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SSH: Introduction to SSH

Purdue Computer Network
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 channels. It is intended as a replacement for rlogin, rsh, rcp, and rdist.

See the file INSTALL for installation instructions. See COPYING for license terms and other legal issues. See RFC for a description of the protocol. There is a WWW page for ssh; see

This file has been updated to match ssh-1.2.15.


  • Strong authentication. Closes several security holes (e.g., IP, routing, and DNS spoofing). New authentication methods: .rhosts together with RSA based host authentication, and pure RSA authentication.

  • Improved privacy. All communications are automatically and transparently encrypted. RSA is used for key exchange, and a conventional cipher (normally IDEA, DES, or triple-DES) for encrypting the session. Encryption is started before authentication, and no passwords or other information is transmitted in the clear. Encryption is also used to protect against spoofed packets.

  • Secure X11 sessions. The program automatically sets DISPLAY on the server machine, and forwards any X11 connections over the secure channel. Fake Xauthority information is automatically generated and forwarded to the remote machine; the local client automatically examines incoming X11 connections and replaces the fake authorization data with the real data (never telling the remote machine the real information).

  • Arbitrary TCP/IP ports can be redirected through the encrypted channel in both directions (e.g., for e-cash transactions).

  • No retraining needed for normal users; everything happens automatically, and old .rhosts files will work with strong authentication if administration installs host key files.

  • Never trusts the network. Minimal trust on the remote side of the connection. Minimal trust on domain name servers. Pure RSA authentication never trusts anything but the private key.

  • 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).

  • Host authentication key distribution can be centrally by the administration, automatically when the first connection is made to a machine (the key obtained on the first connection will be recorded and used for authentication in the future), or manually by each user for his/her own use. The central and per-user host key repositories are both used and complement each other. Host keys can be generated centrally or automatically when the software is installed. Host authentication keys are typically 1024 bits.

  • Any user can create any number of user authentication RSA keys for his/her own use. Each user has a file which lists the RSA public keys for which proof of possession of the corresponding private key is accepted as authentication. User authentication keys are typically 1024 bits.

  • The server program has its own server RSA key which is automatically regenerated every hour. This key is never saved in any file. Exchanged session keys are encrypted using both the server key and the server host key. The purpose of the separate server key is to make it impossible to decipher a captured session by breaking into the server machine at a later time; one hour from the connection even the server machine cannot decipher the session key. The key regeneration interval is configurable. The server key is normally 768 bits.

  • An authentication agent, running in the user's laptop or local workstation, can be used to hold the user's RSA authentication keys. Ssh automatically forwards the connection to the authentication agent over any connections, and there is no need to store the RSA authentication keys on any machine in the network (except the user's own local machine). The authentication protocols never reveal the keys; they can only be used to verify that the user's agent has a certain key. Eventually the agent could rely on a smart card to perform all authentication computations.

  • The software can be installed and used (with restricted functionality) even without root privileges.

  • The client is customizable in system-wide and per-user configuration files. Most aspects of the client's operation can be configured. Different options can be specified on a per-host basis.

  • Automatically executes conventional rsh (after displaying a warning) if the server machine is not running sshd.

  • Optional compression of all data with gzip (including forwarded X11 and TCP/IP port data), which may result in significant speedups on slow connections.

  • Complete replacement for rlogin, rsh, and rcp.

Currently, almost all communications in computer networks are done without encryption. As a consequence, anyone who has access to any machine connected to the network can listen in on any communication. This is being done by hackers, curious administrators, employers, criminals, industrial spies, and governments. Some networks leak off enough electromagnetic radiation that data may be captured even from a distance.

When you log in, your password goes in the network in plain text. Thus, any listener can then use your account to do any evil he likes. Many incidents have been encountered worldwide where crackers have started programs on workstations without the owners knowledge just to listen to the network and collect passwords. Programs for doing this are available on the Internet, or can be built by a competent programmer in a few hours.

