View source for Secure Shell

{{short description|Cryptographic network protocol}}
{{redirect|SSH}}

{{Infobox networking protocol
| is stack = yes
| purpose = secure connection, remote access
| developer = [[Tatu Ylönen]], [[Internet Engineering Task Force]] (IETF)
| osilayer = [[Transport layer]] through [[application layer]]
| ports = 22
| rfcs = {{IETF RFC|4250|4251|4252|4253|4254|plainlink=yes}}
| date = {{Start date and age|1995}}
}}
{{IPstack}}

The '''Secure Shell Protocol''' ('''SSH Protocol''') is a [[cryptographic]] [[network protocol]] for operating [[network service]]s securely over an [[unsecured network]].<ref name="rfc4251"/> Its most notable applications are [[remote login]] and [[command-line]] execution.

SSH was designed for [[Unix-like]] operating systems as a replacement for [[Telnet]] and [[Computer security|unsecured]] remote [[Unix shell]] protocols, such as the Berkeley [[Remote Shell]] (rsh) and the related [[rlogin]] and [[rexec]] protocols, which all use insecure, [[plaintext]] methods of authentication, such as [[password]]s.

Since mechanisms like [[Telnet]] and [[Remote Shell]] are designed to access and operate remote computers, sending the [[authentication]] tokens (e.g. [[username]] and [[password]]) for this access to these computers across a [[Computer network|public network]] in an unsecured way poses a great risk of third parties obtaining the password and achieving the same level of access to the remote system as the telnet user. Secure Shell mitigates this risk through the use of [[encryption]] mechanisms that are intended to hide the contents of the transmission from an observer, even if the observer has access to the entire [[data stream]].<ref>{{cite web |url=https://it.mst.edu/policies/secure-telnet/ |title=Missouri University S&T: Secure Telnet}}</ref>

Finnish computer scientist [[Tatu Ylönen]] designed SSH in 1995 and provided an implementation in the form of two commands, {{mono|ssh}} and {{mono|slogin}}, as secure replacements for {{mono|rsh}} and {{mono|rlogin}}, respectively. Subsequent development of the protocol suite proceeded in several developer groups, producing several variants of implementation. The protocol specification distinguishes two major versions, referred to as SSH-1 and SSH-2. The most commonly implemented software stack is [[OpenSSH]], released in 1999 as open-source software by the [[OpenBSD]] developers. Implementations are distributed for all types of operating systems in common use, including [[embedded system]]s.

SSH applications are based on a [[client–server architecture]], connecting an [[SSH client]] instance with an [[SSH server]].<ref name="rfc4252">{{cite ietf|rfc=4252|author1=T. Ylonen|author2=C. Lonvick|date=January 2006|title=The Secure Shell (SSH) Authentication Protocol|publisher=IETF Trust}}</ref> SSH operates as a layered protocol suite comprising three principal hierarchical components: the ''transport layer'' provides server authentication, confidentiality, and integrity; the ''user authentication protocol'' validates the user to the server; and the ''connection protocol'' [[multiplexes]] the encrypted tunnel into multiple logical communication channels.<ref name="rfc4251">{{cite ietf|rfc=4251|author1=T. Ylonen|author2=C. Lonvick|title=The Secure Shell (SSH) Protocol Architecture|date=January 2006|publisher=IETF Trust}}</ref>

== Definition ==
SSH uses [[public-key cryptography]] to [[authenticate]] the remote computer and allow it to authenticate the user, if necessary.<ref name="rfc4252" />

SSH may be used in several methodologies. In the simplest manner, both ends of a communication channel use automatically generated public-private key pairs to encrypt a network connection, and then use a [[password]] to authenticate the user.

When the public-private key pair is generated by the user manually, the authentication is essentially performed when the key pair is created, and a session may then be opened automatically without a password prompt. In this scenario, the public key is placed on all computers that must allow access to the owner of the matching private key, which the owner keeps private. While authentication is based on the private key, the key is never transferred through the network during authentication. SSH only verifies that the same person offering the public key also owns the matching private key.

In all versions of SSH, it is important to verify unknown [[public key]]s, i.e., [[Public-key cryptography#Associating public keys with identities|associate the public keys with identities]], before accepting them as valid. Accepting an attacker's public key without validation will authorize an unauthorized attacker as a valid user.

