





Network Working Group                                          T. Ylonen
Internet-Draft                         Helsinki University of Technology
draft-ylonen-ssh-protocol-00.txt                        15 November 1995
Expires: 15 May 1996


               The SSH (Secure Shell) Remote Login Protocol

Status of This Memo

   This document is an Internet-Draft.   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.

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   months  and  may  be updated, replaced, or obsoleted by other docu-
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   reference  material  or  to  cite them other than as ``work in pro-
   gress.''

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   ``1id-abstracts.txt'' listing contained in the Internet- Drafts
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   munnari.oz.au (Pacific Rim), ds.internic.net (US East Coast), or
   ftp.isi.edu (US West Coast).

   The distribution of  this  memo  is  unlimited.

Introduction

   SSH (Secure Shell) is a program to log into another computer over a
   network, to execute commands in a remote machine, and to move files
   from one machine to another.  It provides strong authentication and
   secure communications over insecure networks.  Its features include
   the following:

   o    Closes several security holes (e.g., IP, routing, and DNS spoof-
        ing).  New authentication methods: .rhosts together with RSA
        [RSA] based host authentication, and pure RSA authentication.

   o    All communications are automatically and transparently
        encrypted.  Encryption is also used to protect integrity.

   o    X11 connection forwarding provides secure X11 sessions.

   o    Arbitrary TCP/IP ports can be redirected over the encrypted
        channel in both directions.



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   o    Client RSA-authenticates the server machine in the beginning of
        every connection to prevent trojan horses (by routing or DNS
        spoofing) and man-in-the-middle attacks, and the server RSA-
        authenticates the client machine before accepting .rhosts or
        /etc/hosts.equiv authentication (to prevent DNS, routing, or IP
        spoofing).

   o    An authentication agent, running in the user's local workstation
        or laptop, can be used to hold the user's RSA authentication
        keys.

   The goal has been to make the software as easy to use as possible for
   ordinary users.  The protocol has been designed to be as secure as
   possible while making it possible to create implementations that are
   easy to use and install.  The sample implementation has a number of
   convenient features that are not described in this document as they
   are not relevant for the protocol.


Overview of the Protocol

   The software consists of a server program running on a server
   machine, and a client program running on a client machine (plus a few
   auxiliary programs).  The machines are connected by an insecure IP
   [RFC0791] network (that can be monitored, tampered with, and spoofed
   by hostile parties).

   A connection is always initiated by the client side.  The server
   listens on a specific port waiting for connections.  Many clients may
   connect to the same server machine.

   The client and the server are connected via a TCP/IP [RFC0793] socket
   that is used for bidirectional communication.  Other types of tran-
   sport can be used but are currently not defined.

   When the client connects the server, the server accepts the connec-
   tion and responds by sending back its version identification string.
   The client parses the server's identification, and sends its own
   identification.  The purpose of the identification strings is to
   validate that the connection was to the correct port, declare the
   protocol version number used, and to declare the software version
   used on each side (for debugging purposes).  The identification
   strings are human-readable.  If either side fails to understand or
   support the other side's version, it closes the connection.

   After the protocol identification phase, both sides switch to a
   packet based binary protocol.  The server starts by sending its host
   key (every host has an RSA key used to authenticate the host), server



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   key (an RSA key regenerated every hour), and other information to the
   client.  The client then generates a 256 bit session key, encrypts it
   using both RSA keys (see below for details), and sends the encrypted
   session key and selected cipher type to the server.  Both sides then
   turn on encryption using the selected algorithm and key.  The server
   sends an encrypted confirmation message to the client.

   The client then authenticates itself using any of a number of authen-
   tication methods.  The currently supported authentication methods are
   .rhosts or /etc/hosts.equiv authentication (disabled by default), the
   same with RSA-based host authentication, RSA authentication, and
   password authentication.

   After successful authentication, the client makes a number of
   requests to prepare for the session.  Typical requests include allo-
   cating a pseudo tty, starting X11 [X11] or TCP/IP port forwarding,
   starting authentication agent forwarding, and executing the shell or
   a command.

   When a shell or command is executed, the connection enters interac-
   tive session mode.  In this mode, data is passed in both directions,
   new forwarded connections may be opened, etc.  The interactive ses-
   sion normally terminates when the server sends the exit status of the
   program to the client.


   The protocol makes several reservations for future extensibility.
   First of all, the initial protocol identification messages include
   the protocol version number.  Second, the first packet by both sides
   includes a protocol flags field, which can be used to agree on exten-
   sions in a compatible manner.  Third, the authentication and session
   preparation phases work so that the client sends requests to the
   server, and the server responds with success or failure.  If the
   client sends a request that the server does not support, the server
   simply returns failure for it.  This permits compatible addition of
   new authentication methods and preparation operations.  The interac-
   tive session phase, on the other hand, works asynchronously and does
   not permit the use of any extensions (because there is no easy and
   reliable way to signal rejection to the other side and problems would
   be hard to debug).  Any compatible extensions to this phase must be
   agreed upon during any of the earlier phases.

The Binary Packet Protocol

   After the protocol identification strings, both sides only send spe-
   cially formatted packets.  The packet layout is as follows:

   o    Packet length: 32 bit unsigned integer, coded as four 8-bit



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        bytes, msb first.  Gives the length of the packet, not including
        the length field and padding.  The maximum length of a packet
        (not including the length field and padding) is 262144 bytes.

   o    Padding: 1-8 bytes of random data (or zeroes if not encrypting).
        The amount of padding is (8 - (length % 8)) bytes (where %
        stands for the modulo operator).  The rationale for always hav-
        ing some random padding at the beginning of each packet is to
        make known plaintext attacks more difficult.

   o    Packet type: 8-bit unsigned byte.  The value 255 is reserved for
        future extension.

   o    Data: binary data bytes, depending on the packet type.  The
        number of data bytes is the "length" field minus 5.

   o    Check bytes: 32-bit crc, four 8-bit bytes, msb first.  The crc
        is the Cyclic Redundancy Check, with the polynomial 0xedb88320,
        of the Padding, Packet type, and Data fields.  The crc is com-
        puted before any encryption.

   The packet, except for the length field, may be encrypted using any
   of a number of algorithms.  The length of the encrypted part (Padding
   + Type + Data + Check) is always a multiple of 8 bytes.  Typically
   the cipher is used in a chained mode, with all packets chained
   together as if it was a single data stream (the length field is never
   included in the encryption process).  Details of encryption are
   described below.

   When the session starts, encryption is turned off.  Encryption is
   enabled after the client has sent the session key.  The encryption
   algorithm to use is selected by the client.


Packet Compression

   If compression is supported (it is an optional feature, see
   SSH_CMSG_REQUEST_COMPRESSION below), the packet type and data fields
   of the packet are compressed using the gzip deflate algorithm [GZIP].
   If compression is in effect, the packet length field indicates the
   length of the compressed data, plus 4 for the crc.  The amount of
   padding is computed from the compressed data, so that the amount of
   data to be encrypted becomes a multiple of 8 bytes.

   When compressing, the packets (type + data portions) in each direc-
   tion are compressed as if they formed a continuous data stream, with
   only the current compression block flushed between packets.  This
   corresponds to the GNU ZLIB library Z_PARTIAL_FLUSH option.  The



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   compression dictionary is not flushed between packets.  The two
   directions are compressed independently of each other.


Packet Encryption

   The protocol supports several encryption methods.  During session
   initialization, the server sends a bitmask of all encryption methods
   that it supports, and the client selects one of these methods.  The
   client also generates a 256-bit random session key (32 8-bit bytes)
   and sends it to the server.

   The encryption methods supported by the current implementation, and
   their codes are:

          SSH_CIPHER_NONE   0      No encryption
          SSH_CIPHER_IDEA   1      IDEA in CFB mode
          SSH_CIPHER_DES    2      DES in CBC mode
          SSH_CIPHER_3DES   3      Triple-DES in CBC mode
          SSH_CIPHER_RC4    5      RC4



   All implementations are required to support SSH_CIPHER_3DES.  Sup-
   porting SSH_CIPHER_IDEA, SSH_CIPHER_RC4, and SSH_CIPHER_NONE is
   recommended. Other ciphers may be added at a later time; support
   for them is optional.

   For encryption, the encrypted portion of the packet is considered a
   linear byte stream.  The length of the stream is always a multiple of
   8.  The encrypted portions of consecutive packets (in the same direc-
   tion) are encrypted as if they were a continuous buffer (that is, any
   initialization vectors are passed from the previous packet to the
   next packet).  Data in each direction is encrypted independently.

