Renamed README -> README.Ylonen
Renamed README.openssh -> README Minor updated to new README
This commit is contained in:
Damien Miller
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2bbb9d0c99
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README
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README
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@ -1,563 +1,52 @@
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Ssh (Secure Shell) is a program to log into another computer over a
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network, to execute commands in a remote machine, and to move files
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from one machine to another. It provides strong authentication and
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secure communications over insecure channels. It is inteded as a
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replacement for rlogin, rsh, rcp, and rdist.
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This is a Linux port of OpenBSD's excellent OpenSSH.
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See the file INSTALL for installation instructions. See COPYING for
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license terms and other legal issues. See RFC for a description of
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the protocol. There is a WWW page for ssh; see http://www.cs.hut.fi/ssh.
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OpenSSH is based on the last free version of Tatu Ylonen's SSH with
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all patent-encumbered algorithms removed, all known security bugs
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fixed, new features reintroduced and many other clean-ups.
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This file has been updated to match ssh-1.2.12.
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This Linux port basically consists of a few fixes to deal with the way
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that OpenSSL is usually installed on Linux systems, a few replacements
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for OpenBSD library functions and the introduction of PAM support.
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The PAM support is now more functional than the popular packages of
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commercial ssh-1.2.x. It checks "account" and "session" modules for
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all logins, not just when using password authentication. This code is
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very new and needs further testing. I have also added basic libpwdb
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support (detected by autoconf).
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FEATURES
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All new code is released under a XFree style license, which is very
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liberal. This code is released with no warranties of any kind,
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neither I nor my employer (Internet Business Solutions) will take any
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responsibility for any loss, damage or liability arising from the use
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or abuse of this software. The code in strlcpy.c and mktemp.c is from
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the OpenBSD project and has its own license (see source file for
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details).
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o Strong authentication. Closes several security holes (e.g., IP,
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routing, and DNS spoofing). New authentication methods: .rhosts
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together with RSA based host authentication, and pure RSA
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authentication.
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OpenSSH depends on Zlib, OpenSSL and PAM and optionally libpwdb. It now
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uses autoconf to build thanks to Dan Brosemer <odin@linuxfreak.com>
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o Improved privacy. All communications are automatically and
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transparently encrypted. RSA is used for key exchange, and a
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conventional cipher (normally IDEA, DES, or triple-DES) for
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encrypting the session. Encryption is started before
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authentication, and no passwords or other information is
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transmitted in the clear. Encryption is also used to protect
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against spoofed packets.
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Damien Miller <djm@ibs.com.au>
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Internet Business Solutions
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o Secure X11 sessions. The program automatically sets DISPLAY on
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the server machine, and forwards any X11 connections over the
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secure channel. Fake Xauthority information is automatically
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generated and forwarded to the remote machine; the local client
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automatically examines incoming X11 connections and replaces the
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fake authorization data with the real data (never telling the
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remote machine the real information).
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o Arbitrary TCP/IP ports can be redirected through the encrypted channel
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in both directions (e.g., for e-cash transactions).
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Credits -
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o No retraining needed for normal users; everything happens
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automatically, and old .rhosts files will work with strong
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authentication if administration installs host key files.
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The OpenBSD team
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'jonchen' - the original author of PAM support of SSH
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Dan Brosemer <odin@linuxfreak.com> - Autoconf and build fixes & Debian scripts
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Niels Kristian Bech Jensen <nkbj@image.dk> - Makefile patch
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Nalin Dahyabhai <nalin.dahyabhai@pobox.com> - PAM environment patch
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o Never trusts the network. Minimal trust on the remote side of
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the connection. Minimal trust on domain name servers. Pure RSA
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authentication never trusts anything but the private key.
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Miscellania -
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o Client RSA-authenticates the server machine in the beginning of
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every connection to prevent trojan horses (by routing or DNS
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spoofing) and man-in-the-middle attacks, and the server
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RSA-authenticates the client machine before accepting .rhosts or
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/etc/hosts.equiv authentication (to prevent DNS, routing, or
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IP-spoofing).
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This version of SSH is based upon code retrieved from the OpenBSD CVS
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repository on 1999-10-29 patched by Damien Miller <djm@ibs.com.au>,
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which in turn was based on the last free version of SSH released by
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Tatu Ylonen.