Furthermore, it is possible to hijack connections going though the network. This means that an intruder can enter in the middle of an existing connection, and start modifying data in both directions. This can, e.g., be used to insert new commands in sessions authenticated by one-time passwords. A consequence is that no security method based on purely authenticating the user is safe. Furthermore, routing spoofing can be used to bring almost any connection in the Internet to a location where it can be attacked.

Encryption and cryptographic authentication and integrity protection are required to secure networks and computer systems. SSH uses strong cryptographic algorithms to achieve these goals.

Ease of use is critical to the acceptance of a piece of software. SSH attempts to be *easier* to use than its insecure counterparts.

SSH has gained very wide acceptance. It is currently (late 1996) being used in approximately 50 countries at probably tens of thousands of organizations. Its users include top universities, research laboratories, banks, major corporations, and numerous smaller companies and individuals.

SSH is available for almost all Unix platforms, and commercial versions are available for Windows (3.1, 95, NT) and Macintosh. For more information, see


The software consists of a number of programs.

   sshd		Server program run on the server machine.  This
listens for connections from client machines, and
whenever it receives a connection, it performs
authentication and starts serving the client.

ssh This is the client program used to log into another
machine or to execute commands on the other machine.
"slogin" is another name for this program.

scp Securely copies files from one machine to another.

ssh-keygen Used to create RSA keys (host keys and user
authentication keys).

ssh-agent Authentication agent. This can be used to hold RSA
keys for authentication.

ssh-add Used to register new keys with the agent.

Used to create the /etc/ssh_known_hosts file.

Ssh is the program users normally use. It is started as
  • ssh host
  • ssh host command
The first form opens a new shell on the remote machine (after authentication). The latter form executes the command on the remote machine.

When started, the ssh connects sshd on the server machine, verifies that the server machine really is the machine it wanted to connect, exchanges encryption keys (in a manner which prevents an outside listener from getting the keys), performs authentication using .rhosts and /etc/hosts.equiv, RSA authentication, or conventional password based authentication. The server then (normally) allocates a pseudo-terminal and starts an interactive shell or user program.

The TERM environment variable (describing the type of the user's terminal) is passed from the client side to the remote side. Also, terminal modes will be copied from the client side to the remote side to preserve user preferences (e.g., the erase character).

If the DISPLAY variable is set on the client side, the server will create a dummy X server and set DISPLAY accordingly. Any connections to the dummy X server will be forwarded through the secure channel, and will be made to the real X server from the client side. An arbitrary number of X programs can be started during the session, and starting them does not require anything special from the user. (Note that the user must not manually set DISPLAY, because then it would connect directly to the real display instead of going through the encrypted channel). This behavior can be disabled in the configuration file or by giving the -x option to the client.

Arbitrary IP ports can be forwarded over the secure channel. The program then creates a port on one side, and whenever a connection is opened to this port, it will be passed over the secure channel, and a connection will be made from the other side to a specified host:port pair. Arbitrary IP forwarding must always be explicitly requested, and cannot be used to forward privileged ports (unless the user is root). It is possible to specify automatic forwards in a per-user configuration file, for example to make electronic cash systems work securely.

If there is an authentication agent on the client side, connection to it will be automatically forwarded to the server side.

For more information, see the manual pages ssh(1), sshd(8), scp(1), ssh-keygen(1), ssh-agent(1), ssh-add(1), and make-ssh-known-hosts(1) included in this distribution.


X11 forwarding serves two purposes: it is a convenience to the user because there is no need to set the DISPLAY variable, and it provides encrypted X11 connections. I cannot think of any other easy way to make X11 connections encrypted; modifying the X server, clients or libraries would require special work for each machine, vendor and application. Widely used IP-level encryption does not seem likely for several years. Thus what we have left is faking an X server on the same machine where the clients are run, and forwarding the connections to a real X server over the secure channel.

X11 forwarding works as follows. The client extracts Xauthority information for the server. It then creates random authorization data, and sends the random data to the server. The server allocates an X11 display number, and stores the (fake) Xauthority data for this display. Whenever an X11 connection is opened, the server forwards the connection over the secure channel to the client, and the client parses the first packet of the X11 protocol, substitutes real authentication data for the fake data (if the fake data matched), and forwards the connection to the real X server.