   SSH_CIPHER_DES
        The key is taken from the first 8 bytes of the session key.  The
        least significant bit of each byte is ignored.  This results in
        56 bits of key data.  DES [DES] is used in CBC mode.  The iv
        (initialization vector) is initialized to all zeroes.

   SSH_CIPHER_3DES
        The variant of triple-DES used here works as follows: there are
        three independent DES-CBC ciphers, with independent initializa-
        tion vectors.  The data (the whole encrypted data stream) is
        first encrypted with the first cipher, then decrypted with the
        second cipher, and finally encrypted with the third cipher.  All



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        these operations are performed in CBC mode.

        The key for the first cipher is taken from the first 8 bytes of
        the session key; the key for the next cipher from the next 8
        bytes, and the key for the third cipher from the following 8
        bytes.  All three initialization vectors are initialized to
        zero.

        (Note: the variant of 3DES used here differs from some other
        descriptions.)

   SSH_CIPHER_IDEA
        The key is taken from the first 16 bytes of the session key.
        IDEA [IDEA] is used in CFB mode.  The initialization vector is
        initialized to all zeroes.

   SSH_CIPHER_RC4
        The first 16 bytes of the session key are used as the key for
        the server to client direction.  The remaining 16 bytes are used
        as the key for the client to server direction.  This gives
        independent 128-bit keys for each direction.

        This algorithm is the alleged RC4 cipher posted to the Usenet in
        1995.  It is widely believed to be equivalent with the original
        RSADSI RC4 cipher.  This is a very fast algorithm.


Data Type Encodings

   The Data field of each packet contains data encoded as described in
   this section.  There may be several data items; each item is coded as
   described here, and their representations are concatenated together
   (without any alignment or padding).

   Each data type is stored as follows:

   8-bit byte
        The byte is stored directly as a single byte.




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   32-bit unsigned integer
        Stored in 4 bytes, msb first.

   Arbitrary length binary string
        First 4 bytes are the length of the string, msb first (not
        including the length itself).  The following "length" bytes are
        the string value.  There are no terminating null characters.

   Multiple-precision integer
        First 2 bytes are the number of bits in the integer, msb first
        (for example, the value 0x00012345 would have 17 bits).  The
        value zero has zero bits.  It is permissible that the number of
        bits be larger than the real number of bits.

        The number of bits is followed by (bits + 7) / 8 bytes of binary
        data, msb first, giving the value of the integer.


TCP/IP Port Number and Other Options

   The server listens for connections on TCP/IP port 22.

   The client may connect the server from any port.  However, if the
   client wishes to use any form of .rhosts or /etc/hosts.equiv authen-
   tication, it must connect from a privileged port (less than 1024).

   For the IP Type of Service field [RFC0791], it is recommended that
   interactive sessions (those having a user terminal or forwarding X11
   connections) use the IPTOS_LOWDELAY, and non-interactive connections
   use IPTOS_THROUGHPUT.

   It is recommended that keepalives are used, because otherwise pro-
   grams on the server may never notice if the other end of the connec-
   tion is rebooted.


Protocol Version Identification

   After the socket is opened, the server sends an identification
   string, which is of the form "SSH-<protocolmajor>.<protocolminor>-
   <version>\n", where <protocolmajor> and <protocolminor> are integers
   and specify the protocol version number (not software distribution
   version).  <version> is server side software version string (max 40
   characters); it is not interpreted by the remote side but may be use-
   ful for debugging.

   The client parses the server's string, and sends a corresponding
   string with its own information in response.  If the server has lower



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   version number, and the client contains special code to emulate it,
   the client responds with the lower number; otherwise it responds with
   its own number.  The server then compares the version number the
   client sent with its own, and determines whether they can work
   together.  The server either disconnects, or sends the first packet
   using the binary packet protocol and both sides start working accord-
   ing to the lower of the protocol versions.

   By convention, changes which keep the protocol compatible with previ-
   ous versions keep the same major protocol version; changes that are
   not compatible increment the major version (which will hopefully
   never happen).  The version described in this document is 1.5.

Key Exchange and Server Host Authentication

   The first message sent by the server using the packet protocol is
   SSH_SMSG_PUBLIC_KEY.  It declares the server's host key, server pub-
   lic key, supported ciphers, supported authentication methods, and
   flags for protocol extensions.  It also contains a 64-bit random
   number (cookie) that must be returned in the client's reply (to make
   IP spoofing more difficult).  No encryption is used for this message.

   Both sides compute a session id as follows.  The modulus of the host
   key is interpreted as a byte string (without explicit length field,
   with minimum length able to hold the whole value), most significant
   byte first.  This string is concatenated with the server key inter-
   preted the same way.  Additionally, the cookie is concatenated with
   this.  Both sides compute MD5 of the resulting string.  The resulting
   16 bytes (128 bits) are stored by both parties and are called the
   session id.

   In other words,
        session_id = MD5(hostkey->n || servkey->n || cookie)

   The client responds with a SSH_CMSG_SESSION_KEY message, which con-
   tains the selected cipher type, a copy of the 64-bit cookie sent by
   the server, client's protocol flags, and a session key encrypted with
   both the server's host key and server key.  No encryption is used for
   this message.

   The session key is 32 8-bit bytes (a total of 256 random bits gen-
   erated by the client).  The client first xors the 16 bytes of the
   session id with the first 16 bytes of the session key.  The resulting
   string is then encrypted using the smaller key (one with smaller
   modulus), and the result is then encrypted using the other key.  The
   number of bits in the public modulus of the two keys must differ by
   at least 128 bits.




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   At each encryption step, a multiple-precision integer is constructed
   from the data to be encrypted as follows (the integer is here inter-
   preted as a sequence of bytes, msb first; the number of bytes is the
   number of bytes needed to represent the modulus).

   The most significant byte (which is only partial as the value must be
   less than the public modulus, which is never a power of two) is zero.

   The next byte contains the value 2 (which stands for public-key
   encrypted data in the PKCS standard [PKCS#1]).  Then, there are non-
   zero random bytes to fill any unused space, a zero byte, and the data
   to be encrypted in the least significant bytes, the last byte of the
   data in the least significant byte.

   This algorithm is used twice.  First, it is used to encrypt the 32
   random bytes generated by the client to be used as the session key
   (xored by the session id).  This value is converted to an integer as
   described above, and encrypted with RSA using the key with the
   smaller modulus.  The resulting integer is converted to a byte
   stream, msb first.  This byte stream is padded and encrypted identi-
   cally using the key with the larger modulus.

   After the client has sent the session key, it starts to use the
   selected algorithm and key for decrypting any received packets, and
   for encrypting any sent packets.  Separate ciphers are used for dif-
   ferent directions (that is, both directions have separate initializa-
   tion vectors or other state for the ciphers).

   When the server has received the session key message, and has turned
   on encryption, it sends a SSH_SMSG_SUCCESS message to the client.

   The recommended size of the host key is 1024 bits, and 768 bits for
   the server key.  The minimum size is 512 bits for the smaller key.


Declaring the User Name

   The client then sends a SSH_CMSG_USER message to the server.  This
   message specifies the user name to log in as.

   The server validates that such a user exists, checks whether authen-
   tication is needed, and responds with either SSH_SMSG_SUCCESS or
   SSH_SMSG_FAILURE.  SSH_SMSG_SUCCESS indicates that no authentication
   is needed for this user (no password), and authentication phase has
   now been completed.  SSH_SMSG_FAILURE indicates that authentication
   is needed (or the user does not exist).

   If the user does not exist, it is recommended that this returns



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   failure, but the server keeps reading messages from the client, and
   responds to any messages (except SSH_MSG_DISCONNECT, SSH_MSG_IGNORE,
   and SSH_MSG_DEBUG) with SSH_SMSG_FAILURE.  This way the client cannot
   be certain whether the user exists.


Authentication Phase

   Provided the server didn't immediately accept the login, an authenti-
   cation exchange begins.  The client sends messages to the server
   requesting different types of authentication in arbitrary order as
   many times as desired (however, the server may close the connection
   after a timeout).  The server always responds with SSH_SMSG_SUCCESS
   if it has accepted the authentication, and with SSH_SMSG_FAILURE if
   it has denied authentication with the requested method or it does not
   recognize the message.  Some authentication methods cause an exchange
   of further messages before the final result is sent.  The authentica-
   tion phase ends when the server responds with success.

   The recommended value for the authentication timeout (timeout before
   disconnecting if no successful authentication has been made) is 5
   minutes.

