path: root/README
diff options
authorDamien Miller <>1999-10-27 13:42:43 +1000
committerDamien Miller <>1999-10-27 13:42:43 +1000
commitd4a8b7e34dd619a4debf9a206c81db26d1402ea6 (patch)
treea47d770a2f790f40d18b0982d4e55fa7cfb1fa3b /README
Initial revision
Diffstat (limited to 'README')
1 files changed, 563 insertions, 0 deletions
diff --git a/README b/README
new file mode 100644
index 00000000..ed360844
--- /dev/null
+++ b/README
@@ -0,0 +1,563 @@
+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 inteded 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-1.2.12.
+ o 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.
+ o 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.
+ o 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).
+ o Arbitrary TCP/IP ports can be redirected through the encrypted channel
+ in both directions (e.g., for e-cash transactions).
+ o No retraining needed for normal users; everything happens
+ automatically, and old .rhosts files will work with strong
+ authentication if administration installs host key files.
+ o 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.
+ o Client RSA-authenticates the server machine in the beginning of
+ every connection to prevent trojan horses (by routing or DNS
+ spoofing) and man-in-the-middle attacks, and the server
+ RSA-authenticates the client machine before accepting .rhosts or
+ /etc/hosts.equiv authentication (to prevent DNS, routing, or
+ IP-spoofing).
+ o 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.
+ o 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.
+ o 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.
+ o 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.
+ o The software can be installed and used (with restricted
+ functionality) even without root privileges.
+ o 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.
+ o Automatically executes conventional rsh (after displaying a
+ warning) if the server machine is not running sshd.
+ o Optional compression of all data with gzip (including forwarded X11
+ and TCP/IP port data), which may result in significant speedups on
+ slow connections.
+ o 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
+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.
+Any information that you type or is printed on your screen can be
+monitored, recorded, and analyzed. For example, an intruder who has
+penetrated a host connected to a major network can start a program
+that listens to all data flowing in the network, and whenever it
+encounters a 16-digit string, it checks if it is a valid credit card
+number (using the check digit), and saves the number plus any
+surrounding text (to catch expiration date and holder) in a file.
+When the intruder has collected a few thousand credit card numbers, he
+makes smallish mail-order purchases from a few thousand stores around
+the world, and disappears when the goods arrive but before anyone
+suspects anything.
+Businesses have trade secrets, patent applications in preparation,
+pricing information, subcontractor information, client data, personnel
+data, financial information, etc. Currently, anyone with access to
+the network (any machine on the network) can listen to anything that
+goes in the network, without any regard to normal access restrictions.
+Many companies are not aware that information can so easily be
+recovered from the network. They trust that their data is safe
+since nobody is supposed to know that there is sensitive information
+in the network, or because so much other data is transferred in the
+network. This is not a safe policy.
+Individual persons also have confidential information, such as
+diaries, love letters, health care documents, information about their
+personal interests and habits, professional data, job applications,
+tax reports, political documents, unpublished manuscripts, etc.
+One should also be aware that economical intelligence and industrial
+espionage has recently become a major priority of the intelligence
+agencies of major governments. President Clinton recently assigned
+economical espionage as the primary task of the CIA, and the French
+have repeatedly been publicly boasting about their achievements on
+this field.
+There is also another frightening aspect about the poor security of
+communications. Computer storage and analysis capability has
+increased so much that it is feasible for governments, major
+companies, and criminal organizations to automatically analyze,
+identify, classify, and file information about millions of people over
+the years. Because most of the work can be automated, the cost of
+collecting this information is getting very low.
+Government agencies may be able to monitor major communication
+systems, telephones, fax, computer networks, etc., and passively
+collect huge amounts of information about all people with any
+significant position in the society. Most of this information is not
+sensitive, and many people would say there is no harm in someone
+getting that information. However, the information starts to get
+sensitive when someone has enough of it. You may not mind someone
+knowing what you bought from the shop one random day, but you might
+not like someone knowing every small thing you have bought in the last
+ten years.
+If the government some day starts to move into a more totalitarian
+direction (one should remember that Nazi Germany was created by
+democratic elections), there is considerable danger of an ultimate
+totalitarian state. With enough information (the automatically
+collected records of an individual can be manually analyzed when the
+person becomes interesting), one can form a very detailed picture of
+the individual's interests, opinions, beliefs, habits, friends,
+lovers, weaknesses, etc. This information can be used to 1) locate
+any persons who might oppose the new system 2) use deception to
+disturb any organizations which might rise against the government 3)
+eliminate difficult individuals without anyone understanding what
+happened. Additionally, if the government can monitor communications
+too effectively, it becomes too easy to locate and eliminate any
+persons distributing information contrary to the official truth.
