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 unsecure channels. It is intended as a replacement for telnet, rlogin, rsh, and rcp. For SSH2, there is a replacement for FTP: sftp.

Additionally, Secure Shell provides secure X connections and secure forwarding of arbitrary TCP connections. You can also use Secure Shell 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.

There are two versions of Secure Shell available: SSH1 and SSH2. This FAQ does its best to distinguish when the situation calls for the difference between the two.

What encryption algorithms does Secure Shell use?

Secure Shell uses the following ciphers for encryption: Cipher

DES
3DES
IDEA
Blowfish
Twofish
Arcfour
Cast128-cbc