==Authentication: OpenSSH key management==
On [[Unix-like]] systems, the list of authorized public keys is typically stored in the home directory of the user that is allowed to log in remotely, in the file <code>~/.ssh/authorized_keys</code>.<ref>{{cite web|url=http://wiki.qnap.com/wiki/How_To_Set_Up_Authorized_Keys|title=How To Set Up Authorized Keys|url-status=live|archive-url=https://web.archive.org/web/20110510111514/http://wiki.qnap.com/wiki/How_To_Set_Up_Authorized_Keys|archive-date=2011-05-10}}</ref> This file is respected by SSH only if it is not writable by anything apart from the owner and root. When the public key is present on the remote end, and the matching private key is present on the local end, typing in the password is no longer required. However, for additional security, the private key itself can be locked with a passphrase.

The private key can also be looked for in standard places, and its full path can be specified as a command-line setting (the option <code>-i</code> for ssh). The [[ssh-keygen]] utility produces the public and private keys, always in pairs.

==Use==
{{Original research section|date=June 2024|reason=Most information is original search based on primary sources, there are few secondary sources}}
SSH is typically used to log into a remote computer's [[Shell (computing)|shell]] or [[command-line interface]] (CLI) and to execute commands on a remote server. It also supports mechanisms for [[tunneling protocol|tunneling]], [[Port forwarding|forwarding]] of [[TCP and UDP port|TCP ports]] and [[X11]] connections and it can be used to transfer files using the associated [[SSH File Transfer Protocol]] (SFTP) or [[Secure Copy Protocol]] (SCP).<ref name="rfc4252" />

SSH uses the [[client–server model]]. An SSH [[client (computing)|client]] program is typically used for establishing connections to an SSH [[daemon (computing)|daemon]], such as sshd, accepting remote connections. Both are commonly present on most modern [[operating systems]], including [[macOS]], most distributions of [[Linux]], [[OpenBSD]], [[FreeBSD]], [[NetBSD]], [[Solaris (operating system)|Solaris]] and [[OpenVMS]]. Notably, versions of [[Windows]] prior to Windows 10 version 1709 do not include SSH by default, but [[proprietary software|proprietary]], [[freeware]] and [[Open-source software|open source]] versions of various levels of complexity and completeness did and do exist (see [[Comparison of SSH clients]]). In 2018 [[Microsoft]] began porting the [[OpenSSH]] source code to Windows<ref>[https://github.com/PowerShell/Win32-OpenSSH Win-32 OpenSSH]</ref> and in [[Windows 10 version 1709]], an official Win32 port of OpenSSH is now available.

File managers for UNIX-like systems (e.g., [[Konqueror]]) can use the [[Files transferred over shell protocol|FISH]] protocol to provide a split-pane GUI with drag-and-drop. The open source Windows program [[WinSCP]]<ref>{{cite web|url=http://winscp.net/eng/index.php|title=WinSCP home page|url-status=live|archive-url=https://web.archive.org/web/20140217163252/http://winscp.net/eng/index.php|archive-date=2014-02-17}}</ref> provides similar file management (synchronization, copy, remote delete) capability using [[PuTTY]] as a back-end. Both WinSCP<ref>{{cite web|url=http://portableapps.com/apps/internet/winscp_portable|title=WinSCP page for PortableApps.com|url-status=live|archive-url=https://web.archive.org/web/20140216120049/http://portableapps.com/apps/internet/winscp_portable|archive-date=2014-02-16}}</ref> and PuTTY<ref>{{cite web|url=http://portableapps.com/apps/internet/putty_portable|title=PuTTY page for PortableApps.com|url-status=live|archive-url=https://web.archive.org/web/20140216214310/http://portableapps.com/apps/internet/putty_portable|archive-date=2014-02-16}}</ref> are available packaged to run directly off a USB drive, without requiring installation on the client machine. Crostini on [[ChromeOS]] comes with OpenSSH by default. Setting up an SSH server in Windows typically involves enabling a feature in the Settings app.

SSH is important in [[cloud computing]] to solve connectivity problems, avoiding the security issues of exposing a cloud-based virtual machine directly on the Internet. An SSH tunnel can provide a secure path over the Internet, through a firewall to a virtual machine.<ref>{{cite web|last1=Amies|first1=A|last2=Wu|first2=C F|last3=Wang|first3=G C|last4=Criveti|first4=M|year=2012|url=http://www.ibm.com/developerworks/cloud/library/cl-networkingtools/index.html|title=Networking on the cloud|website=IBM developerWorks|url-status=live|archive-url=https://web.archive.org/web/20130614123106/http://www.ibm.com/developerworks/cloud/library/cl-networkingtools/index.html|archive-date=2013-06-14}}</ref>