   The following authentication methods are currently supported:

      SSH_AUTH_RHOSTS       1   .rhosts or /etc/hosts.equiv
      SSH_AUTH_RSA          2   pure RSA authentication
      SSH_AUTH_PASSWORD     3   password authentication
      SSH_AUTH_RHOSTS_RSA   4   .rhosts with RSA host authentication


   SSH_AUTH_RHOSTS

        This is the authentication method used by rlogin and rsh
        [RFC1282].

        The client sends SSH_CMSG_AUTH_RHOSTS with the client-side user
        name as an argument.

        The server checks whether to permit authentication.  On UNIX
        systems, this is usually done by checking /etc/hosts.equiv, and
        .rhosts in the user's home directory.  The connection must come
        from a privileged port.

        It is recommended that the server checks that there are no IP
        options (such as source routing) specified for the socket before
        accepting this type of authentication.  The client host name
        should be reverse-mapped and then forward mapped to ensure that



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        it has the proper IP-address.

        This authentication method trusts the remote host (root on the
        remote host can pretend to be any other user on that host), the
        name services, and partially the network: anyone who can see
        packets coming out from the server machine can do IP-spoofing
        and pretend to be any machine; however, the protocol prevents
        blind IP-spoofing (which used to be possible with rlogin).

        Many sites probably want to disable this authentication method
        because of the fundamental insecurity of conventional .rhosts or
        /etc/hosts.equiv authentication when faced with spoofing.  It is
        recommended that this method not be supported by the server by
        default.

   SSH_AUTH_RHOSTS_RSA

        In addition to conventional .rhosts and hosts.equiv authentica-
        tion, this method additionally requires that the client host be
        authenticated using RSA.

        The client sends SSH_CMSG_AUTH_RHOSTS_RSA specifying the
        client-side user name, and the public host key of the client
        host.

        The server first checks if normal .rhosts or /etc/hosts.equiv
        authentication would be accepted, and if not, responds with
        SSH_SMSG_FAILURE.  Otherwise, it checks whether it knows the
        host key for the client machine (using the same name for the
        host that was used for checking the .rhosts and /etc/hosts.equiv
        files).  If it does not know the RSA key for the client, access
        is denied and SSH_SMSG_FAILURE is sent.

        If the server knows the host key of the client machine, it veri-
        fies that the given host key matches that known for the client.
        If not, access is denied and SSH_SMSG_FAILURE is sent.

        The server then sends a SSH_SMSG_AUTH_RSA_CHALLENGE message con-
        taining an encrypted challenge for the client.  The challenge is
        32 8-bit random bytes (256 bits).  When encrypted, the highest
        (partial) byte is left as zero, the next byte contains the value
        2, the following are non-zero random bytes, followed by a zero
        byte, and the challenge put in the remaining bytes.  This is
        then encrypted using RSA with the client host's public key.
        (The padding and encryption algorithm is the same as that used
        for the session key.)

        The client decrypts the challenge using its private host key,



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        concatenates this with the session id, and computes an MD5
        checksum of the resulting 48 bytes.  The MD5 output is returned
        as 16 bytes in a SSH_CMSG_AUTH_RSA_RESPONSE message.  (MD5 is
        used to deter chosen plaintext attacks against RSA; the session
        id binds it to a specific session).

        The server verifies that the MD5 of the decrypted challenge
        returned by the client matches that of the original value, and
        sends SSH_SMSG_SUCCESS if so.  Otherwise it sends
        SSH_SMSG_FAILURE and refuses the authentication attempt.

        This authentication method trusts the client side machine in
        that root on that machine can pretend to be any user on that
        machine.  Additionally, it trusts the client host key.  The name
        and/or IP address of the client host is only used to select the
        public host key.  The same host name is used when scanning
        .rhosts or /etc/hosts.equiv and when selecting the host key.  It
        would in principle be possible to eliminate the host name
        entirely and substitute it directly by the host key.  IP and/or
        DNS [RFC1034] spoofing can only be used to pretend to be a host
        for which the attacker has the private host key.

   SSH_AUTH_RSA

        The idea behind RSA authentication is that the server recognizes
        the public key offered by the client, generates a random chal-
        lenge, and encrypts the challenge with the public key.  The
        client must then prove that it has the corresponding private key
        by decrypting the challenge.

        The client sends SSH_CMSG_AUTH_RSA with public key modulus (n)
        as an argument.

        The server may respond immediately with SSH_SMSG_FAILURE if it
        does not permit authentication with this key.  Otherwise it gen-
        erates a challenge, encrypts it using the user's public key
        (stored on the server and identified using the modulus), and
        sends SSH_SMSG_AUTH_RSA_CHALLENGE with the challenge (mp-int) as
        an argument.

        The challenge is 32 8-bit random bytes (256 bits).  When
        encrypted, the highest (partial) byte is left as zero, the next
        byte contains the value 2, the following are non-zero random
        bytes, followed by a zero byte, and the challenge put in the
        remaining bytes.  This is then encrypted with the public key.
        (The padding and encryption algorithm is the same as that used
        for the session key.)




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        The client decrypts the challenge using its private key, con-
        catenates it with the session id, and computes an MD5 checksum
        of the resulting 48 bytes.  The MD5 output is returned as 16
        bytes in a SSH_CMSG_AUTH_RSA_RESPONSE message.  (Note that the
        MD5 is necessary to avoid chosen plaintext attacks against RSA;
        the session id binds it to a specific session.)

        The server verifies that the MD5 of the decrypted challenge
        returned by the client matches that of the original value, and
        sends SSH_SMSG_SUCCESS if so.  Otherwise it sends
        SSH_SMSG_FAILURE and refuses the authentication attempt.

        This authentication method does not trust the remote host, the
        network, name services, or anything else.  Authentication is
        based solely on the possession of the private identification
        keys.  Anyone in possession of the private keys can log in, but
        nobody else.

        The server may have additional requirements for a successful
        authentiation.  For example, to limit damage due to a comprom-
        ised RSA key, a server might restrict access to a limited set of
        hosts.

   SSH_AUTH_PASSWORD

        The client sends a SSH_CMSG_AUTH_PASSWORD message with the plain
        text password.  (Note that even though the password is plain
        text inside the message, it is normally encrypted by the packet
        mechanism.)

        The server verifies the password, and sends SSH_SMSG_SUCCESS if
        authentication was accepted and SSH_SMSG_FAILURE otherwise.

        Note that the password is read from the user by the client; the
        user never interacts with a login program.

        This authentication method does not trust the remote host, the
        network, name services or anything else.  Authentication is
        based solely on the possession of the password.  Anyone in pos-
        session of the password can log in, but nobody else.

Preparatory Operations

   After successful authentication, the server waits for a request from
   the client, processes the request, and responds with SSH_SMSG_SUCCESS
   whenever a request has been successfully processed.  If it receives a
   message that it does not recognize or it fails to honor a request, it
   returns SSH_SMSG_FAILURE.  It is expected that new message types



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   might be added to this phase in future.

   The following messages are currently defined for this phase.

   SSH_CMSG_REQUEST_COMPRESSION
        Requests that compression be enabled for this session.  A gzip-
        compatible compression level (1-9) is passed as an argument.

   SSH_CMSG_REQUEST_PTY
        Requests that a pseudo terminal device be allocated for this
        session.  The user terminal type and terminal modes are supplied
        as arguments.

   SSH_CMSG_X11_REQUEST_FORWARDING
        Requests forwarding of X11 connections from the remote machine
        to the local machine over the secure channel.  Causes an
        internet-domain socket to be allocated and the DISPLAY variable
        to be set on the server.  X11 authentication data is automati-
        cally passed to the server, and the client may implement spoof-
        ing of authentication data for added security.  The authentica-
        tion data is passed as arguments.

   SSH_CMSG_PORT_FORWARD_REQUEST
        Requests forwarding of a TCP/IP port on the server host over the
        secure channel.  What happens is that whenever a connection is
        made to the port on the server, a connection will be made from
        the client end to the specified host/port.  Any user can forward
        unprivileged ports; only the root can forward privileged ports
        (as determined by authentication done earlier).

   SSH_CMSG_AGENT_REQUEST_FORWARDING
        Requests forwarding of the connection to the authentication
        agent.

   SSH_CMSG_EXEC_SHELL
        Starts a shell (command interpreter) for the user, and moves
        into interactive session mode.

   SSH_CMSG_EXEC_CMD
        Executes the given command (actually "<shell> -c <command>" or
        equivalent) for the user, and moves into interactive session
        mode.