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o Host authentication key distribution can be centrally by the
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administration, automatically when the first connection is made
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to a machine (the key obtained on the first connection will be
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recorded and used for authentication in the future), or manually
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by each user for his/her own use. The central and per-user host
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key repositories are both used and complement each other. Host
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keys can be generated centrally or automatically when the software
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is installed. Host authentication keys are typically 1024 bits.
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Code in helper.[ch] is Copyright 1999 Internet Business Solutions and
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is released under a X11-style license (see source file for details).
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o Any user can create any number of user authentication RSA keys for
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his/her own use. Each user has a file which lists the RSA public
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keys for which proof of possession of the corresponding private
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key is accepted as authentication. User authentication keys are
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typically 1024 bits.
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(A)RC4 code in rc4.[ch] is Copyright 1999 Damien Miller. It too is
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under a X11-style license (see source file for details).
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o The server program has its own server RSA key which is
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automatically regenerated every hour. This key is never saved in
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any file. Exchanged session keys are encrypted using both the
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server key and the server host key. The purpose of the separate
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server key is to make it impossible to decipher a captured session by
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breaking into the server machine at a later time; one hour from
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the connection even the server machine cannot decipher the session
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key. The key regeneration interval is configurable. The server
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key is normally 768 bits.
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o An authentication agent, running in the user's laptop or local
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workstation, can be used to hold the user's RSA authentication
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keys. Ssh automatically forwards the connection to the
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authentication agent over any connections, and there is no need to
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store the RSA authentication keys on any machine in the network
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(except the user's own local machine). The authentication
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protocols never reveal the keys; they can only be used to verify
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that the user's agent has a certain key. Eventually the agent
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could rely on a smart card to perform all authentication
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computations.
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o The software can be installed and used (with restricted
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functionality) even without root privileges.
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o The client is customizable in system-wide and per-user
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configuration files. Most aspects of the client's operation can
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be configured. Different options can be specified on a per-host basis.
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o Automatically executes conventional rsh (after displaying a
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warning) if the server machine is not running sshd.
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o Optional compression of all data with gzip (including forwarded X11
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and TCP/IP port data), which may result in significant speedups on
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slow connections.
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o Complete replacement for rlogin, rsh, and rcp.
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WHY TO USE SECURE SHELL
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Currently, almost all communications in computer networks are done
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without encryption. As a consequence, anyone who has access to any
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machine connected to the network can listen in on any communication.
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This is being done by hackers, curious administrators, employers,
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criminals, industrial spies, and governments. Some networks leak off
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enough electromagnetic radiation that data may be captured even from a
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distance.
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When you log in, your password goes in the network in plain
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text. Thus, any listener can then use your account to do any evil he
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likes. Many incidents have been encountered worldwide where crackers
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have started programs on workstations without the owners knowledge
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just to listen to the network and collect passwords. Programs for
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doing this are available on the Internet, or can be built by a
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competent programmer in a few hours.
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Any information that you type or is printed on your screen can be
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monitored, recorded, and analyzed. For example, an intruder who has
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penetrated a host connected to a major network can start a program
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that listens to all data flowing in the network, and whenever it
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encounters a 16-digit string, it checks if it is a valid credit card
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number (using the check digit), and saves the number plus any
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surrounding text (to catch expiration date and holder) in a file.
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When the intruder has collected a few thousand credit card numbers, he
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makes smallish mail-order purchases from a few thousand stores around
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the world, and disappears when the goods arrive but before anyone
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suspects anything.
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Businesses have trade secrets, patent applications in preparation,
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pricing information, subcontractor information, client data, personnel
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data, financial information, etc. Currently, anyone with access to
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the network (any machine on the network) can listen to anything that
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goes in the network, without any regard to normal access restrictions.
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Many companies are not aware that information can so easily be
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recovered from the network. They trust that their data is safe
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since nobody is supposed to know that there is sensitive information
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in the network, or because so much other data is transferred in the
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network. This is not a safe policy.
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Individual persons also have confidential information, such as
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diaries, love letters, health care documents, information about their
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personal interests and habits, professional data, job applications,
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tax reports, political documents, unpublished manuscripts, etc.