The [[IANA]] has assigned [[Transmission Control Protocol|TCP]] [[Port (computer networking)|port]] 22, [[User Datagram Protocol|UDP]] port 22 and [[SCTP]] port 22 for this protocol.<ref>{{cite web|title=Service Name and Transport Protocol Port Number Registry|url=https://www.iana.org/assignments/service-names-port-numbers/service-names-port-numbers.xhtml}}</ref> IANA had listed the standard TCP port 22 for SSH servers as one of the [[well-known ports]] as early as 2001.<ref>{{cite web|url=https://www.iana.org/assignments/port-numbers|title=Service Name and Transport Protocol Port Number Registry|website=iana.org|url-status=live|archive-url=https://web.archive.org/web/20010604223215/https://www.iana.org/assignments/port-numbers|archive-date=2001-06-04}}</ref> SSH can also be run using [[SCTP]] rather than TCP as the connection-oriented transport layer protocol.<ref>{{cite conference|last=Seggelmann|first=R.|author2=Tuxen, M. |author3=Rathgeb, E.P. |title=SSH over SCTP — Optimizing a multi-channel protocol by adapting it to SCTP|conference=8th International Symposium on Communication Systems, Networks & Digital Signal Processing (CSNDSP)|date=18–20 July 2012|pages=1–6|doi=10.1109/CSNDSP.2012.6292659|isbn=978-1-4577-1473-3|s2cid=8415240}}</ref>

== Historical development ==
=== Version 1 ===
In 1995, '''Tatu Ylönen''', a researcher at [[Helsinki University of Technology]] in Finland designed the first version of the protocol (now called '''SSH-1''') prompted by a password-[[Packet analyzer|sniffing]] attack at his [[university network]].<ref>{{cite web|author=Tatu Ylönen|url=https://www.scmagazineuk.com/the-new-skeleton-key-changing-the-locks-in-your-network-environment/article/545848/|title=The new skeleton key: changing the locks in your network environment|url-status=dead|archive-url=https://web.archive.org/web/20170820162632/https://www.scmagazineuk.com/the-new-skeleton-key-changing-the-locks-in-your-network-environment/article/545848/|archive-date=2017-08-20}}</ref> The goal of SSH was to replace the earlier [[rlogin]], [[telnet]], [[FTP]]<ref>{{cite web|url=https://www.ssh.com/ssh/port|title=SSH Port|author=Tatu Ylönen|url-status=live|archive-url=https://web.archive.org/web/20170803235736/https://www.ssh.com/ssh/port|archive-date=2017-08-03}}</ref> and [[Remote Shell|rsh]] protocols, which did not provide strong authentication nor guarantee confidentiality. He chose the port number 22 because it is between <code>telnet</code> (port 23) and <code>ftp</code> (port 21).<ref>{{Cite web |last=Ylönen |first=Tatu |title=The story of the SSH port is 22. |url=https://www.ssh.com/academy/ssh/port |access-date=2023-11-30 |website=www.ssh.com |language=en}}</ref>

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.<ref>{{Cite book |last1=Barrett |first1=Daniel J. |title=SSH, the secure shell: the definitive guide |last2=Silverman |first2=Richard E. |date=2001 |publisher=O'Reilly |isbn=978-0-596-00011-0 |edition=1st |location=Cambridge [Mass.] |page=11}}</ref>

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 Multi-Precision Library|GNU libgmp]], but later versions released by SSH Communications Security evolved into increasingly [[proprietary software]].

It was estimated that by 2000, the number of users had grown to 2 million.<ref name="Nicholas Rosasco and David Larochelle">{{cite web
 |author = Nicholas Rosasco and David Larochelle
 |title = How and Why More Secure Technologies Succeed in Legacy Markets: Lessons from the Success of SSH
 |publisher = Dept. of Computer Science, Univ. of Virginia
 |work = Quoting [[Daniel J. Barrett|Barrett]] and Silverman, SSH, the Secure Shell: The Definitive Guide, O'Reilly & Associates (2001)
 |url = http://www.cs.virginia.edu/~drl7x/sshVsTelnetWeb3.pdf
 |access-date = 2006-05-19
 |url-status = live
 |archive-url = https://web.archive.org/web/20060625065258/http://www.cs.virginia.edu/~drl7x/sshVsTelnetWeb3.pdf
 |archive-date = 2006-06-25
}}</ref>