Interactive Session and Exchange of Data

   During the interactive session, any data written by the shell or com-
   mand running on the server machine is forwarded to stdin or stderr on



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   the client machine, and any input available from stdin on the client
   machine is forwarded to the program on the server machine.

   All exchange is asynchronous; either side can send at any time, and
   there are no acknowledgements (TCP/IP already provides reliable tran-
   sport, and the packet protocol protects against tampering or IP
   spoofing).

   When the client receives EOF from its standard input, it will send
   SSH_CMSG_EOF; however, this in no way terminates the exchange.  The
   exchange terminates and interactive mode is left when the server
   sends SSH_SMSG_EXITSTATUS to indicate that the client program has
   terminated.  Alternatively, either side may disconnect at any time by
   sending SSH_MSG_DISCONNECT or closing the connection.

   The server may send any of the following messages:

   SSH_SMSG_STDOUT_DATA
        Data written to stdout by the program running on the server.
        The data is passed as a string argument.  The client writes this
        data to stdout.

   SSH_SMSG_STDERR_DATA
        Data written to stderr by the program running on the server.
        The data is passed as a string argument.  The client writes this
        data to stderr.  (Note that if the program is running on a tty,
        it is not possible to separate stdout and stderr data, and all
        data will be sent as stdout data.)

   SSH_SMSG_EXITSTATUS
        Indicates that the shell or command has exited.  Exit status is
        passed as an integer argument.  This message causes termination
        of the interactive session.

   SSH_SMSG_AGENT_OPEN
        Indicates that someone on the server side is requesting a con-
        nection to the authentication agent.  The server-side channel
        number is passed as an argument.  The client must respond with
        either SSH_CHANNEL_OPEN_CONFIRMATION or
        SSH_CHANNEL_OPEN_FAILURE.

   SSH_SMSG_X11_OPEN
        Indicates that a connection has been made to the X11 socket on
        the server side and should be forwarded to the real X server.
        An integer argument indicates the channel number allocated for
        this connection on the server side.  The client should send back
        either SSH_MSG_CHANNEL_OPEN_CONFIRMATION or
        SSH_MSG_CHANNEL_OPEN_FAILURE with the same server side channel



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        number.

   SSH_MSG_PORT_OPEN
        Indicates that a connection has been made to a port on the
        server side for which forwarding has been requested.  Arguments
        are server side channel number, host name to connect to, and
        port to connect to.  The client should send back either
        SSH_MSG_CHANNEL_OPEN_CONFIRMATION or
        SSH_MSG_CHANNEL_OPEN_FAILURE with the same server side channel
        number.

   SSH_MSG_CHANNEL_OPEN_CONFIRMATION
        This is sent by the server to indicate that it has opened a con-
        nection as requested in a previous message.  The first argument
        indicates the client side channel number, and the second argu-
        ment is the channel number that the server has allocated for
        this connection.

   SSH_MSG_CHANNEL_OPEN_FAILURE
        This is sent by the server to indicate that it failed to open a
        connection as requested in a previous message.  The client-side
        channel number is passed as an argument.  The client will close
        the descriptor associated with the channel and free the channel.

   SSH_MSG_CHANNEL_DATA
        This packet contains data for a channel from the server.  The
        first argument is the client-side channel number, and the second
        argument (a string) is the data.

   SSH_MSG_CHANNEL_CLOSE
        This is sent by the server to indicate that whoever was in the
        other end of the channel has closed it.  The argument is the
        client side channel number.  The client will let all buffered
        data in the channel to drain, and when ready, will close the
        socket, free the channel, and send the server a
        SSH_MSG_CHANNEL_CLOSE_CONFIRMATION message for the channel.

   SSH_MSG_CHANNEL_CLOSE_CONFIRMATION
        This is send by the server to indicate that a channel previously
        closed by the client has now been closed on the server side as
        well.  The argument indicates the client channel number.  The
        client frees the channel.

   The client may send any of the following messages:

   SSH_CMSG_STDIN_DATA
        This is data to be sent as input to the program running on the
        server.  The data is passed as a string.



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   SSH_CMSG_EOF
        Indicates that the client has encountered EOF while reading
        standard input.  The server will allow any buffered input data
        to drain, and will then close the input to the program.

   SSH_CMSG_WINDOW_SIZE
        Indicates that window size on the client has been changed.  The
        server updates the window size of the tty and causes SIGWINCH to
        be sent to the program.  The new window size is passed as four
        integer arguments:  row, col, xpixel, ypixel.

   SSH_MSG_PORT_OPEN
        Indicates that a connection has been made to a port on the
        client side for which forwarding has been requested.  Arguments
        are client side channel number, host name to connect to, and
        port to connect to.  The server should send back either
        SSH_MSG_CHANNEL_OPEN_CONFIRMATION or
        SSH_MSG_CHANNEL_OPEN_FAILURE with the same client side channel
        number.

   SSH_MSG_CHANNEL_OPEN_CONFIRMATION
        This is sent by the client to indicate that it has opened a con-
        nection as requested in a previous message.  The first argument
        indicates the server side channel number, and the second argu-
        ment is the channel number that the client has allocated for
        this connection.

   SSH_MSG_CHANNEL_OPEN_FAILURE
        This is sent by the client to indicate that it failed to open a
        connection as requested in a previous message.  The server side
        channel number is passed as an argument.  The server will close
        the descriptor associated with the channel and free the channel.

   SSH_MSG_CHANNEL_DATA
        This packet contains data for a channel from the client.  The
        first argument is the server side channel number, and the second
        argument (a string) is the data.

   SSH_MSG_CHANNEL_CLOSE
        This is sent by the client to indicate that whoever was in the
        other end of the channel has closed it.  The argument is the
        server channel number.  The server will allow buffered data to
        drain, and when ready, will close the socket, free the channel,
        and send the client a SSH_MSG_CHANNEL_CLOSE_CONFIRMATION message
        for the channel.

   SSH_MSG_CHANNEL_CLOSE_CONFIRMATION
        This is send by the client to indicate that a channel previously



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        closed by the server has now been closed on the client side as
        well.  The argument indicates the server channel number.  The
        server frees the channel.

   Any unsupported messages during interactive mode cause the connection
   to be terminated with SSH_MSG_DISCONNECT and an error message.  Com-
   patible protocol upgrades should agree about any extensions during
   the preparation phase or earlier.


Termination of the Connection

   Normal termination of the connection is always initiated by the
   server by sending SSH_SMSG_EXITSTATUS after the program has exited.
   The client responds to this message by sending
   SSH_CMSG_EXIT_CONFIRMATION and closes the socket; the server then
   closes the socket.  There are two purposes for the confirmation: some
   systems may lose previously sent data when the socket is closed, and
   closing the client side first causes any TCP/IP TIME_WAIT [RFC0793]
   waits to occur on the client side, not consuming server resources.

   If the program terminates due to a signal, the server will send
   SSH_MSG_DISCONNECT with an appropriate message.  If the connection is
   closed, all file descriptors to the program will be closed and the
   server will exit.  If the program runs on a tty, the kernel sends it
   the SIGHUP signal when the pty master side is closed.

Protocol Flags

   Both the server and the client pass 32 bits of protocol flags to the
   other side.  The flags are intended for compatible protocol exten-
   sion; the server first announces which added capabilities it sup-
   ports, and the client then sends the capabilities that it supports.

   The following flags are currently defined (the values are bit masks):

   1 SSH_PROTOFLAG_SCREEN_NUMBER
        This flag can only be sent by the client.  It indicates that the
        X11 forwarding requests it sends will include the screen number.

   2 SSH_PROTOFLAG_HOST_IN_FWD_OPEN
        If both sides specify this flag, SSH_SMSG_X11_OPEN and
        SSH_MSG_PORT_OPEN messages will contain an additional field con-
        taining a description of the host at the other end of the con-
        nection.

Detailed Description of Packet Types and Formats




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   The supported packet types and the corresponding message numbers are
   given in the following table.  Messages with _MSG_ in their name may
   be sent by either side.  Messages with _CMSG_ are only sent by the
   client, and messages with _SMSG_ only by the server.

   A packet may contain additional data after the arguments specified
   below.  Any such data should be ignored by the receiver.  However, it
   is recommended that no such data be stored without good reason.
   (This helps build compatible extensions.)

   0 SSH_MSG_NONE
        This code is reserved.  This message type is never sent.

   1 SSH_MSG_DISCONNECT

        string   Cause of disconnection

        This message may be sent by either party at any time.  It causes
        the immediate disconnection of the connection.  The message is
        intended to be displayed to a human, and describes the reason
        for disconnection.

