447, February 2003.

   [RFC3526]      Kivinen, T. and M. Kojo, "More Modular Exponential
                  (MODP) Diffie-Hellman groups for Internet Key Exchange
                  (IKE)", RFC 3526, May 2003.

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

   [FIPS-180-2]   US National Institute of Standards and Technology,
                  "Secure Hash Standard (SHS)", Federal Information
                  Processing Standards Publication 180-2, August 2002.

   [FIPS-186-2]   US National Institute of Standards and Technology,
                  "Digital Signature Standard (DSS)", Federal
                  Information Processing Standards Publication 186-2,
                  January 2000.

   [FIPS-197]     US National Institute of Standards and Technology,
                  "Advanced Encryption Standard (AES)", Federal
                  Information Processing Standards Publication 197,
                  November 2001.

   [FIPS-46-3]    US National Institute of Standards and Technology,
                  "Data Encryption Standard (DES)", Federal Information
                  Processing Standards Publication 46-3, October 1999.

   [SCHNEIER]     Schneier, B., "Applied Cryptography Second Edition:
                  protocols algorithms and source in code in C", John
                  Wiley and Sons, New York, NY, 1996.

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

15.2.  Informative References

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

   [RFC1661]      Simpson, W., "The Point-to-Point Protocol (PPP)", STD
                  51, RFC 1661, July 1994.

   [RFC2412]      Orman, H., "The OAKLEY Key Determination Protocol",
                  RFC 2412, November 1998.

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

Authors' Addresses

   Tatu Ylonen
   SSH Communications Security Corp
   Valimotie 17
   00380 Helsinki
   Finland

   EMail: ylo@ssh.com

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

   EMail: clonvick@cisco.com

Trademark Notice

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