NetBSD/crypto/dist/ssh/RFC.nroff

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

.in 3

.ce
The SSH (Secure Shell) Remote Login Protocol

.ti 0
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.

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

To learn the current status of any Internet-Draft, please check the
``1id-abstracts.txt'' listing contained in the Internet- Drafts Shadow
Directories on ftp.is.co.za (Africa), nic.nordu.net (Europe),
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.

.ti 0
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:
.IP o
Closes several security holes (e.g., IP, routing, and DNS spoofing).
New authentication methods: .rhosts together with RSA [RSA] based host
authentication, and pure RSA authentication.
.IP o
All communications are automatically and transparently encrypted.
Encryption is also used to protect integrity.
.IP o
X11 connection forwarding provides secure X11 sessions.
.IP o
Arbitrary TCP/IP ports can be redirected over the encrypted channel
in both directions.
.IP 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).
.IP o
An authentication agent, running in the user's local workstation or
laptop, can be used to hold the user's RSA authentication keys.
.RT

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.


.ti 0
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
transport can be used but are currently not defined.

When the client connects the server, the server accepts the connection
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 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
authentication 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 allocating 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 interactive
session mode.  In this mode, data is passed in both directions, 
new forwarded connections may be opened, etc.  The interactive session
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
extensions 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
interactive 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.

.ti 0
The Binary Packet Protocol

After the protocol identification strings, both sides only send
specially formatted packets.  The packet layout is as follows:
.IP o
Packet length: 32 bit unsigned integer, coded as four 8-bit 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.
.IP 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 having some random padding
at the beginning of each packet is to make known plaintext attacks
more difficult.
.IP o
Packet type: 8-bit unsigned byte.  The value 255 is reserved for
future extension.
.IP o
Data: binary data bytes, depending on the packet type.  The number of
data bytes is the "length" field minus 5.
.IP 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 computed before
any encryption.
.RT

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.


.ti 0
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 direction
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 compression
dictionary is not flushed between packets.  The two directions are
compressed independently of each other.


.ti 0
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:
.TS
center;
l r l.
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_TSS	4	   An experimental stream cipher
SSH_CIPHER_RC4	5	   RC4
.TE

All implementations are required to support SSH_CIPHER_DES and
SSH_CIPHER_3DES.  Supporting SSH_CIPHER_IDEA, SSH_CIPHER_RC4, and
SSH_CIPHER_NONE is recommended.  Support for SSH_CIPHER_TSS is
optional (and it is not described in this document).  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
direction) 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.
.IP 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.
.IP SSH_CIPHER_3DES
The variant of triple-DES used here works as follows: there are three
independent DES-CBC ciphers, with independent initialization 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 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.)
.IP 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.
.IP SSH_CIPHER_TSS
All 32 bytes of the session key are used as the key.

There is no reference available for the TSS algorithm; it is currently
only documented in the sample implementation source code.  The
security of this cipher is unknown (but it is quite fast).  The cipher
is basically a stream cipher that uses MD5 as a random number
generator and takes feedback from the data.
.IP 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.
.RT


.ti 0
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:
.IP "8-bit byte"
The byte is stored directly as a single byte.
.IP "32-bit unsigned integer"
Stored in 4 bytes, msb first.
.IP "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.
.IP "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.
.RT


.ti 0
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
authentication, 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 programs
on the server may never notice if the other end of the connection is
rebooted.


.ti 0
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 useful 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
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
according to the lower of the protocol versions.

By convention, changes which keep the protocol compatible with
previous 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 1.3.

The client will 

.ti 0
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 public
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 server
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 host key
interpreted 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.

The client responds with a SSH_CMSG_SESSION_KEY message, which
contains 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
generated 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.

At each encryption step, a multiple-precision integer is constructed
from the data to be encrypted as follows (the integer is here
interpreted 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 identically 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
different directions (that is, both directions have separate
initialization 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.


.ti 0
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
authentication 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
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.