   2 SSH_SMSG_PUBLIC_KEY

        8 bytes      anti_spoofing_cookie
        32-bit int   server_key_bits
        mp-int       server_key_public_exponent
        mp-int       server_key_public_modulus
        32-bit int   host_key_bits
        mp-int       host_key_public_exponent
        mp-int       host_key_public_modulus
        32-bit int   protocol_flags
        32-bit int   supported_ciphers_mask
        32-bit int   supported_authentications_mask

        Sent as the first message by the server.  This message gives the
        server's host key, server key, protocol flags (intended for com-
        patible protocol extension), supported_ciphers_mask (which is
        the bitwise or of (1 << cipher_number), where << is the left
        shift operator, for all supported ciphers), and
        supported_authentications_mask (which is the bitwise or of (1 <<
        authentication_type) for all supported authentication types).
        The anti_spoofing_cookie is 64 random bits, and must be sent
        back verbatim by the client in its reply.  It is used to make
        IP-spoofing more difficult (encryption and host keys are the
        real defense against spoofing).

   3 SSH_CMSG_SESSION_KEY



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        1 byte       cipher_type (must be one of the supported values)
        8 bytes      anti_spoofing_cookie (must match data sent by the server)
        mp-int       double-encrypted session key
        32-bit int   protocol_flags

        Sent by the client as the first message in the session.  Selects
        the cipher to use, and sends the encrypted session key to the
        server.  The anti_spoofing_cookie must be the same bytes that
        were sent by the server.  Protocol_flags is intended for nego-
        tiating compatible protocol extensions.

   4 SSH_CMSG_USER

        string   user login name on server

        Sent by the client to begin authentication.  Specifies the user
        name on the server to log in as.  The server responds with
        SSH_SMSG_SUCCESS if no authentication is needed for this user,
        or SSH_SMSG_FAILURE if authentication is needed (or the user
        does not exist).  [Note to the implementator: the user name is
        of arbitrary size.  The implementation must be careful not to
        overflow internal buffers.]

   5 SSH_CMSG_AUTH_RHOSTS

        string   client-side user name

        Requests authentication using /etc/hosts.equiv and .rhosts (or
        equivalent mechanisms).  This authentication method is normally
        disabled in the server because it is not secure (but this is the
        method used by rsh and rlogin).  The server responds with
        SSH_SMSG_SUCCESS if authentication was successful, and
        SSH_SMSG_FAILURE if access was not granted.  The server should
        check that the client side port number is less than 1024 (a
        privileged port), and immediately reject authentication if it is
        not.  Supporting this authentication method is optional.  This
        method should normally not be enabled in the server because it
        is not safe.  (However, not enabling this only helps if rlogind
        and rshd are disabled.)

   6 SSH_CMSG_AUTH_RSA

        mp-int   identity_public_modulus

        Requests authentication using pure RSA authentication.  The
        server checks if the given key is permitted to log in, and if
        so, responds with SSH_SMSG_AUTH_RSA_CHALLENGE.  Otherwise, it
        responds with SSH_SMSG_FAILURE.  The client often tries several



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        different keys in sequence until one supported by the server is
        found.  Authentication is accepted if the client gives the
        correct response to the challenge.  The server is free to add
        other criteria for authentication, such as a requirement that
        the connection must come from a certain host.  Such additions
        are not visible at the protocol level.  Supporting this authen-
        tication method is optional but recommended.

   7 SSH_SMSG_AUTH_RSA_CHALLENGE

        mp-int   encrypted challenge

        Presents an RSA authentication challenge to the client.  The
        challenge is a 256-bit random value encrypted as described else-
        where in this document.  The client must decrypt the challenge
        using the RSA private key, compute MD5 of the challenge plus
        session id, and send back the resulting 16 bytes using
        SSH_CMSG_AUTH_RSA_RESPONSE.

   8 SSH_CMSG_AUTH_RSA_RESPONSE

        16 bytes   MD5 of decrypted challenge

        This message is sent by the client in response to an RSA chal-
        lenge.  The MD5 checksum is returned instead of the decrypted
        challenge to deter known-plaintext attacks against the RSA key.
        The server responds to this message with either SSH_SMSG_SUCCESS
        or SSH_SMSG_FAILURE.

   9 SSH_CMSG_AUTH_PASSWORD

        string   plain text password

        Requests password authentication using the given password.  Note
        that even though the password is plain text inside the packet,
        the whole packet is normally encrypted by the packet layer.  It
        would not be possible for the client to perform password
        encryption/hashing, because it cannot know which kind of
        encryption/hashing, if any, the server uses.  The server
        responds to this message with SSH_SMSG_SUCCESS or
        SSH_SMSG_FAILURE.

   10 SSH_CMSG_REQUEST_PTY

        string       TERM environment variable value (e.g. vt100)
        32-bit int   terminal height, rows (e.g., 24)
        32-bit int   terminal width, columns (e.g., 80)
        32-bit int   terminal width, pixels (0 if no graphics) (e.g., 480)



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        32-bit int   terminal height, pixels (0 if no graphics) (e.g., 640)
        n bytes      tty modes encoded in binary

        Requests a pseudo-terminal to be allocated for this command.
        This message can be used regardless of whether the session will
        later execute the shell or a command.  If a pty has been
        requested with this message, the shell or command will run on a
        pty.  Otherwise it will communicate with the server using pipes,
        sockets or some other similar mechanism.

        The terminal type gives the type of the user's terminal.  In the
        UNIX environment it is passed to the shell or command in the
        TERM environment variable.

        The width and height values give the initial size of the user's
        terminal or window.  All values can be zero if not supported by
        the operating system.  The server will pass these values to the
        kernel if supported.

        Terminal modes are encoded into a byte stream in a portable for-
        mat.  The exact format is described later in this document.

        The server responds to the request with either SSH_SMSG_SUCCESS
        or SSH_SMSG_FAILURE.  If the server does not have the concept of
        pseudo terminals, it should return success if it is possible to
        execute a shell or a command so that it looks to the client as
        if it was running on a pseudo terminal.

   11 SSH_CMSG_WINDOW_SIZE

        32-bit int   terminal height, rows
        32-bit int   terminal width, columns
        32-bit int   terminal width, pixels
        32-bit int   terminal height, pixels

        This message can only be sent by the client during the interac-
        tive session.  This indicates that the size of the user's window
        has changed, and provides the new size.  The server will update
        the kernel's notion of the window size, and a SIGWINCH signal or
        equivalent will be sent to the shell or command (if supported by
        the operating system).

   12 SSH_CMSG_EXEC_SHELL

        (no arguments)

        Starts a shell (command interpreter), and enters interactive
        session mode.



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   13 SSH_CMSG_EXEC_CMD

        string   command to execute

        Starts executing the given command, and enters interactive ses-
        sion mode.  On UNIX, the command is run as "<shell> -c <com-
        mand>", where <shell> is the user's login shell.

   14 SSH_SMSG_SUCCESS

        (no arguments)

        This message is sent by the server in response to the session
        key, a successful authentication request, and a successfully
        completed preparatory operation.

   15 SSH_SMSG_FAILURE

        (no arguments)

        This message is sent by the server in response to a failed
        authentication operation to indicate that the user has not yet
        been successfully authenticated, and in response to a failed
        preparatory operation.  This is also sent in response to an
        authentication or preparatory operation request that is not
        recognized or supported.

   16 SSH_CMSG_STDIN_DATA

        string   data

        Delivers data from the client to be supplied as input to the
        shell or program running on the server side.  This message can
        only be used in the interactive session mode.  No acknowledge-
        ment is sent for this message.

   17 SSH_SMSG_STDOUT_DATA

        string   data

        Delivers data from the server that was read from the standard
        output of the shell or program running on the server side.  This
        message can only be used in the interactive session mode.  No
        acknowledgement is sent for this message.

   18 SSH_SMSG_STDERR_DATA

        string   data



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        Delivers data from the server that was read from the standard
        error of the shell or program running on the server side.  This
        message can only be used in the interactive session mode.  No
        acknowledgement is sent for this message.

   19 SSH_CMSG_EOF

        (no arguments)

        This message is sent by the client to indicate that EOF has been
        reached on the input.  Upon receiving this message, and after
        all buffered input data has been sent to the shell or program,
        the server will close the input file descriptor to the program.
        This message can only be used in the interactive session mode.
        No acknowledgement is sent for this message.

