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Network Working Group                                          T. Ylonen
INTERNET-DRAFT                                                T. Kivinen
draft-ietf-secsh-transport-07.txt                            M. Saarinen
Expires in six months                                           T. Rinne
                                                             S. Lehtinen
                                             SSH Communications Security
                                                             11 May 2000


                      SSH Transport Layer Protocol

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.

Abstract

SSH is a protocol for secure remote login and other secure network ser-
vices over an insecure network.  This document describes the SSH trans-
port 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 pro-
vides strong encryption, server authentication, and integrity protec-
tion. It may also provide compression.  Key exchange method, public key
algorithm, symmetric encryption algorithm, message authentication algo-
rithm, 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 proto-
col.











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Table of Contents

1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . .  2
2.  Conventions Used in This Document   . . . . . . . . . . . . . . .  3
3.  Connection Setup  . . . . . . . . . . . . . . . . . . . . . . . .  3
  3.1.  Use over TCP/IP   . . . . . . . . . . . . . . . . . . . . . .  3
  3.2.  Protocol Version Exchange   . . . . . . . . . . . . . . . . .  3
  3.3.  Compatibility with Old SSH Versions   . . . . . . . . . . . .  4
    3.3.1.  Old Client, New Server  . . . . . . . . . . . . . . . . .  4
    3.3.2.  New Client, Old Server  . . . . . . . . . . . . . . . . .  4
4.  Binary Packet Protocol  . . . . . . . . . . . . . . . . . . . . .  4
  4.1.  Maximum Packet Length   . . . . . . . . . . . . . . . . . . .  5
  4.2.  Compression   . . . . . . . . . . . . . . . . . . . . . . . .  5
  4.3.  Encryption  . . . . . . . . . . . . . . . . . . . . . . . . .  6
  4.4.  Data Integrity  . . . . . . . . . . . . . . . . . . . . . . .  7
  4.5.  Key Exchange Methods  . . . . . . . . . . . . . . . . . . . .  8
  4.6.  Public Key Algorithms   . . . . . . . . . . . . . . . . . . .  8
5.  Key Exchange  . . . . . . . . . . . . . . . . . . . . . . . . . . 10
  5.1.  Algorithm Negotiation   . . . . . . . . . . . . . . . . . . . 10
  5.2.  Output from Key Exchange  . . . . . . . . . . . . . . . . . . 12
  5.3.  Taking Keys into Use  . . . . . . . . . . . . . . . . . . . . 13
6.  Diffie-Hellman Key Exchange   . . . . . . . . . . . . . . . . . . 14
  6.1.  diffie-hellman-group1-sha1  . . . . . . . . . . . . . . . . . 15
7.  Key Re-Exchange   . . . . . . . . . . . . . . . . . . . . . . . . 16
8.  Service Request   . . . . . . . . . . . . . . . . . . . . . . . . 16
9.  Additional Messages   . . . . . . . . . . . . . . . . . . . . . . 17
  9.1.  Disconnection Message   . . . . . . . . . . . . . . . . . . . 17
  9.2.  Ignored Data Message  . . . . . . . . . . . . . . . . . . . . 18
  9.3.  Debug Message   . . . . . . . . . . . . . . . . . . . . . . . 18
  9.4.  Reserved Messages   . . . . . . . . . . . . . . . . . . . . . 18
10.  Summary of Message Numbers   . . . . . . . . . . . . . . . . . . 18
11.  Security Considerations  . . . . . . . . . . . . . . . . . . . . 19
12.  Trademark Issues   . . . . . . . . . . . . . . . . . . . . . . . 19
13.  References   . . . . . . . . . . . . . . . . . . . . . . . . . . 19
14.  Authors' Addresses   . . . . . . . . . . . . . . . . . . . . . . 20



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


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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
[RFC-2119].

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
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 (Section ``Diffie-
Hellman Key Exchange'').

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 [RFC-2044] (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.



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

3.3.1.  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 protocol version 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 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
desired.  The server MUST NOT send additional data before the version
string, in the compatibility mode.

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

3.3.2.  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 of the following format.

  uint32    packet_length
  byte      padding_length
  byte[n1]  payload; n1 = packet_length - padding_length - 1


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  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.

    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).  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.




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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.

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        GNU ZLIB (LZ77) compression

The "zlib" compression is described in [RFC-1950] and in [RFC-1951]. 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 [SSH-ARCH].

4.3.  Encryption

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

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

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

The following ciphers are currently defined:

  3des-cbc         REQUIRED          three-key 3DES in CBC mode
  blowfish-cbc     RECOMMENDED       Blowfish in CBC mode
  twofish-cbc      RECOMMENDED       Twofish in CBC mode
  arcfour          OPTIONAL          the ARCFOUR stream cipher
  idea-cbc         OPTIONAL          IDEA in CBC mode
  cast128-cbc      OPTIONAL          CAST-128 in CBC mode
  none             OPTIONAL          no encryption; NOT RECOMMENDED

The "3des-cbc" cipher is three-key triple-DES (encrypt-decrypt-encrypt),
where the first 8 bytes of the key are used for the first encryption,


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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" cipher is Twofish in CBC mode, with 256 bit keys as
described in Twofish AES submission. This is a block cipher with 16 byte
blocks.