.ti 0
Authentication Phase

Provided the server didn't immediately accept the login, an
authentication 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
authentication 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:
.TS
center;
l r l.
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
.TE
.IP 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 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.
.IP SSH_AUTH_RHOSTS_RSA

In addition to conventional .rhosts and hosts.equiv authentication,
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 verifies
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 containing
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,
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.
.IP SSH_AUTH_RSA

The idea behind RSA authentication is that the server recognizes the
public key offered by the client, generates a random challenge, 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 generates 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.)

The client decrypts the challenge using its private key, concatenates
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 compromised RSA
key, a server might restrict access to a limited set of hosts.
.IP 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 possession of the
password can log in, but nobody else.
.RT

.ti 0
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 might
be added to this phase in future.

The following messages are currently defined for this phase.
.IP SSH_CMSG_REQUEST_COMPRESSION
Requests that compression be enabled for this session.  A
gzip-compatible compression level (1-9) is passed as an argument.
.IP 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.
.IP 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 automatically passed to the server, and the
client may implement spoofing of authentication data for added
security.  The authentication data is passed as arguments.
.IP 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).
.IP SSH_CMSG_AGENT_REQUEST_FORWARDING
Requests forwarding of the connection to the authentication agent.
.IP SSH_CMSG_EXEC_SHELL
Starts a shell (command interpreter) for the user, and moves into
interactive session mode.
.IP SSH_CMSG_EXEC_CMD
Executes the given command (actually "<shell> -c <command>" or
equivalent) for the user, and moves into interactive session mode.
.RT


.ti 0
Interactive Session and Exchange of Data

During the interactive session, any data written by the shell or
command running on the server machine is forwarded to stdin or
stderr on 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
transport, 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:
.IP 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.
.IP 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.)
.IP 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.
.IP SSH_SMSG_AGENT_OPEN
Indicates that someone on the server side is requesting a connection
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.
.IP 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 number.
.IP 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.
.IP SSH_MSG_CHANNEL_OPEN_CONFIRMATION
This is sent by the server to indicate that it has opened a connection
as requested in a previous message.  The first argument indicates the
client side channel number, and the second argument is the channel number
that the server has allocated for this connection.
.IP 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.
.IP 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.
.IP 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.
.IP 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.
.RT

The client may send any of the following messages:
.IP 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.
.IP 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.
.IP 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.
.IP 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.
.IP SSH_MSG_CHANNEL_OPEN_CONFIRMATION
This is sent by the client to indicate that it has opened a connection
as requested in a previous message.  The first argument indicates the
server side channel number, and the second argument is the channel
number that the client has allocated for this connection.
.IP 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.
.IP 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.
.IP 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.
.IP SSH_MSG_CHANNEL_CLOSE_CONFIRMATION
This is send by the client to indicate that a channel previously
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.
.RT

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


.ti 0
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.

.ti 0
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 extension;
the server first announces which added capabilities it supports, and
the client then sends the capabilities that it supports.

The following flags are currently defined (the values are bit masks):
.IP "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.
.IP "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 containing
a description of the host at the other end of the connection.
.RT

.ti 0
Detailed Description of Packet Types and Formats

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.)
.IP "0 SSH_MSG_NONE"
This code is reserved.  This message type is never sent.
.IP "1 SSH_MSG_DISCONNECT"
.TS
;
l l.
string	Cause of disconnection
.TE
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.
.IP "2 SSH_SMSG_PUBLIC_KEY"
.TS
;
l l.
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
.TE
Sent as the first message by the server.  This message gives the
server's host key, server key, protocol flags (intended for compatible
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 bytes, 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).
.IP "3 SSH_CMSG_SESSION_KEY"
.TS
;
l l.
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
.TE
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 negotiating compatible
protocol extensions.
.IP "4 SSH_CMSG_USER"
.TS
;
l l.
string	user login name on server
.TE
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.]
.IP "5 SSH_CMSG_AUTH_RHOSTS"
.TS
;
l l.
string	client-side user name
.TE
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.)
.IP "6 SSH_CMSG_AUTH_RSA"
.TS
;
l l.
mp-int	identity_public_modulus
.TE
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 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
authentication method is optional but recommended.
.IP "7 SSH_SMSG_AUTH_RSA_CHALLENGE"
.TS
;
l l.
mp-int	encrypted challenge
.TE
Presents an RSA authentication challenge to the client.  The challenge
is a 256-bit random value encrypted as described elsewhere 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.
.IP "8 SSH_CMSG_AUTH_RSA_RESPONSE"
.TS
;
l l.
16 bytes	MD5 of decrypted challenge
.TE
This message is sent by the client in response to an RSA challenge.
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.
.IP "9 SSH_CMSG_AUTH_PASSWORD"
.TS
;
l l.
string	plain text password
.TE
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.
.IP "10 SSH_CMSG_REQUEST_PTY"
.TS
;
l l.
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)
32-bit int	terminal height, pixels (0 if no graphics) (e.g., 640)
n bytes	tty modes encoded in binary
.TE
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 format.
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.
.IP "11 SSH_CMSG_WINDOW_SIZE"
.TS
;
l l.
32-bit int	terminal height, rows
32-bit int	terminal width, columns
32-bit int	terminal width, pixels
32-bit int	terminal height, pixels
.TE
This message can only be sent by the client during the interactive
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).
.IP "12 SSH_CMSG_EXEC_SHELL"