   20 SSH_SMSG_EXITSTATUS

        32-bit int   exit status of the command

        Returns the exit status of the shell or program after it has
        exited.  The client should respond with
        SSH_CMSG_EXIT_CONFIRMATION when it has received this message.
        This will be the last message sent by the server.  If the pro-
        gram being executed dies with a signal instead of exiting nor-
        mally, the server should terminate the session with
        SSH_MSG_DISCONNECT (which can be used to pass a human-readable
        string indicating that the program died due to a signal) instead
        of using this message.

   21 SSH_MSG_CHANNEL_OPEN_CONFIRMATION

        32-bit int   remote_channel
        32-bit int   local_channel

        This is sent in response to any channel open request if the
        channel has been successfully opened.  Remote_channel is the
        channel number received in the initial open request;
        local_channel is the channel number the side sending this mes-
        sage has allocated for the channel.  Data can be transmitted on
        the channel after this message.

   22 SSH_MSG_CHANNEL_OPEN_FAILURE

        32-bit int   remote_channel

        This message indicates that an earlier channel open request by
        the other side has failed or has been denied.  Remote_channel is



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        the channel number given in the original request.

   23 SSH_MSG_CHANNEL_DATA

        32-bit int   remote_channel
        string       data

        Data is transmitted in a channel in these messages.  A channel
        is bidirectional, and both sides can send these messages.  There
        is no acknowledgement for these messages.  It is possible that
        either side receives these messages after it has sent
        SSH_MSG_CHANNEL_CLOSE for the channel.  These messages cannot be
        received after the party has sent or received
        SSH_MSG_CHANNEL_CLOSE_CONFIRMATION.

   24 SSH_MSG_CHANNEL_CLOSE

        32-bit int   remote_channel

        When a channel is closed at one end of the connection, that side
        sends this message.  Upon receiving this message, the channel
        should be closed.  When this message is received, if the channel
        is already closed (the receiving side has sent this message for
        the same channel earlier), the channel is freed and no further
        action is taken; otherwise the channel is freed and
        SSH_MSG_CHANNEL_CLOSE_CONFIRMATION is sent in response.  (It is
        possible that the channel is closed simultaneously at both
        ends.)

   25 SSH_MSG_CHANNEL_CLOSE_CONFIRMATION

        32-bit int   remote_channel

        This message is sent in response to SSH_MSG_CHANNEL_CLOSE unless
        the channel was already closed.  When this message is sent or
        received, the channel is freed.

   26 (OBSOLETED; was unix-domain X11 forwarding)

   27 SSH_SMSG_X11_OPEN

        32-bit int   local_channel
        string       originator_string (see below)

        This message can be sent by the server during the interactive
        session mode to indicate that a client has connected the fake X
        server.  Local_channel is the channel number that the server has
        allocated for the connection.  The client should try to open a



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        connection to the real X server, and respond with
        SSH_MSG_CHANNEL_OPEN_CONFIRMATION or
        SSH_MSG_CHANNEL_OPEN_FAILURE.

        The field originator_string is present if both sides specified
        SSH_PROTOFLAG_HOST_IN_FWD_OPEN in the protocol flags.  It con-
        tains a description of the host originating the connection.

   28 SSH_CMSG_PORT_FORWARD_REQUEST

        32-bit int   server_port
        string       host_to_connect
        32-bit int   port_to_connect

        Sent by the client in the preparatory phase, this message
        requests that server_port on the server machine be forwarded
        over the secure channel to the client machine, and from there to
        the specified host and port.  The server should start listening
        on the port, and send SSH_MSG_PORT_OPEN whenever a connection is
        made to it.  Supporting this message is optional, and the server
        is free to reject any forward request.  For example, it is
        highly recommended that unless the user has been authenticated
        as root, forwarding any privileged port numbers (below 1024) is
        denied.

   29 SSH_MSG_PORT_OPEN

        32-bit int   local_channel
        string       host_name
        32-bit int   port
        string       originator_string (see below)

        Sent by either party in interactive session mode, this message
        indicates that a connection has been opened to a forwarded
        TCP/IP port.  Local_channel is the channel number that the send-
        ing party has allocated for the connection.  Host_name is the
        host the connection should be be forwarded to, and the port is
        the port on that host to connect.  The receiving party should
        open the connection, and respond with
        SSH_MSG_CHANNEL_OPEN_CONFIRMATION or
        SSH_MSG_CHANNEL_OPEN_FAILURE.  It is recommended that the
        receiving side check the host_name and port for validity to
        avoid compromising local security by compromised remote side
        software.  Particularly, it is recommended that the client per-
        mit connections only to those ports for which it has requested
        forwarding with SSH_CMSG_PORT_FORWARD_REQUEST.

        The field originator_string is present if both sides specified



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        SSH_PROTOFLAG_HOST_IN_FWD_OPEN in the protocol flags.  It con-
        tains a description of the host originating the connection.

   30 SSH_CMSG_AGENT_REQUEST_FORWARDING

        (no arguments)

        Requests that the connection to the authentication agent be for-
        warded over the secure channel.  The method used by clients to
        contact the authentication agent within each machine is imple-
        mentation and machine dependent.  If the server accepts this
        request, it should arrange that any clients run from this ses-
        sion will actually contact the server program when they try to
        contact the authentication agent.  The server should then send a
        SSH_SMSG_AGENT_OPEN to open a channel to the agent, and the
        client should forward the connection to the real authentication
        agent.  Supporting this message is optional.

   31 SSH_SMSG_AGENT_OPEN

        32-bit int   local_channel

        Sent by the server in interactive session mode, this message
        requests opening a channel to the authentication agent.  The
        client should open a channel, and respond with either
        SSH_MSG_CHANNEL_OPEN_CONFIRMATION or
        SSH_MSG_CHANNEL_OPEN_FAILURE.

   32 SSH_MSG_IGNORE

        string   data

        Either party may send this message at any time.  This message,
        and the argument string, is silently ignored.  This message
        might be used in some implementations to make traffic analysis
        more difficult.  This message is not currently sent by the
        implementation, but all implementations are required to recog-
        nize and ignore it.

   33 SSH_CMSG_EXIT_CONFIRMATION

        (no arguments)

        Sent by the client in response to SSH_SMSG_EXITSTATUS.  This is
        the last message sent by the client.

   34 SSH_CMSG_X11_REQUEST_FORWARDING




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        string       x11_authentication_protocol
        string       x11_authentication_data
        32-bit int   screen number (if SSH_PROTOFLAG_SCREEN_NUMBER)

        Sent by the client during the preparatory phase, this message
        requests that the server create a fake X11 display and set the
        DISPLAY environment variable accordingly.  An internet-domain
        display is preferable.  The given authentication protocol and
        the associated data should be recorded by the server so that it
        is used as authentication on connections (e.g., in .Xauthority).
        The authentication protocol must be one of the supported X11
        authentication protocols, e.g., "MIT-MAGIC-COOKIE-1".  Authenti-
        cation data must be a lowercase hex string of even length.  Its
        interpretation is protocol dependent.  The data is in a format
        that can be used with e.g. the xauth program.  Supporting this
        message is optional.

        The client is permitted (and recommended) to generate fake
        authentication information and send fake information to the
        server.  This way, a corrupt server will not have access to the
        user's terminal after the connection has terminated.  The
        correct authorization codes will also not be left hanging around
        in files on the server (many users keep the same X session for
        months, thus protecting the authorization data becomes impor-
        tant).

        X11 authentication spoofing works by initially sending fake
        (random) authentication data to the server, and interpreting the
        first packet sent by the X11 client after the connection has
        been opened.  The first packet contains the client's authentica-
        tion.  If the packet contains the correct fake data, it is
        replaced by the client by the correct authentication data, and
        then sent to the X server.

   35 SSH_CMSG_AUTH_RHOSTS_RSA

        string       clint-side user name
        32-bit int   client_host_key_bits
        mp-int       client_host_key_public_exponent
        mp-int       client_host_key_public_modulus

        Requests authentication using /etc/hosts.equiv and .rhosts (or
        equivalent) together with RSA host authentication.  The server
        should check that the client side port number is less than 1024
        (a privileged port), and immediately reject authentication if it
        is not.  The server responds with SSH_SMSG_FAILURE or
        SSH_SMSG_AUTH_RSA_CHALLENGE.  The client must respond to the
        challenge with the proper SSH_CMSG_AUTH_RSA_RESPONSE.  The



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        server then responds with success if access was granted, or
        failure if the client gave a wrong response.  Supporting this
        authentication method is optional but recommended in most
        environments.