+Fighting crime and terrorism are often used as grounds for domestic
+surveillance and restricting encryption. These are good goals, but
+there is considerable danger that the surveillance data starts to get
+used for questionable purposes. I find that it is better to tolerate
+a small amount of crime in the society than to let the society become
+fully controlled. I am in favor of a fairly strong state, but the
+state must never get so strong that people become unable to spread
+contra-offical information and unable to overturn the government if it
+is bad. The danger is that when you notice that the government is
+too powerful, it is too late. Also, the real power may not be where
+the official government is.
+For these reasons (privacy, protecting trade secrets, and making it
+more difficult to create a totalitarian state), I think that strong
+cryptography should be integrated to the tools we use every day.
+Using it causes no harm (except for those who wish to monitor
+everything), but not using it can cause huge problems. If the society
+changes in undesirable ways, then it will be to late to start
+Encryption has had a "military" or "classified" flavor to it. There
+are no longer any grounds for this. The military can and will use its
+own encryption; that is no excuse to prevent the civilians from
+protecting their privacy and secrets. Information on strong
+encryption is available in every major bookstore, scientific library,
+and patent office around the world, and strong encryption software is
+available in every country on the Internet.
+Some people would like to make it illegal to use encryption, or to
+force people to use encryption that governments can break. This
+approach offers no protection if the government turns bad. Also, the
+"bad guys" will be using true strong encryption anyway. Good
+encryption techniques are too widely known to make them disappear.
+Thus, any "key escrow encryption" or other restrictions will only help
+monitor ordinary people and petty criminals. It does not help against
+powerful criminals, terrorists, or espionage, because they will know
+how to use strong encryption anyway. (One source for internationally
+available encryption software is
+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.
+ make-ssh-known-hosts
+ 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
+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
+If there is an authentication agent on the client side, connection to
+it will be automatically forwarded to the server side.
+For more infomation, 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.
+RSA authentication is based on public key cryptograpy. The idea is
+that there are two encryption keys, one for encryption and another for
+decryption. It is not possible (on human timescale) 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 IDEA, 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 compromized), 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
+One should understand that while this software may provide
+cryptographically secure communications, it may be easy to
+monitor the communications at their endpoints.
+Basically, anyone with root access on the local machine on which you
+are running the software may be able to do anything. Anyone with root
+access on the server machine may be able to monitor your
+communications, and a very talented root user might even be able to
+send his/her own requests to your authentication agent.
+One should also be aware that computers send out electromagnetic
+radition that can sometimes be picked up hundreds of meters away.
+Your keyboard is particularly easy to listen to. The image on your
+monitor might also be seen on another monitor in a van parked behind
+your house.
+Beware that unwanted visitors might come to your home or office and
+use your machine while you are away. They might also make
+modifications or install bugs in your hardware or software.
+Beware that the most effective way for someone to decrypt your data
+may be with a rubber hose.
+As far as I am concerned, anyone is permitted to use this software
+freely. However, see the file COPYING for detailed copying,
+licensing, and distribution information.
+In some countries, particularly France, Russia, Iraq, and Pakistan,
+it may be illegal to use any encryption at all without a special
+permit, and the rumor has it that you cannot get a permit for any
+strong encryption.
+This software may be freely imported into the United States; however,
+the United States Government may consider re-exporting it a criminal
+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.
+COPYING for more information.
+There is a mailing list for ossh. 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.
+Bug reports should be sent to
+This software was written by Tatu Ylonen <>. I work as a
+researcher at Helsinki University of Technology, Finland. For more
+information, see My PGP public key is
+available via finger from and from the key servers. I
+prefer PGP encrypted mail.
+The author can be contacted via ordinary mail at
+ Tatu Ylonen
+ Helsinki University of Technology
+ Otakaari 1
+ FIN-02150 ESPOO
+ Finland
+ Fax. +358-0-4513293
+I thank Tero Kivinen, Timo Rinne, Janne Snabb, and Heikki Suonsivu for
+their help and comments in the design, implementation and porting of
+this software. I also thank numerous contributors, including but not
+limited to Walker Aumann, Jurgen Botz, Hans-Werner Braun, Stephane
+Bortzmeyer, Adrian Colley, Michael Cooper, David Dombek, Jerome
+Etienne, Bill Fithen, Mark Fullmer, Bert Gijsbers, Andreas Gustafsson,
+Michael Henits, Steve Johnson, Thomas Koenig, Felix Leitner, Gunnar
+Lindberg, Andrew Macpherson, Marc Martinec, Paul Mauvais, Donald
+McKillican, Leon Mlakar, Robert Muchsel, Mark Treacy, Bryan
+O'Sullivan, Mikael Suokas, Ollivier Robert, Jakob Schlyter, Tomasz
+Surmacz, Alvar Vinacua, Petri Virkkula, Michael Warfield, and
+Cristophe Wolfhugel.
+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) 1995 Tatu Ylonen, Espoo, Finland.