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One should also be aware that economical intelligence and industrial
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espionage has recently become a major priority of the intelligence
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agencies of major governments. President Clinton recently assigned
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economical espionage as the primary task of the CIA, and the French
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have repeatedly been publicly boasting about their achievements on
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this field.
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There is also another frightening aspect about the poor security of
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communications. Computer storage and analysis capability has
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increased so much that it is feasible for governments, major
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companies, and criminal organizations to automatically analyze,
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identify, classify, and file information about millions of people over
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the years. Because most of the work can be automated, the cost of
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collecting this information is getting very low.
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Government agencies may be able to monitor major communication
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systems, telephones, fax, computer networks, etc., and passively
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collect huge amounts of information about all people with any
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significant position in the society. Most of this information is not
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sensitive, and many people would say there is no harm in someone
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getting that information. However, the information starts to get
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sensitive when someone has enough of it. You may not mind someone
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knowing what you bought from the shop one random day, but you might
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not like someone knowing every small thing you have bought in the last
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ten years.
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If the government some day starts to move into a more totalitarian
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direction (one should remember that Nazi Germany was created by
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democratic elections), there is considerable danger of an ultimate
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totalitarian state. With enough information (the automatically
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collected records of an individual can be manually analyzed when the
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person becomes interesting), one can form a very detailed picture of
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the individual's interests, opinions, beliefs, habits, friends,
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lovers, weaknesses, etc. This information can be used to 1) locate
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any persons who might oppose the new system 2) use deception to
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disturb any organizations which might rise against the government 3)
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eliminate difficult individuals without anyone understanding what
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happened. Additionally, if the government can monitor communications
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too effectively, it becomes too easy to locate and eliminate any
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persons distributing information contrary to the official truth.
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Fighting crime and terrorism are often used as grounds for domestic
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surveillance and restricting encryption. These are good goals, but
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there is considerable danger that the surveillance data starts to get
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used for questionable purposes. I find that it is better to tolerate
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a small amount of crime in the society than to let the society become
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fully controlled. I am in favor of a fairly strong state, but the
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state must never get so strong that people become unable to spread
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contra-offical information and unable to overturn the government if it
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is bad. The danger is that when you notice that the government is
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too powerful, it is too late. Also, the real power may not be where
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the official government is.
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For these reasons (privacy, protecting trade secrets, and making it
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more difficult to create a totalitarian state), I think that strong
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cryptography should be integrated to the tools we use every day.
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Using it causes no harm (except for those who wish to monitor
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everything), but not using it can cause huge problems. If the society
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changes in undesirable ways, then it will be to late to start
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encrypting.
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Encryption has had a "military" or "classified" flavor to it. There
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are no longer any grounds for this. The military can and will use its
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own encryption; that is no excuse to prevent the civilians from
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protecting their privacy and secrets. Information on strong
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encryption is available in every major bookstore, scientific library,
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and patent office around the world, and strong encryption software is
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available in every country on the Internet.
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Some people would like to make it illegal to use encryption, or to
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force people to use encryption that governments can break. This
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approach offers no protection if the government turns bad. Also, the
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"bad guys" will be using true strong encryption anyway. Good
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encryption techniques are too widely known to make them disappear.
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Thus, any "key escrow encryption" or other restrictions will only help
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monitor ordinary people and petty criminals. It does not help against
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powerful criminals, terrorists, or espionage, because they will know
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how to use strong encryption anyway. (One source for internationally
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available encryption software is http://www.cs.hut.fi/crypto.)
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OVERVIEW OF SECURE SHELL
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The software consists of a number of programs.
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sshd Server program run on the server machine. This
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listens for connections from client machines, and
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whenever it receives a connection, it performs
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authentication and starts serving the client.
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ssh This is the client program used to log into another
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machine or to execute commands on the other machine.
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"slogin" is another name for this program.
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scp Securely copies files from one machine to another.
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ssh-keygen Used to create RSA keys (host keys and user
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authentication keys).
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ssh-agent Authentication agent. This can be used to hold RSA
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keys for authentication.
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ssh-add Used to register new keys with the agent.
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make-ssh-known-hosts
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Used to create the /etc/ssh_known_hosts file.
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Ssh is the program users normally use. It is started as
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ssh host
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or
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ssh host command
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The first form opens a new shell on the remote machine (after
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authentication). The latter form executes the command on the remote
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machine.