If the display does not have Xauthority data, the server will create a unix domain socket in /tmp/.X11-unix, and use the unix domain socket as the display. No authentication information is forwarded in this case. X11 connections are again forwarded over the secure channel. To the X server the connections appear to come from the client machine, and the server must have connections allowed from the local machine. Using authentication data is always recommended because not using it makes the display insecure. If XDM is used, it automatically generates the authentication data.

One should be careful not to use "xin" or "xstart" or other similar scripts that explicitly set DISPLAY to start X sessions in a remote machine, because the connection will then not go over the secure channel. The recommended way to start a shell in a remote machine is

  • xterm -e ssh host &

and the recommended way to execute an X11 application in a remote machine is

  • ssh -n host emacs &

If you need to type a password/passphrase for the remote machine,

  • ssh -f host emacs

may be useful.

A few other documents that may be useful are located at:


RSA authentication is based on public key cryptography. The idea is that there are two encryption keys, one for encryption and another for decryption. It is not possible (on human time scale) to derive the decryption key from the encryption key. The encryption key is called the public key, because it can be given to anyone and it is not secret. The decryption key, on the other hand, is secret, and is called the private key.

RSA authentication is based on the impossibility of deriving the private key from the public key. The public key is stored on the server machine in the user's $HOME/.ssh/authorized_keys file. The private key is only kept on the user's local machine, laptop, or other secure storage. Then the user tries to log in, the client tells the server the public key that the user wishes to use for authentication. The server then checks if this public key is admissible. If so, it generates a 256 bit random number, encrypts it with the public key, and sends the value to the client. The client then decrypts the number with its private key, computes a 128 bit MD5 checksum from the resulting data, and sends the checksum back to the server. (Only a checksum is sent to prevent chosen-plaintext attacks against RSA.) The server checks computes a checksum from the correct data, and compares the checksums. Authentication is accepted if the checksums match. (Theoretically this indicates that the client only probably knows the correct key, but for all practical purposes there is no doubt.)

The RSA private key can be protected with a passphrase. The passphrase can be any string; it is hashed with MD5 to produce an encryption key for 3DES, which is used to encrypt the private part of the key file. With passphrase, authorization requires access to the key file and the passphrase. Without passphrase, authorization only depends on possession of the key file.

RSA authentication is the most secure form of authentication supported by this software. It does not rely on the network, routers, domain name servers, or the client machine. The only thing that matters is access to the private key.

All this, of course, depends on the security of the RSA algorithm itself. RSA has been widely known since about 1978, and no effective methods for breaking it are known if it is used properly. Care has been taken to avoid the well-known pitfalls. Breaking RSA is widely believed to be equivalent to factoring, which is a very hard mathematical problem that has received considerable public research. So far, no effective methods are known for numbers bigger than about 512 bits. However, as computer speeds and factoring methods are increasing, 512 bits can no longer be considered secure. The factoring work is exponential, and 768 or 1024 bits are widely considered to be secure in the near future.


Conventional .rhosts and hosts.equiv based authentication mechanisms are fundamentally insecure due to IP, DNS (domain name server) and routing spoofing attacks. Additionally this authentication method relies on the integrity of the client machine. These weaknesses is tolerable, and been known and exploited for a long time.

Ssh provides an improved version of these types of authentication, because they are very convenient for the user (and allow easy transition from rsh and rlogin). It permits these types of authentication, but additionally requires that the client host be authenticated using RSA.

The server has a list of host keys stored in /etc/ssh_known_host, and additionally each user has host keys in $HOME/.ssh/known_hosts. Ssh uses the name servers to obtain the canonical name of the client host, looks for its public key in its known host files, and requires the client to prove that it knows the private host key. This prevents IP and routing spoofing attacks (as long as the client machine private host key has not been compromised), but is still vulnerable to DNS attacks (to a limited extent), and relies on the integrity of the client machine as to who is requesting to log in. This prevents outsiders from attacking, but does not protect against very powerful attackers. If maximal security is desired, only RSA authentication should be used.

