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
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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: 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. All
communications are automatically and transparently encrypted.
Encryption is also used to protect integrity. X11 connection
forwarding provides secure X11 sessions. Arbitrary TCP/IP ports can
be redirected over the encrypted channel in both directions. 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). 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
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.
The Binary Packet Protocol
After the protocol identification strings, both sides only send
specially formatted packets. The packet layout is as follows: 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. 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.
Packet type: 8-bit unsigned byte. The value 255 is reserved for
future extension. Data: binary data bytes, depending on the packet
type. The number of data bytes is the "length" field minus 5. 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.
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
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.
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: 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
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.
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. 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.) 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. 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. 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: The byte is stored directly as a
single byte. Stored in 4 bytes, msb first. 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. 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
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.
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
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.
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.
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: 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
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.
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.
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.
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.
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.
Requests that compression be enabled for this session. A gzip-
compatible compression level (1-9) is passed as an argument.
Requests that a pseudo terminal device be allocated for this session.
The user terminal type and terminal modes are supplied as arguments.
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.
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). Requests forwarding of the connection
to the authentication agent. Starts a shell (command interpreter)
for the user, and moves into interactive session mode. 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
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: 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. 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.) Indicates that the shell or command has exited. Exit status
is passed as an integer argument. This message causes termination of
the interactive session. 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. 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. 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. 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. 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. 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. 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. 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: This is data to be
sent as input to the program running on the server. The data is
passed as a string. 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.
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. 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. 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.
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. 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. 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. 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.
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.
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
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):
This flag can only be sent by the client. It indicates that the X11
forwarding requests it sends will include the screen number. 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.
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.) This code is reserved.
This message type is never sent. ; l l. 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. ; 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 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). ; 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 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. ; l l.
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.] ; l l. 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.) ; l l. 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 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. ; l l. mp-int encrypted challenge 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. ; l l. 16 bytes MD5
of decrypted challenge 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. ; l l. 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. ; 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
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. ; 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 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).
(no arguments)
Starts a shell (command interpreter), and enters interactive session
mode. ; l l. string command to execute 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.
(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.
(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. ;
l l. 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
acknowledgement is sent for this message. ; l l. 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. ; l l. string data 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.
(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. ; l l. 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 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. ; l l. 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 message has
allocated for the channel. Data can be transmitted on the channel
after this message. ; l l. 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 the channel
number given in the original request. ; l l. 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. ; l l. 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.) ;
l l. 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. ; l l. 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 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. ; l l. 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.
; l l. 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 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.
(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. ; l
l. 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. ; l l. 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 recognize and ignore it.
(no arguments)
Sent by the client in response to SSH_SMSG_EXITSTATUS. This is the
last message sent by the client. ; l l.
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". 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. ; 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 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. ; l l. 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 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. ; l l. 32-bit
int gzip compression level (1-9) 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.
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. Indicates end of
options. Interrupt character; 255 if none. Similarly for the other
characters. Not all of these characters are supported on all
systems. The quit character (sends SIGQUIT signal on UNIX systems).
Erase the character to left of the cursor. Kill the current input
line. End-of-file character (sends EOF from the terminal). End-of-
line character in addition to carriage return and/or linefeed.
Additional end-of-line character. Continues paused output (normally
^Q). Pauses output (^S). Suspends the current program. Another
suspend character. Reprints the current input line. Erases a word
left of cursor. More special input characters; these are probably
not supported on most systems.
The ignore parity flag. The next byte should be 0 if this flag is
not set, and 1 if it is set. More flags. The exact definitions can
be found in the POSIX standard.
Specifies the input baud rate in bits per second. Specifies the
output baud rate in bits per second.
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: ; 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.
The following message types are currently defined:
(no arguments)
Requests the agent to send a list of all RSA keys for which it can
answer a challenge. ; l l. 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. ; 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 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. ; l l. 16
bytes MD5 of decrypted challenge Answers an RSA authentication
challenge. The response is 16 bytes: the MD5 checksum of the 32-byte
challenge.
(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.
(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. ; 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 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. ; l l.
32-bit int bits 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 supported 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
connections active simultaneously).
References
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. 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. 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. PKCS
#1: RSA Encryption Standard. Version 1.5, RSA Laboratories, November
1993. Available for anonymous ftp at ftp.rsa.com. Portable
Operating System Interface (POSIX) - Part 1: Application Program
Interface (API) [C language], ISO/IEC 9945-1, IEEE Std 1003.1, 1990.
J. Postel: Internet Protocol, RFC 791, USC/ISI, September 1981. J.
Postel: Transmission Control Protocol, RFC 793, USC/ISI, September
1981. P. Mockapetris: Domain Names - Concepts and Facilities, RFC
1034, USC/ISI, November 1987. B. Kantor: BSD Rlogin, RFC 1258, UCSD,
December 1991. 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. 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
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 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. DNS spoofing is made
ineffective (by host keys). Routing spoofing is made ineffective (by
host keys). 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. Strong
authentication methods: .rhosts combined with RSA host
authentication, and pure RSA authentication. X11 connections and
arbitrary TCP/IP ports can be forwarded securely. Man-in-the-middle
attacks are deterred by using the server host key to encrypt the
session key. 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
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.
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
; 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
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