(no arguments)

Starts a shell (command interpreter), and enters interactive session
mode.
.IP "13 SSH_CMSG_EXEC_CMD"
.TS
;
l l.
string	command to execute
.TE
Starts executing the given command, and enters interactive session
mode.  On UNIX, the command is run as "<shell> -c <command>", where
<shell> is the user's login shell.
.IP "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.
.IP "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.
.IP "16 SSH_CMSG_STDIN_DATA"
.TS
;
l l.
string	data
.TE
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 acknowledgement is sent for this
message.
.IP "17 SSH_SMSG_STDOUT_DATA"
.TS
;
l l.
string	data
.TE
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.
.IP "18 SSH_SMSG_STDERR_DATA"
.TS
;
l l.
string	data
.TE
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.
.IP "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.
.IP "20 SSH_SMSG_EXITSTATUS"
.TS
;
l l.
32-bit int	exit status of the command
.TE
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 program being executed dies with a signal instead of
exiting normally, 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.
.IP "21 SSH_MSG_CHANNEL_OPEN_CONFIRMATION"
.TS
;
l l.
32-bit int	remote_channel
32-bit int	local_channel
.TE
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 message has allocated for the channel.
Data can be transmitted on the channel after this message.
.IP "22 SSH_MSG_CHANNEL_OPEN_FAILURE"
.TS
;
l l.
32-bit int	remote_channel
.TE
This message indicates that an earlier channel open request by the
other side has failed or has been denied.  Remote_channel is the
channel number given in the original request.
.IP "23 SSH_MSG_CHANNEL_DATA"
.TS
;
l l.
32-bit int	remote_channel
string	data
.TE
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.
.IP "24 SSH_MSG_CHANNEL_CLOSE"
.TS
;
l l.
32-bit int	remote_channel
.TE
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.)
.IP "25 SSH_MSG_CHANNEL_CLOSE_CONFIRMATION"
.TS
;
l l.
32-bit int	remote_channel
.TE
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.
.IP "26 (OBSOLETED; was unix-domain X11 forwarding)
.IP "27 SSH_SMSG_X11_OPEN"
.TS
;
l l.
32-bit int	local_channel
string	originator_string (see below)
.TE
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 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
contains a description of the host originating the connection.
.IP "28 SSH_CMSG_PORT_FORWARD_REQUEST"
.TS
;
l l.
32-bit int	server_port
string	host_to_connect
32-bit int	port_to_connect
.TE
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.
.IP "29 SSH_MSG_PORT_OPEN"
.TS
;
l l.
32-bit int	local_channel
string	host_name
32-bit int	port
string	originator_string (see below)
.TE
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 sending 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 permit 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 SSH_PROTOFLAG_HOST_IN_FWD_OPEN in the protocol flags.  It
contains a description of the host originating the connection.
.IP "30 SSH_CMSG_AGENT_REQUEST_FORWARDING"

(no arguments)

