1. About Secure Shell (SSH)

This section should answer general questions about SSH and what it does and doesn't do. Click here for the contents of this section.

1.1. What is Secure Shell?

To quote the README file:

Secure Shell (SSH) 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 unsecure channels. It is intended as a replacement for rlogin, rsh, and rcp.

Additionally, SSH provides secure X connections and secure forwarding of arbitrary TCP connections. You can also use SSH as a tool for things like rsync and secure network backups.

The traditional BSD 'r' - commmands (rsh, rlogin, rcp) are vulnerable to different kinds of attacks. Somebody who has root access to machines on the network, or physical access to the wire, can gain unauthorized access to systems in a variety of ways. It is also possible for such a person to log all the traffic to and from your system, including passwords (which ssh never sends in the clear).

The X Window System also has a number of severe vulnerabilities. With ssh, you can create secure remote X sessions which are transparent to the user. As a side effect, using remote X clients with ssh is more convenient for users.

1.2 How widespread is its use?

The most current figures available are over 2 million Secure Shell users in over 60 countries. This is not an accurate amount, but an estimate. It also does not necessarily include the different implementations of Secure Shell for different operating systems.

1.3 What protocols does SSH use?

For the SSH1 protocol, you can find this information in an old IETF draft available here. It is also available with the source distribution.

For the SSH2 protocol, you can find this information in the SSH2 IETF drafts:

1.4 What encryption algorithms does SSH use?

SSH uses the following ciphers for encryption:

SSH uses the following ciphers for authentication:
RSAyesyes *

* Note: This is only for the commercial release (Datafellows) of SSH2.

1.5 How does SSH authenticate?

SSH authenticates using one or more of the following:

1.6 What does SSH protect against?

Ssh protects against (again, from the README):

In other words, ssh never trusts the net; somebody hostile who has taken over the network can only force ssh to disconnect, but cannot decrypt or play back the traffic, or hijack the connection.

The above only holds if you actually use encryption. Ssh does have an option to use encryption of type "none" this is only for debugging purposes, and should not be used.

1.7 What doesn't SSH protect against?

Ssh will not help you with anything that compromises your host's security in some other way. Once an attacker has gained root access to a machine, he can then subvert ssh, too.

If somebody malevolent has access to your home directory, then security is nonexistent. This is very much the case if your home directory is exported via NFS.

1.8 What is the difference between SSH1 and SSH2?

The difference between SSH1 and SSH2 is they are two entirely different protocols. SSH1 and SSH2 encrypt at different parts of the packets, and SSH1 uses server and host keys to authenticate systems where SSH2 only uses host keys. SSH2 is a complete rewrite of the protocol, and it does not use the same networking implementation that SSH1 does. Also, SSH2 is more secure.

Because of the different protocol implementation, they are not compatible.

1.9 Who maintains SSH?

SSH Communications Security, is the developer of secure shell (SSH) protocol and maintains the releases of SSH1 and SSH2. SSH Communications Security, F-Secure, and Van Dyke all sell their own releases of the application.

For more information on support, see section 8 of this FAQ.

1.10 Can I run SSH legally?

Most likely. It depends on your country's laws for cryptography and which version of SSH that you're using. Check out the information on licensing, cryptography laws, and patents on cryptographic algorithms below.

1.10.1 Licensing

Please direct any licensing concerns to SSH Communications Security.

The following lists the licensing information from the official distribution site at ftp.cs.hut.fi/pub/ssh.

SSH1: The UNIX version of ssh 1.2.27 may be used freely for non-commercial purposes and may not be sold commercially as a separate product, as part of a bigger product or project, or otherwise used for financial gain without a separate license The definition of "commercial use" is generally interpreted as using ssh for anything that would generate financial gain, such as logging into a customers system to do administration, or providing ssh as a secure login to your partners or vendors. See the COPYING file for more information.

1.10.2 Cryptography laws

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

If you are in the United States, you should be aware that, while ssh was written outside the United States using information publicly available everywhere, the US Government may consider it a criminal offence to export this software from the US once it has been imported, including putting it on a ftp site. Contact the Office of Defence Trade Controls if you need more information.

There's a really good link that keeps up to date with the Wassenaar Agreement and the cryptography laws throughout the world. Check out Bert-Jaap Koops Crypto Law Survey.

1.10.3 Patents on Cryptographic algorithms

The algorithms RSA and IDEA, which are used by ssh, are claimed as patented in different countries, including the US. Linking against the RSAREF library, which is possible, may or may not make it legal to use ssh for non-commercial purposes in the US. You may need to obtain licenses for commercial use of IDEA; ssh can be configured without IDEA and works perfectly fine without it.

For information on software patents in general, see the League for Programming Freedom's homepage at http://lpf.ai.mit.edu/.

1.11 What operating systems does SSH run on?

From the SSH home page:

SSH2 currently runs on the following platforms:

There may be other implementations that have developed for SSH2; however, I do not have that information. If you do, please let me know.

The non-commercial Unix version of SSH1 works on almost all unix variants, including at least the following:

There are also non-commercial ports of SSH for SSH1 including PalmOS, Windows, Macintosh, OS/2, BeOS, WindowsCE, Java, and OpenVMS. See section 2 of this FAQ for information on how to SSH.

1.12 . Shouldn't I be using only SSH2?

Because of the licensing restrictions on SSH2, many companies like ISPs are not running SSH2. Instead, they are still running SSH1. You should check which version of SSH you are connecting to before installing a client.

If you are installing a daemon, check to make sure your remote clients are connecting to you with the right version of SSH.

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