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When started, the ssh connects sshd on the server machine, verifies
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that the server machine really is the machine it wanted to connect,
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exchanges encryption keys (in a manner which prevents an outside
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listener from getting the keys), performs authentication using .rhosts
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and /etc/hosts.equiv, RSA authentication, or conventional password
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based authentication. The server then (normally) allocates a
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pseudo-terminal and starts an interactive shell or user program.
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The TERM environment variable (describing the type of the user's
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terminal) is passed from the client side to the remote side. Also,
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terminal modes will be copied from the client side to the remote side
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to preserve user preferences (e.g., the erase character).
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If the DISPLAY variable is set on the client side, the server will
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create a dummy X server and set DISPLAY accordingly. Any connections
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to the dummy X server will be forwarded through the secure channel,
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and will be made to the real X server from the client side. An
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arbitrary number of X programs can be started during the session, and
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starting them does not require anything special from the user. (Note
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that the user must not manually set DISPLAY, because then it would
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connect directly to the real display instead of going through the
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encrypted channel). This behavior can be disabled in the
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configuration file or by giving the -x option to the client.
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Arbitrary IP ports can be forwarded over the secure channel. The
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program then creates a port on one side, and whenever a connection is
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opened to this port, it will be passed over the secure channel, and a
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connection will be made from the other side to a specified host:port
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pair. Arbitrary IP forwarding must always be explicitly requested,
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and cannot be used to forward privileged ports (unless the user is
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root). It is possible to specify automatic forwards in a per-user
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configuration file, for example to make electronic cash systems work
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securely.
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If there is an authentication agent on the client side, connection to
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it will be automatically forwarded to the server side.
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For more infomation, see the manual pages ssh(1), sshd(8), scp(1),
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ssh-keygen(1), ssh-agent(1), ssh-add(1), and make-ssh-known-hosts(1)
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included in this distribution.
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X11 CONNECTION FORWARDING
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X11 forwarding serves two purposes: it is a convenience to the user
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because there is no need to set the DISPLAY variable, and it provides
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encrypted X11 connections. I cannot think of any other easy way to
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make X11 connections encrypted; modifying the X server, clients or
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libraries would require special work for each machine, vendor and
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application. Widely used IP-level encryption does not seem likely for
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several years. Thus what we have left is faking an X server on the
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same machine where the clients are run, and forwarding the connections
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to a real X server over the secure channel.
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X11 forwarding works as follows. The client extracts Xauthority
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information for the server. It then creates random authorization
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data, and sends the random data to the server. The server allocates
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an X11 display number, and stores the (fake) Xauthority data for this
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display. Whenever an X11 connection is opened, the server forwards
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the connection over the secure channel to the client, and the client
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parses the first packet of the X11 protocol, substitutes real
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authentication data for the fake data (if the fake data matched), and
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forwards the connection to the real X server.
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If the display does not have Xauthority data, the server will create a
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unix domain socket in /tmp/.X11-unix, and use the unix domain socket
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as the display. No authentication information is forwarded in this
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case. X11 connections are again forwarded over the secure channel.
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To the X server the connections appear to come from the client
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machine, and the server must have connections allowed from the local
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machine. Using authentication data is always recommended because not
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using it makes the display insecure. If XDM is used, it automatically
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generates the authentication data.
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One should be careful not to use "xin" or "xstart" or other similar
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scripts that explicitly set DISPLAY to start X sessions in a remote
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machine, because the connection will then not go over the secure
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channel. The recommended way to start a shell in a remote machine is
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xterm -e ssh host &
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and the recommended way to execute an X11 application in a remote
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machine is
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ssh -n host emacs &
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If you need to type a password/passphrase for the remote machine,
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ssh -f host emacs
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may be useful.
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RSA AUTHENTICATION
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RSA authentication is based on public key cryptograpy. The idea is
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that there are two encryption keys, one for encryption and another for
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decryption. It is not possible (on human timescale) to derive the
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decryption key from the encryption key. The encryption key is called
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the public key, because it can be given to anyone and it is not
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secret. The decryption key, on the other hand, is secret, and is
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called the private key.
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RSA authentication is based on the impossibility of deriving the
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private key from the public key. The public key is stored on the
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server machine in the user's $HOME/.ssh/authorized_keys file. The
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private key is only kept on the user's local machine, laptop, or other
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secure storage. Then the user tries to log in, the client tells the
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server the public key that the user wishes to use for authentication.