Requests that the connection to the authentication agent be forwarded
over the secure channel.  The method used by clients to contact the
authentication agent within each machine is implementation and machine
dependent.  If the server accepts this request, it should arrange that
any clients run from this session 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.
.IP "31 SSH_SMSG_AGENT_OPEN"
.TS
;
l l.
32-bit int	local_channel
.TE
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.
.IP "32 SSH_MSG_IGNORE"
.TS
;
l l.
string	data
.TE
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 recognize and ignore it.
.IP "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.
.IP "34 SSH_CMSG_X11_REQUEST_FORWARDING"
.TS
;
l l.
string	x11_authentication_protocol
string	x11_authentication_data
32-bit int	screen number (if SSH_PROTOFLAG_SCREEN_NUMBER)
.TE
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".  Authentication 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 important).

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 authentication.  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.
.IP "35 SSH_CMSG_AUTH_RHOSTS_RSA"
.TS
;
l l.
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
.TE
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 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.
.IP "36 SSH_MSG_DEBUG"
.TS
;
l l.
string	debugging message sent to the other side
.TE
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 authentication 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 reasons.  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
implementation must be able to receive this message, but no
implementation is required to send these.
.IP "37 SSH_CMSG_REQUEST_COMPRESSION"
.TS
;
l l.
32-bit int	gzip compression level (1-9)
.TE
This message can be sent by the client in the preparatory operations
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 direction will be compressed by gzip.
.RT


.ti 0
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
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.
.IP "0 TTY_OP_END"
Indicates end of options.
.IP "1 VINTR"
Interrupt character; 255 if none.  Similarly for the other characters.
Not all of these characters are supported on all systems.
.IP "2 VQUIT"
The quit character (sends SIGQUIT signal on UNIX systems).
.IP "3 VERASE"
Erase the character to left of the cursor.
.IP "4 VKILL"
Kill the current input line.
.IP "5 VEOF "
End-of-file character (sends EOF from the terminal).
.IP "6 VEOL "
End-of-line character in addition to carriage return and/or linefeed.
.IP "7 VEOL2"
Additional end-of-line character.
.IP "8 VSTART"
Continues paused output (normally ^Q).
.IP "9 VSTOP"
Pauses output (^S).
.IP "10 VSUSP"
Suspends the current program.
.IP "11 VDSUSP"
Another suspend character.
.IP "12 VREPRINT"
Reprints the current input line.
.IP "13 VWERASE"
Erases a word left of cursor.
.IP "14 VLNEXT"
More special input characters; these are probably not supported on
most systems.
.IP "15 VFLUSH"
.IP "16 VSWTCH"
.IP "17 VSTATUS"
.IP "18 VDISCARD"

.IP "30 IGNPAR"
The ignore parity flag.  The next byte should be 0 if this flag is not
set, and 1 if it is set.
.IP "31 PARMRK"
More flags.  The exact definitions can be found in the POSIX standard.
.IP "32 INPCK"
.IP "33 ISTRIP"
.IP "34 INLCR"
.IP "35 IGNCR"
.IP "36 ICRNL"
.IP "37 IUCLC"
.IP "38 IXON"
.IP "39 IXANY"
.IP "40 IXOFF"
.IP "41 IMAXBEL"

.IP "50 ISIG"
.IP "51 ICANON"
.IP "52 XCASE"
.IP "53 ECHO"
.IP "54 ECHOE"
.IP "55 ECHOK"
.IP "56 ECHONL"
.IP "57 NOFLSH"
.IP "58 TOSTOP"
.IP "59 IEXTEN"
.IP "60 ECHOCTL"
.IP "61 ECHOKE"
.IP "62 PENDIN"

.IP "70 OPOST"
.IP "71 OLCUC"
.IP "72 ONLCR"
.IP "73 OCRNL"
.IP "74 ONOCR"
.IP "75 ONLRET"

.IP "90 CS7"
.IP "91 CS8"
.IP "92 PARENB"
.IP "93 PARODD"

.IP "192 TTY_OP_ISPEED"
Specifies the input baud rate in bits per second.
.IP "193 TTY_OP_OSPEED"
Specifies the output baud rate in bits per second.
.RT


.ti 0
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
challenge.  How the connection is made to the agent (or its
representative) 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
channel.

