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Secure ShellFrom CryptoDox, The Online Encyclopedia on Cryptography and Information SecurityTemplate:IPstack Template:Redirect Secure Shell or SSH is a network protocol that allows data to be exchanged over a secure channel between two computers. Encryption provides confidentiality and integrity of data. SSH uses public-key cryptography to authenticate the remote computer and allow the remote computer to authenticate the user, if necessary. SSH is typically used to log into a remote machine and execute commands, but it also supports tunneling, forwarding arbitrary TCP ports and X11 connections; it can transfer files using the associated SFTP or SCP protocols. An SSH server, by default, listens on the standard TCP port 22. An ssh client program is typically used for establishing connections to an sshd daemon accepting remote connections. Both are commonly present on most modern operating systems, including Mac OS X, Linux, Solaris and OpenVMS. Proprietary, freeware and open source versions of various levels of complexity and completeness exist.
HistoryIn 1995, Tatu Ylönen, a researcher at Helsinki University of Technology, Finland, designed the first version of the protocol (now called SSH-1) prompted by a password-sniffing attack at his university network. The goal of SSH was to replace the earlier rlogin, TELNET and rsh protocols, which did not provide strong authentication or guarantee confidentiality. Ylönen released his implementation as freeware in July 1995, and the tool quickly gained in popularity. Towards the end of 1995, the SSH user base had grown to 20,000 users in fifty countries. In December 1995, Ylönen founded SSH Communications Security to market and develop SSH. The original version of the SSH software used various pieces of free software, such as GNU libgmp, but later versions released by SSH Secure Communications evolved into increasingly proprietary software. In 1996, a revised version of the protocol, SSH-2, was designed, incompatible with SSH-1. SSH-2 features both security and feature improvements over SSH-1. Better security, for example, comes through Diffie-Hellman key exchange and strong integrity checking via message authentication codes. New features of SSH-2 include the ability to run any number of shell sessions over a single SSH connection.[1] In 1999, developers wanting a free software version to be available went back to the older 1.2.12 release of the original ssh program, which was the last released under an open source license. Björn Grönvall's OSSH was subsequently developed from this codebase. Shortly thereafter, OpenBSD developers forked Björn's code and did extensive work on it, creating OpenSSH, which shipped with the 2.6 release of OpenBSD. From this version, a "portability" branch was formed to port OpenSSH to other operating systems. It is estimated that, at the end of 2000, there were 2,000,000 users of SSH.[2] As of 2005, OpenSSH is the single most popular ssh implementation, coming by default in a large number of operating systems. OSSH meanwhile has become obsolete.[3] In 2006, the aforementioned SSH-2 protocol became a proposed Internet standard with the publication by the IETF "secsh" working group of RFCs (see references). SSH architecture
 The SSH-2 protocol has a clean internal architecture (defined in RFC 4251) with well-separated layers. These are:
This open architecture provides considerable flexibility, allowing SSH to be used for a variety of purposes beyond secure shell. The functionality of the transport layer alone is comparable to TLS; the user authentication layer is highly extensible with custom authentication methods; and the connection layer provides the ability to multiplex many secondary sessions into a single SSH connection, a feature comparable to BEEP and not available in TLS. Security cautionsSince SSH-1 has inherent design flaws which make it vulnerable to, e.g., man-in-the-middle attacks, it is now generally considered obsolete and should be avoided by explicitly disabling fallback to SSH-1. While most modern servers and clients support SSH-2, some organizations still use software with no support for SSH-2, and thus SSH-1 cannot always be avoided. In all versions of SSH, it is important to verify unknown public keys before accepting them as valid. Accepting an attacker's public key as a valid public key has the effect of disclosing the transmitted password and allowing man in the middle attacks. As with any encrypted protocol, SSH can be considered a security risk by companies or governments who do not trust their users and wish to eavesdrop on their communications. Furthermore SSH has built in tunneling features which make it easier for users to achieve passage of large volumes of information or to establish an entry point for unauthorized inward access over a SSH link than with other protocols. How SSH uses public-key cryptography (with analogy)Main article: Public-key cryptography
First, a pair of cryptographic keys is generated. One is the private key, the other is the public key. As an analogy, they can be thought of as a matching private-key and a public padlock. The public padlock is what is installed on the remote machine and is used by ssh to authenticate users which use the matching private key. As a user of the system, you don’t care who can see or copy the padlock (ie the public key), since only the secret private key fits it. The private key is the part you keep secret inside a secure box that can only be opened with the correct passphrase. When the user wants to access a remote system, he opens the secure box with his passphrase, and uses the private-key to authenticate him with the padlock on the remote computer. Neither the passphrase nor the private key leave the user's machine. However, the user still needs to trust the local machine not to scrape his passphrase or copy his private-key while it's out of the secure box. See alsoTemplate:Wikibooks Template:Portal
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