   36 SSH_MSG_DEBUG

        string   debugging message sent to the other side

        This message may be sent by either party at any time.  It is
        used to send debugging messages that may be informative to the
        user in solving various problems.  For example, if authentica-
        tion fails because of some configuration error (e.g., incorrect
        permissions for some file), it can be very helpful for the user
        to make the cause of failure available.  On the other hand, one
        should not make too much information available for security rea-
        sons.  It is recommended that the client provides an option to
        display the debugging information sent by the sender (the user
        probably does not want to see it by default).  The server can
        log debugging data sent by the client (if any).  Either party is
        free to ignore any received debugging data.  Every implementa-
        tion must be able to receive this message, but no implementation
        is required to send these.

   37 SSH_CMSG_REQUEST_COMPRESSION

        32-bit int   gzip compression level (1-9)

        This message can be sent by the client in the preparatory opera-
        tions phase.  The server responds with SSH_SMSG_FAILURE if it
        does not support compression or does not want to compress; it
        responds with SSH_SMSG_SUCCESS if it accepted the compression
        request.  In the latter case the response to this packet will
        still be uncompressed, but all further packets in either direc-
        tion will be compressed by gzip.

   38 SSH_CMSG_MAX_PACKET_SIZE

        32-bit int   maximum packet size, bytes (4096-1024k)

        This message can be sent by the client in the preparatory opera-
        tions phase.  The server responds with SSH_SMSG_FAILURE if it
        does not support limiting packet size, or with SSH_SMSG_SUCCESS
        if it has limited the maximum packet size (as determined by the
        value in the size field) to the specified value.

   39 SSH_CMSG_AUTH_TIS




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        (no arguments)

        This message starts TIS authentication. The server responds with
        SSH_SMSG_FAILURE or SSH_SMSG_AUTH_TIS_CHALLENGE.

   40 SSH_SMSG_AUTH_TIS_CHALLENGE

        string   tis challenge

        Server sends TIS challenge to user and client should show it to
        user and ask for response, which is sent back using
        SSH_CMSG_AUTH_TIS_RESPONSE message.

   41 SSH_CMSG_AUTH_TIS_RESPONSE

        string   user response to tis challenge

        When client receives SSH_SMSG_AUTH_TIS_CHALLENGE and ask users
        response to challenge it sends it back this message. The server
        answers with SSH_SMSG_FAILURE or SSH_SMSG_SUCCESS.

   42 SSH_CMSG_AUTH_KERBEROS

        string   authentication info

        Client sends authentication info to server, which replies with
        SSH_SMSG_AUTH_KERBEROS_RESPONSE message having correct response
        data encrypted with the session key.

   43 SSH_SMSG_AUTH_KERBEROS_RESPONSE

        string   response data

        Server replies to SSH_CMSG_AUTH_KERBEROS message with this mes-
        sage so that the response data is encrypted with session key.

   44 SSH_CMSG_HAVE_KERBEROS_TGT

        string   kerberos credentials

        Client sends kerberos credentials to server and the server
        replies with SSH_SMSG_SUCCESS or SSH_SMSG_FAILURE.


Encoding of Terminal Modes

   Terminal modes (as passed in SSH_CMSG_REQUEST_PTY) are encoded into a
   byte stream.  It is intended that the coding be portable across



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   different environments.

   The tty mode description is a stream of bytes.  The stream consists
   of opcode-argument pairs.  It is terminated by opcode TTY_OP_END (0).
   Opcodes 1-127 have one-byte arguments.  Opcodes 128-159 have 32-bit
   integer arguments (stored msb first).  Opcodes 160-255 are not yet
   defined, and cause parsing to stop (they should only be used after
   any other data).

   The client puts in the stream any modes it knows about, and the
   server ignores any modes it does not know about.  This allows some
   degree of machine-independence, at least between systems that use a
   POSIX-like [POSIX] tty interface.  The protocol can support other
   systems as well, but the client may need to fill reasonable values
   for a number of parameters so the server pty gets set to a reasonable
   mode (the server leaves all unspecified mode bits in their default
   values, and only some combinations make sense).

   The following opcodes have been defined.  The naming of opcodes
   mostly follows the POSIX terminal mode flags.

   0 TTY_OP_END
        Indicates end of options.

   1 VINTR
        Interrupt character; 255 if none.  Similarly for the other char-
        acters.  Not all of these characters are supported on all sys-
        tems.

   2 VQUIT
        The quit character (sends SIGQUIT signal on UNIX systems).

   3 VERASE
        Erase the character to left of the cursor.

   4 VKILL
        Kill the current input line.

   5 VEOF
        End-of-file character (sends EOF from the terminal).

   6 VEOL
        End-of-line character in addition to carriage return and/or
        linefeed.

   7 VEOL2
        Additional end-of-line character.




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   8 VSTART
        Continues paused output (normally ^Q).

   9 VSTOP
        Pauses output (^S).

   10 VSUSP
        Suspends the current program.

   11 VDSUSP
        Another suspend character.

   12 VREPRINT
        Reprints the current input line.

   13 VWERASE
        Erases a word left of cursor.

   14 VLNEXT
        More special input characters; these are probably not supported
        on most systems.

   15 VFLUSH

   16 VSWTCH

   17 VSTATUS

   18 VDISCARD


   30 IGNPAR
        The ignore parity flag.  The next byte should be 0 if this flag
        is not set, and 1 if it is set.

   31 PARMRK
        More flags.  The exact definitions can be found in the POSIX
        standard.

   32 INPCK

   33 ISTRIP

   34 INLCR

   35 IGNCR

   36 ICRNL



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   37 IUCLC

   38 IXON

   39 IXANY

   40 IXOFF

   41 IMAXBEL


   50 ISIG

   51 ICANON

   52 XCASE

   53 ECHO

   54 ECHOE

   55 ECHOK

   56 ECHONL

   57 NOFLSH

   58 TOSTOP

   59 IEXTEN

   60 ECHOCTL

   61 ECHOKE

   62 PENDIN


   70 OPOST

   71 OLCUC

   72 ONLCR

   73 OCRNL

   74 ONOCR




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   75 ONLRET


   90 CS7

   91 CS8

   92 PARENB

   93 PARODD


   192 TTY_OP_ISPEED
        Specifies the input baud rate in bits per second (as a 32-bit
        int, msb first).

   193 TTY_OP_OSPEED
        Specifies the output baud rate in bits per second (as a 32-bt
        int, msb first).


The Authentication Agent Protocol

   The authentication agent is a program that can be used to hold RSA
   authentication keys for the user (in future, it might hold data for
   other authentication types as well).  An authorized program can send
   requests to the agent to generate a proper response to an RSA chal-
   lenge.  How the connection is made to the agent (or its representa-
   tive) inside a host and how access control is done inside a host is
   implementation-dependent; however, how it is forwarded and how one
   interacts with it is specified in this protocol.  The connection to
   the agent is normally automatically forwarded over the secure chan-
   nel.

   A program that wishes to use the agent first opens a connection to
   its local representative (typically, the agent itself or an SSH
   server).  It then writes a request to the connection, and waits for
   response.  It is recommended that at least five minutes of timeout
   are provided waiting for the agent to respond to an authentication
   challenge (this gives sufficient time for the user to cut-and-paste
   the challenge to a separate machine, perform the computation there,
   and cut-and-paste the result back if so desired).

   Messages sent to and by the agent are in the following format:

   4 bytes     Length, msb first.  Does not include length itself.
   1 byte      Packet type.  The value 255 is reserved for future extensions.
   data        Any data, depending on packet type.  Encoding as in the ssh packet



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   protocol.


   The following message types are currently defined:

   1 SSH_AGENTC_REQUEST_RSA_IDENTITIES

        (no arguments)

        Requests the agent to send a list of all RSA keys for which it
        can answer a challenge.

   2 SSH_AGENT_RSA_IDENTITIES_ANSWER

        32-bit int       howmany
        howmany times:
        32-bit int       bits
        mp-int           public exponent
        mp-int           public modulus
        string           comment

        The agent sends this message in response to the to
        SSH_AGENTC_REQUEST_RSA_IDENTITIES.  The answer lists all RSA
        keys for which the agent can answer a challenge.  The comment
        field is intended to help identify each key; it may be printed
        by an application to indicate which key is being used.  If the
        agent is not holding any keys, howmany will be zero.

   3 SSH_AGENTC_RSA_CHALLENGE

        32-bit int   bits
        mp-int       public exponent
        mp-int       public modulus
        mp-int       challenge
        16 bytes     session_id
        32-bit int   response_type

        Requests RSA decryption of random challenge to authenticate the
        other side.  The challenge will be decrypted with the RSA
        private key corresponding to the given public key.