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.

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 [RFC-2144].

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

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

4.4.  Data Integrity

Data integrity is protected by including with each packet a message
authentication code (MAC) that is computed from a shared secret, packet
sequence number, and the contents of the packet.
The message authentication algorithm and key are negotiated during key
exchange.  Initially, no MAC will be in effect, and its length MUST be
zero.  After key exchange, the selected MAC will be computed before
encryption from the concatenation of packet data:

  mac = MAC(key, sequence_number || unencrypted_packet)

where unencrypted_packet is the entire packet without MAC (the length
fields, payload and padding), and sequence_number is an implicit packet
sequence number represented as uint32.  The sequence number is initial-
ized 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.


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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.

The following MAC algorithms are currently defined:

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

The "hmac-*" algorithms are described in [RFC-2104].  The "*-n" MACs use
only the first n bits of the resulting value.

The hash algorithms are described in [Schneier].

The "none" method is NOT RECOMMENDED.  An active attacker may be able to
modify transmitted data if this is used.

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

4.5.  Key Exchange Methods

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

Only one REQUIRED key exchange method has been defined:

  diffie-hellman-group1-sha1       REQUIRED

This method is described later in this document.

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

4.6.  Public Key Algorithms

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

There are several aspects that define a public key type:

o  Key format: how is the key encoded and how are certificates
   represented.  The key blobs in this protocol MAY contain certificates
   in addition to keys.

o  Signature and/or encryption algorithms.  Some key types may not
   support both signing and encryption.  Key usage may also be
   restricted by policy statements in e.g. certificates.  In this case,
   different key types SHOULD be defined for the different policy
   alternatives.


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

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

ssh-dss              REQUIRED     sign      Simple DSS
x509v3               RECOMMENDED  sign      X.509 certificates
spki                 OPTIONAL     sign      SPKI certificates
pgp                  OPTIONAL     sign      OpenPGP certificates

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

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

Certificates and public keys are encoded as:

  uint32   combined length of the format identifier and associated data
  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:

  uint32    length
  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 are 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:

  uint32    length
  string    "ssh-dss"
  string    dss_signature_blob

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

The "x509v3" method indicates that the certificates, the public key, and


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the resulting signature are in X.509v3 compatible DER-encoded format.
The formats used in X.509v3 is described in [PKIX-Part1].

The "spki" method indicates that the certificate blob contains a
sequence of SPKI certificates. The format of SPKI certificates is
described in [SPKI].

The "pgp" method indicates the the certificates, the public key, and the
signature are in OpenPGP compatible binary format.  [RFC-2440]

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.  If all algorithms were guessed
right, 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
  boolean   first_kex_packet_follows
  uint32    0 (reserved for future extension)

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

The first algorithm in each list MUST be the preferred (guessed)


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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 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:

   o  the server also supports the algorithm,

   o  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

   o  if the algorithm requires a signature-capable host key, there is a
      signature-capable algorithm on the server's
      server_host_key_algorithms  that is also supported by the client.

      If no algorithm satisfying all these conditions can be found, the
      connection fails, and both sides MUST disconnect.

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

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

      Algorithm selection depends on whether the chosen key exchange
      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


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      disconnect.

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

    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 ``Data
      Integrity''.

    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
      ``Compression''.

    languages
      This is a comma-separated list of language tags in order of
      preference [RFC-1766]. 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.  If the other
      party's guess was wrong, and this field was true, the next packet
      MUST be silently ignored, and both sides MUST then act as
      determined by the negotiated key exchange method.  If the guess
      was right, key exchange MUST continue using the guessed packet.

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

5.2.  Output from Key Exchange

The key exchange produces two values: a shared secret K, and an exchange
hash H.  Encryption and authentication keys are derived from these. The
exchange hash H from the first key exchange is additionally used as the
session identifier, which is a unique identifier for this connection.


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It is used by authentication methods as a part of the data that is
signed as a proof of possession of a private key.  Once computed, the
session identifier is not changed, even if keys are later re-exchanged.

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

Encryption keys MUST be computed as HASH of a known value and K as
follows:

o  Initial IV client to server: HASH(K || H || "A" || session_id) (Here
   K is encoded as mpint and "A" as byte and session_id as raw data."A"
   means the single character A, ascii 65).

o  Initial IV server to client: HASH(K || H || "B" || session_id)

o  Encryption key client to server: HASH(K || H || "C" || session_id)

o  Encryption key server to client: HASH(K || H || "D" || session_id)

o  Integrity key client to server: HASH(K || H || "E" || session_id)

o  Integrity key server to client: HASH(K || H || "F" || session_id)

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

  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 || ...

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


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with a disconnect message that the other party can understand if
something goes wrong with the key exchange.  Implementations MUST NOT
accept any other messages after key exchange before receiving
SSH_MSG_NEWKEYS.

  byte      SSH_MSG_NEWKEYS

6.  Diffie-Hellman Key Exchange

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

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

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

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:

  byte      SSH_MSG_KEXDH_INIT
  mpint     e

The server responds with:



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

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

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

This value is called the exchange hash, and it is used to authenticate
the key exchange.