It is possible to enable conventional .rhosts and /etc/hosts.equiv authentication (without host authentication) at compile time by giving the option --with-rhosts to configure. However, this is not recommended, and is not done by default.

These weaknesses are present in rsh and rlogin. No improvement in security will be obtained unless rlogin and rsh are completely disabled (commented out in /etc/inetd.conf). This is highly recommended.


See the file COPYING for distribution conditions. To summarize, you can use this software freely for non-commercial purposes. However, this software cannot be sold or used for directly revenue-generating purposes without licensing. THERE IS NO WARRANTY FOR THIS PROGRAM.

In some countries, particularly Russia, Iraq, Pakistan, and France, it may be illegal to use any encryption at all without a special permit.

This software may be freely imported into the United States; however, the United States Government may consider re-exporting it a criminal offense. Thus, if you are outside the US, please retrieve this software from outside the US.

Note that any information and cryptographic algorithms used in this software are publicly available on the Internet and at any major bookstore, scientific library, or patent office worldwide.


There is a mailing list for ssh. It is If you would like to join, send a message to with "subscribe ssh" in body.

The WWW home page for ssh is It contains an archive of the mailing list, and detailed information about new releases, mailing lists, and other relevant issues.

For information about Windows, Macintosh, and commercial licensing, see, or mail to

Bug reports should be sent to


This software was originally written by Tatu Ylonen in Finland. It is now being maintained by SSH Communications Security ( and Data Fellows (


Many people have contributed to the development of this software.

Martin Abadi, Satoshi Adachi, Tim Adam, Walker Aumann, Jurgen Botz, Hans-Werner Braun, Stephane Bortzmeyer, Bill Broadley, Andrey Chernov, Adrian Colley, Michael Cooper, David Dombek, Ian Donaldson, Danek Duvall, Jerome Etienne, Bill Fithen, Mark Fullmer, Jean-Loup Gailly, Bert Gijsbers, Torbjorn Granlund, Klaus Guntermann, Andreas Gustafsson, Michael Henits, Ton Hospel, Antti Huima, Cedomir Igaly, Steve Johnson, Tero Kivinen, Mika Kojo, Thomas Knig, David Kgedal, Joseph Lappa, Felix Leitner, Gunnar Lindberg, Harald Lundberg, Andrew Macpherson, Marc Martinec, Paul Mauvais, David Mazieres, Donald McKillican, Pedro Melo, Leon Mlakar, Robert Muchsel, Pekka Nikander, Matt Power, Timo Rinne, Ollivier Robert, Tomi Salo, Janne Snabb, Bryan O'Sullivan, Mikael Suokas, Heikki Suonsivu, Corey Satten, Jakob Schlyter, Wayne Schroeder, Harlan Stenn, Tomasz Surmacz, Holger Trapp, Mark Treacy, Dragan Vecerina, Wietse Venema, Alvar Vinacua, Russell Vincent, Petri Virkkula, Michael Warfield, Brian Weaver, Peter Wemm, Mike Williams, Christophe Wolfhugel, Andre April, Arne Juul, Andy Polyakov, LaMont Jones, Witse Venema, Charles M. Hannum, Jay Schuster, Kojima Hajime, Glenn Machin, Brian Cully, Hannu Napari, Charles Karney, Markus Linnala, and Doug Engert.

My apologies to people whom I have forgotten to list.

Thanks also go to Philip Zimmermann, whose PGP software and the associated legal battle provided inspiration, motivation, and many useful techniques, and to Bruce Schneier whose book Applied Cryptography has done a great service in widely distributing knowledge about cryptographic methods.

Copyright (c) 1996 SSH Communications Security, Espoo, Finland.

Last Modified: Dec 19, 2016 11:12 am US/Eastern
Created: Mar 7, 2007 11:43 am US/Eastern by admin