        The decrypted challenge must contain a zero in the highest (par-
        tial) byte, 2 in the next byte, followed by non-zero random
        bytes, a zero byte, and then the real challenge value in the
        lowermost bytes.  The real challenge must be 32 8-bit bytes (256
        bits).

        Response_type indicates the format of the response to be



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        returned.  Currently the only supported value is 1, which means
        to compute MD5 of the real challenge plus session id, and return
        the resulting 16 bytes in a SSH_AGENT_RSA_RESPONSE message.

   4 SSH_AGENT_RSA_RESPONSE

        16 bytes   MD5 of decrypted challenge

        Answers an RSA authentication challenge.  The response is 16
        bytes:  the MD5 checksum of the 32-byte challenge.

   5 SSH_AGENT_FAILURE

        (no arguments)

        This message is sent whenever the agent fails to answer a
        request properly.  For example, if the agent cannot answer a
        challenge (e.g., no longer has the proper key), it can respond
        with this.  The agent also responds with this message if it
        receives a message it does not recognize.

   6 SSH_AGENT_SUCCESS

        (no arguments)

        This message is sent by the agent as a response to certain
        requests that do not otherwise cause a message be sent.
        Currently, this is only sent in response to
        SSH_AGENTC_ADD_RSA_IDENTITY and SSH_AGENTC_REMOVE_RSA_IDENTITY.

   7 SSH_AGENTC_ADD_RSA_IDENTITY

        32-bit int   bits
        mp-int       public modulus
        mp-int       public exponent
        mp-int       private exponent
        mp-int       multiplicative inverse of p mod q
        mp-int       p
        mp-int       q
        string       comment

        Registers an RSA key with the agent.  After this request, the
        agent can use this RSA key to answer requests.  The agent
        responds with SSH_AGENT_SUCCESS or SSH_AGENT_FAILURE.

   8 SSH_AGENT_REMOVE_RSA_IDENTITY

        32-bit int   bits



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        mp-int       public exponent
        mp-int       public modulus

        Removes an RSA key from the agent.  The agent will no longer
        accept challenges for this key and will not list it as a sup-
        ported identity.  The agent responds with SSH_AGENT_SUCCESS or
        SSH_AGENT_FAILURE.

   If the agent receives a message that it does not understand, it
   responds with SSH_AGENT_FAILURE.  This permits compatible future
   extensions.

   It is possible that several clients have a connection open to the
   authentication agent simultaneously.  Each client will use a separate
   connection (thus, any SSH connection can have multiple agent connec-
   tions active simultaneously).


References


   [DES] FIPS PUB 46-1: Data Encryption Standard.  National Bureau of
        Standards, January 1988.  FIPS PUB 81: DES Modes of Operation.
        National Bureau of Standards, December 1980.  Bruce Schneier:
        Applied Cryptography.  John Wiley & Sons, 1994.  J. Seberry and
        J. Pieprzyk:  Cryptography: An Introduction to Computer Secu-
        rity.  Prentice-Hall, 1989.

   [GZIP]
        The GNU GZIP program; available for anonymous ftp at
        prep.ai.mit.edu.  Please let me know if you know a paper
        describing the algorithm.

   [IDEA]
        Xuejia Lai: On the Design and Security of Block Ciphers, ETH
        Series in Information Processing, vol. 1, Hartung-Gorre Verlag,
        Konstanz, Switzerland, 1992.  Bruce Schneier: Applied Cryptogra-
        phy, John Wiley & Sons, 1994.  See also the following patents:
        PCT/CH91/00117, EP 0 482 154 B1, US Pat. 5,214,703.

   [PKCS#1]
        PKCS #1: RSA Encryption Standard.  Version 1.5, RSA Labora-
        tories, November 1993.  Available for anonymous ftp at
        ftp.rsa.com.

   [POSIX]
        Portable Operating System Interface (POSIX) - Part 1: Applica-
        tion Program Interface (API) [C language], ISO/IEC 9945-1, IEEE



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        Std 1003.1, 1990.

   [RFC0791]
        J. Postel: Internet Protocol, RFC 791, USC/ISI, September 1981.

   [RFC0793]
        J. Postel: Transmission Control Protocol, RFC 793, USC/ISI, Sep-
        tember 1981.

   [RFC1034]
        P. Mockapetris: Domain Names - Concepts and Facilities, RFC
        1034, USC/ISI, November 1987.

   [RFC1282]
        B. Kantor: BSD Rlogin, RFC 1258, UCSD, December 1991.

   [RSA] Bruce Schneier: Applied Cryptography.  John Wiley & Sons, 1994.
        See also R. Rivest, A. Shamir, and L. M. Adleman: Cryptographic
        Communications System and Method.  US Patent 4,405,829, 1983.

   [X11] R. Scheifler: X Window System Protocol, X Consortium Standard,
        Version 11, Release 6.  Massachusetts Institute of Technology,
        Laboratory of Computer Science, 1994.


Security Considerations

   This protocol deals with the very issue of user authentication and
   security.

   First of all, as an implementation issue, the server program will
   have to run as root (or equivalent) on the server machine.  This is
   because the server program will need be able to change to an arbi-
   trary user id.  The server must also be able to create a privileged
   TCP/IP port.

   The client program will need to run as root if any variant of .rhosts
   authentication is to be used.  This is because the client program
   will need to create a privileged port.  The client host key is also
   usually stored in a file which is readable by root only.  The client
   needs the host key in .rhosts authentication only.  Root privileges
   can be dropped as soon as the privileged port has been created and
   the host key has been read.

   The SSH protocol offers major security advantages over existing tel-
   net and rlogin protocols.

   o    IP spoofing is restricted to closing a connection (by



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        encryption, host keys, and the special random cookie).  If
        encryption is not used, IP spoofing is possible for those who
        can hear packets going out from the server.

   o    DNS spoofing is made ineffective (by host keys).

   o    Routing spoofing is made ineffective (by host keys).

   o    All data is encrypted with strong algorithms to make eavesdrop-
        ping as difficult as possible.  This includes encrypting any
        authentication information such as passwords.  The information
        for decrypting session keys is destroyed every hour.

   o    Strong authentication methods: .rhosts combined with RSA host
        authentication, and pure RSA authentication.

   o    X11 connections and arbitrary TCP/IP ports can be forwarded
        securely.

   o    Man-in-the-middle attacks are deterred by using the server host
        key to encrypt the session key.

   o    Trojan horses to catch a password by routing manipulation are
        deterred by checking that the host key of the server machine
        matches that stored on the client host.

   The security of SSH against man-in-the-middle attacks and the secu-
   rity of the new form of .rhosts authentication, as well as server
   host validation, depends on the integrity of the host key and the
   files containing known host keys.

   The host key is normally stored in a root-readable file.  If the host
   key is compromised, it permits attackers to use IP, DNS and routing
   spoofing as with current rlogin and rsh.  It should never be any
   worse than the current situation.

   The files containing known host keys are not sensitive.  However, if
   an attacker gets to modify the known host key files, it has the same
   consequences as a compromised host key, because the attacker can then
   change the recorded host key.

   The security improvements obtained by this protocol for X11 are of
   particular significance.  Previously, there has been no way to pro-
   tect data communicated between an X server and a client running on a
   remote machine.  By creating a fake display on the server, and for-
   warding all X11 requests over the secure channel, SSH can be used to
   run any X11 applications securely without any cooperation with the
   vendors of the X server or the application.



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   Finally, the security of this program relies on the strength of the
   underlying cryptographic algorithms.  The RSA algorithm is used for
   authentication key exchange.  It is widely believed to be secure.  Of
   the algorithms used to encrypt the session, DES has a rather small
   key these days, probably permitting governments and organized crimi-
   nals to break it in very short time with specialized hardware.  3DES
   is probably safe (but slower).  IDEA is widely believed to be secure.
   People have varying degrees of confidence in the other algorithms.
   This program is not secure if used with no encryption at all.


Additional Information

   Additional information (especially on the implementation and mailing
   lists) is available via WWW at http://www.cs.hut.fi/ssh.

   Comments should be sent to Tatu Ylonen <ylo@cs.hut.fi> or the SSH
   Mailing List <ssh@clinet.fi>.

Author's Address


   Tatu Ylonen
   Helsinki University of Technology
   Otakaari 1
   FIN-02150 Espoo, Finland

   Phone: +358-9-4354-3205
   Fax: +358-9-4354-3206
   EMail: ylo@cs.hut.fi





















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