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:
.TS
;
l l.
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
protocol.
.TE

The following message types are currently defined:
.IP "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.
.IP "2 SSH_AGENT_RSA_IDENTITIES_ANSWER"
.TS
;
l l.
32-bit int	howmany
howmany times:
32-bit int	bits
mp-int	public exponent
mp-int	public modulus
string	comment
.TE
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.
.IP "3 SSH_AGENTC_RSA_CHALLENGE
.TS
;
l l.
32-bit int	bits
mp-int	public exponent
mp-int	public modulus
mp-int	challenge
16 bytes	session_id
32-bit int	response_type
.TE
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 (partial)
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 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.
.IP "4 SSH_AGENT_RSA_RESPONSE"
.TS
;
l l.
16 bytes	MD5 of decrypted challenge
.TE
Answers an RSA authentication challenge.  The response is 16 bytes:
the MD5 checksum of the 32-byte challenge.
.IP "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.
.IP "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.
.IP "7 SSH_AGENTC_ADD_RSA_IDENTITY"
.TS
;
l l.
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
.TE
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.
.IP "8 SSH_AGENT_REMOVE_RSA_IDENTITY"
.TS
;
l l.
32-bit int	bits
mp-int	public exponent
mp-int	public modulus
.TE
Removes an RSA key from the agent.  The agent will no longer accept
challenges for this key and will not list it as a supported identity.
The agent responds with SSH_AGENT_SUCCESS or SSH_AGENT_FAILURE.
.RT

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
connections active simultaneously).


.ti 0
References

.IP "[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 Security.  Prentice-Hall,
1989.
.IP "[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.
.IP "[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 Cryptography, John Wiley &
Sons, 1994.  See also the following patents: PCT/CH91/00117, EP 0 482
154 B1, US Pat. 5,214,703.
.IP [PKCS#1]
PKCS #1: RSA Encryption Standard.  Version 1.5, RSA Laboratories,
November 1993.  Available for anonymous ftp at ftp.rsa.com.
.IP [POSIX]
Portable Operating System Interface (POSIX) - Part 1: Application
Program Interface (API) [C language], ISO/IEC 9945-1, IEEE Std 1003.1,
1990.
.IP [RFC0791]
J. Postel: Internet Protocol, RFC 791, USC/ISI, September 1981.
.IP [RFC0793]
J. Postel: Transmission Control Protocol, RFC 793, USC/ISI, September
1981.
.IP [RFC1034]
P. Mockapetris: Domain Names - Concepts and Facilities, RFC 1034,
USC/ISI, November 1987.
.IP [RFC1282]
B. Kantor: BSD Rlogin, RFC 1258, UCSD, December 1991.
.IP "[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.
.IP "[X11] "
R. Scheifler: X Window System Protocol, X Consortium Standard, Version
11, Release 6.  Massachusetts Institute of Technology, Laboratory of
Computer Science, 1994.
.RT


.ti 0
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 arbitrary 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 telnet
and rlogin protocols.
.IP o
IP spoofing is restricted to closing a connection (by 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.
.IP o
DNS spoofing is made ineffective (by host keys).
.IP o
Routing spoofing is made ineffective (by host keys).
.IP o
All data is encrypted with strong algorithms to make eavesdropping as
difficult as possible.  This includes encrypting any authentication
information such as passwords.  The information for decrypting session
keys is destroyed every hour.
.IP o
Strong authentication methods: .rhosts combined with RSA host
authentication, and pure RSA authentication.
.IP o
X11 connections and arbitrary TCP/IP ports can be forwarded securely.
.IP o
Man-in-the-middle attacks are deterred by using the server host key to
encrypt the session key.
.IP 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.
.RT

The security of SSH against man-in-the-middle attacks and the security
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 protect
data communicated between an X server and a client running on a remote
machine.  By creating a fake display on the server, and forwarding 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.

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 criminals 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.


.ti 0
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>.

.ti 0
Author's Address

.TS
;
l.
Tatu Ylonen
Helsinki University of Technology
Otakaari 1
FIN-02150 Espoo, Finland

Phone: +358-0-451-3374
Fax: +358-0-451-3293
EMail: ylo@cs.hut.fi
.TE