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The server then checks if this public key is admissible. If so, it
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generates a 256 bit random number, encrypts it with the public key,
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and sends the value to the client. The client then decrypts the
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number with its private key, computes a 128 bit MD5 checksum from the
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resulting data, and sends the checksum back to the server. (Only a
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checksum is sent to prevent chosen-plaintext attacks against RSA.)
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The server checks computes a checksum from the correct data,
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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.
|
||||
|
||||
|
||||
RHOSTS AUTHENTICATION
|
||||
|
||||
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
|
||||
recommended.
|
||||
|
||||
|
||||
WEAKEST LINKS IN SECURITY
|
||||
|
||||
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.
|
||||
|
||||
|
||||
LEGAL ISSUES
|
||||
|
||||
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
|
||||
offence.
|
||||
|
||||
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 NO WARRANTY FOR THIS PROGRAM. Please consult the file
|
||||
COPYING for more information.
|
||||
|
||||
|
||||
MAILING LISTS AND OTHER INFORMATION
|
||||
|
||||
There is a mailing list for ossh. It is ossh@sics.se. If you would
|
||||
like to join, send a message to majordomo@sics.se with "subscribe
|
||||
ssh" in body.
|
||||
|
||||
The WWW home page for ssh is http://www.cs.hut.fi/ssh. 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 ossh-bugs@sics.se.
|
||||
|
||||
|
||||
ABOUT THE AUTHOR
|
||||
|
||||
This software was written by Tatu Ylonen <ylo@cs.hut.fi>. I work as a
|
||||
researcher at Helsinki University of Technology, Finland. For more
|
||||
information, see http://www.cs.hut.fi/~ylo/. My PGP public key is
|
||||
available via finger from ylo@cs.hut.fi 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
|
||||
|
||||
|
||||
ACKNOWLEDGEMENTS
|
||||
|
||||
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.
|
||||
|
|
|
|||
|
|
@ -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 http://www.cs.hut.fi/ssh.
|
||||
|
||||
This file has been updated to match ssh-1.2.12.
|
||||
|
||||
|
||||
FEATURES
|
||||
|
||||
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.
|
||||
|
||||
|
||||
WHY TO USE SECURE SHELL
|
||||
|
||||
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.
|
||||
|
||||
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
|
||||
encrypting.
|
||||
|
||||
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 http://www.cs.hut.fi/crypto.)
|
||||
|
||||
|
||||
OVERVIEW OF SECURE SHELL
|
||||
|
||||
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
|
||||
|
||||
or
|
||||
|
||||
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 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 CONNECTION FORWARDING
|
||||
|
||||
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
|
||||
|
||||
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.
|
||||
|
||||
|
||||
RHOSTS AUTHENTICATION
|
||||
|
||||
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
|
||||
recommended.
|
||||
|
||||
|
||||
WEAKEST LINKS IN SECURITY
|
||||
|
||||
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.
|
||||
|
||||
|
||||
LEGAL ISSUES
|
||||
|
||||
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
|
||||
offence.
|
||||
|
||||
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 NO WARRANTY FOR THIS PROGRAM. Please consult the file
|
||||
COPYING for more information.
|
||||
|
||||
|
||||
MAILING LISTS AND OTHER INFORMATION
|
||||
|
||||
There is a mailing list for ossh. It is ossh@sics.se. If you would
|
||||
like to join, send a message to majordomo@sics.se with "subscribe
|
||||
ssh" in body.
|
||||
|
||||
The WWW home page for ssh is http://www.cs.hut.fi/ssh. 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 ossh-bugs@sics.se.
|
||||
|
||||
|
||||
ABOUT THE AUTHOR
|
||||
|
||||
This software was written by Tatu Ylonen <ylo@cs.hut.fi>. I work as a
|
||||
researcher at Helsinki University of Technology, Finland. For more
|
||||
information, see http://www.cs.hut.fi/~ylo/. My PGP public key is
|
||||
available via finger from ylo@cs.hut.fi 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
|
||||
|
||||
|
||||
ACKNOWLEDGEMENTS
|
||||
|
||||
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.
|
||||
Loading…
Reference in New Issue