The signature algorithm MUST be applied over H, not the original data.
Most signature algorithms include hashing and additional padding.  For
example, a "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.



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

7.  Key Re-Exchange

Key re-exchange is started by sending a SSH_MSG_KEXINIT packet when not
already doing a key exchange (as described in Section ``Algorithm
Negotiation'').  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 [SSH-ARCH].

Currently, the following names have been reserved:

  ssh-userauth
  ssh-connection

Similar local naming policy is applied to the service names that is
applied to the algorithm names; a local service should use the
servicename@domain syntax.
  byte      SSH_MSG_SERVICE_REQUEST
  string    service name

If the server rejects the service request, it SHOULD send an appropriate
SSH_MSG_DISCONNECT message and MUST disconnect.


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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

  byte      SSH_MSG_SERVICE_ACCEPT
  string    service name

Message numbers used by services should be in the area reserved for them
(see Section ``Summary of Message 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 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 [RFC-2044]
  string    language tag [RFC-1766]

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
  #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


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

9.2.  Ignored Data Message

  byte      SSH_MSG_IGNORE
  string    data

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

9.3.  Debug Message

  byte      SSH_MSG_DEBUG
  boolean   always_display
  string    message [RFC-2044]
  string    language tag [RFC-1766]

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

The message doesn't need to contain a newline. It is, however, allowed
to consist of multiple lines separated by CRLF (carriage return - line
feed) pairs.

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

9.4.  Reserved Messages

An implementation MUST respond to all unrecognized messages with an
SSH_MSG_UNIMPLEMENTED message in the order in which they 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


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  #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 unsecure
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.

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

SSH is a registered trademark and Secure Shell is a trademark of SSH
Communications Security Corp.  SSH Communications Security Corp permits
the use of these trademarks as the name of this standard and protocol,
and permits their use to describe that a product conforms to this
standard, provided that the following acknowledgement is included where
the trademarks are used: ``SSH is a registered trademark and Secure
Shell is a trademark of SSH Communications Security Corp
(www.ssh.com)''.  These trademarks may not be used as part of a product
name or in otherwise confusing manner without prior written permission
of SSH Communications Security Corp.

13.  References



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[FIPS-186] Federal Information Processing Standards Publication (FIPS
PUB) 186, Digital Signature Standard, 18 May 1994.

[Orm96] Orman, H., "The Oakley Key Determination Protocol", version 1,
TR97-92, Department of Computer Science Technical Report, University of
Arizona.

[PKIX-Part1] Housley, R., et al, "Internet X.509 Public Key
Infrastructure, Certificate and CRL Profile", Internet Draft, draft-
ietf-pkix-ipki-part1-11.txt

[RFC-1034] Mockapetris, P., "Domain Names - Concepts and Facilities",
November 1987.
[RFC-1766] Alvestrand, H., "Tags for the Identification of Languages",
March 1995.

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

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

[RFC-2044] Yergeau, F., "UTF-8, a Transformation Format of Unicode and
ISO 10646", October 1996.

[RFC-2104] Krawczyk, H., Bellare, M., and Canetti, R., "HMAC: Keyed-
Hashing for Message Authentication", February 1997

[RFC-2119] Bradner, S., "Key words for use in RFCs to indicate
Requirement Levels", March 1997.

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

[RFC-2440] Callas, J., et al, "OpenPGP Message Format", November 1998.

[Schneier] Schneier, B., "Applied Cryptography Second Edition:
protocols, algorithms, and source code in C", 2nd edition, John Wiley &
Sons, New York, NY, 1996.

[SSH-ARCH] Ylonen, T., et al, "SSH Protocol Architecture", Internet
Draft, draft-ietf-secsh-architecture-05.txt

[SSH-USERAUTH] Ylonen, T., et al, "SSH Authentication Protocol",
Internet Draft, draft-ietf-secsh-userauth-07.txt

[SSH-CONNECT] Ylonen, T., et al, "SSH Connection Protocol", Internet
Draft, draft-ietf-secsh-connect-07.txt

14.  Authors' Addresses

    Tatu Ylonen
    SSH Communications Security Corp
    Fredrikinkatu 42


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    FIN-00100 HELSINKI
    Finland
    E-mail: ylo@ssh.com

    Tero Kivinen
    SSH Communications Security Corp
    Fredrikinkatu 42
    FIN-00100 HELSINKI
    Finland
    E-mail: kivinen@ssh.com

    Markku-Juhani O. Saarinen
    University of Jyvaskyla

    Timo J. Rinne
    SSH Communications Security Corp
    Fredrikinkatu 42
    FIN-00100 HELSINKI
    Finland
    E-mail: tri@ssh.com

    Sami Lehtinen
    SSH Communications Security Corp
    Fredrikinkatu 42
    FIN-00100 HELSINKI
    Finland
    E-mail: sjl@ssh.com



























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