Secure shell or SSH is both a program and a network protocol for logging into and executing commands on a remote computer. It is intended to replace rlogin, telnet and rsh, and provides secure encrypted communications between two untrusted hosts over an insecure network. X11 connections and arbitrary TCP/IP ports can also be forwarded over the secure channel.
The program is a common Unix shell program, but there exists implementations for most modern platforms, including Microsoft Windows (where one of the most popular is PuTTY) and Mac OS. There are commercial versions, freeware versions, and open source versions.
A later version of the protocol was released under the name SSH2.
An IETF working group, secsh, is currently in the process of standardizing the protocol.
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