===Version 2===
In 2006, after being discussed in a working group named "secsh",<ref>[https://datatracker.ietf.org/wg/secsh/documents/ IETF (Internet Engineering Task Force): datatracker for secsh ]</ref> a revised version of the SSH protocol, '''SSH-2''' was adopted as a standard.<ref name="rfc-ssh">[https://datatracker.ietf.org/doc/html/rfc4252 RFC4252: The Secure Shell (SSH) Authentication Protocol, Jan 2006]</ref> This version offers improved security and new features, but is not compatible with SSH-1. For example, it introduces new key-exchange mechanisms like [[Diffie–Hellman key exchange]], improved [[data integrity]] checking via [[message authentication code]]s like [[MD5]] or [[SHA-1]], which can be negotiated between client and server. SSH-2 also adds stronger encryption methods like [[Advanced_Encryption_Standard|AES]] which eventually replaced weaker and compromised ciphers from the previous standard like [[3DES]].<ref>[https://docstore.mik.ua/orelly/networking_2ndEd/ssh/ch03_05.htm O'Reily: Secure Shell, The Definitive Guide]</ref><ref>[https://datatracker.ietf.org/doc/html/rfc4250#page-16 RFC4250: The Secure Shell (SSH) Protocol: Assigned names, Jan 2006, page 16]</ref><ref name="rfc-ssh">[https://datatracker.ietf.org/doc/html/rfc4252 RFC4252: The Secure Shell (SSH) Authentication Protocol, Jan 2006]</ref> New features of SSH-2 include the ability to run any number of [[Unix shell|shell]] sessions over a single SSH connection.<ref>{{cite web|url=http://www.snailbook.com/faq/ssh-1-vs-2.auto.html|title=SSH Frequently Asked Questions|url-status=live|archive-url=https://web.archive.org/web/20041010035705/http://www.snailbook.com/faq/ssh-1-vs-2.auto.html|archive-date=2004-10-10 }}</ref> Due to SSH-2's superiority and popularity over SSH-1, some implementations such as libssh (v0.8.0+),<ref>{{cite web|url=https://www.libssh.org/2018/08/10/libssh-0-8-0/|title=libssh}}</ref> [[Lsh]]<ref>{{cite web|url=http://www.lysator.liu.se/~nisse/lsh/|title=A GNU implementation of the Secure Shell protocols|url-status=live|archive-url=https://web.archive.org/web/20120204035753/http://www.lysator.liu.se/~nisse/lsh/|archive-date=2012-02-04}}</ref> and [[Dropbear (software)|Dropbear]]<ref>{{cite web|url=https://matt.ucc.asn.au/dropbear/dropbear.html|title=Dropbear SSH|url-status=live|archive-url=https://web.archive.org/web/20111014091250/http://matt.ucc.asn.au/dropbear/dropbear.html|archive-date=2011-10-14}}</ref> eventually supported only the SSH-2 protocol.

===Version 1.99===
In January 2006, well after version 2.1 was established, {{IETF RFC|4253}} specified that an SSH server supporting 2.0 as well as prior versions should identify its protocol version as 1.99.<ref>{{cite ietf|rfc=4253|title=The Secure Shell (SSH) Transport Layer Protocol|section=5.1|sectionname=Old Client, New Server|first1=T.|last1=Ylonen|first2=C.|last2=Lonvick|publisher=IETF}}</ref> This version number does not reflect a historical software revision, but a method to identify [[backward compatibility]].

==={{Anchor|OSSH}}OSSH and OpenSSH===
In 1999, developers, desiring availability of a free software version, restarted software development from the 1.2.12 release of the original SSH program, which was the last released under an [[open source license]].<ref>[https://marc.info/?l=secure-shell&m=88561413417101 ssh-1.2.13 now available: copying policy changed (permission now required to sell ssh commercially, use is still permitted for any purpose)]</ref> This served as a code base for Björn Grönvall's OSSH software.<ref>[http://www.mirrorservice.org/sites/ftp.wiretapped.net/pub/security/cryptography/apps/ssh/OSSH/ OSSH sources]</ref> Shortly thereafter, [[OpenBSD]] developers [[fork (software development)|fork]]ed Grönvall's code and created [[OpenSSH]], which shipped with Release 2.6 of OpenBSD. From this version, a "portability" branch was formed to port OpenSSH to other operating systems.<ref>{{cite web |url=http://www.openssh.com/history.html |title=OpenSSH: Project History and Credits |date=2004-12-22 |access-date=2014-04-27 |publisher=openssh.com |url-status=live |archive-url=https://web.archive.org/web/20131224105341/http://openssh.com/history.html |archive-date=2013-12-24 }}</ref>

{{As of|2005}}, [[OpenSSH]] was the single most popular SSH implementation, being the default version in a large number of operating system distributions. OSSH, meanwhile, has become obsolete.<ref>{{cite web|date=2006-02-15|title=OSSH Information for VU#419241|url=https://www.kb.cert.org/vuls/id/MIMG-6L4LBL|url-status=live|archive-url=https://web.archive.org/web/20070927231942/https://www.kb.cert.org/vuls/id/MIMG-6L4LBL|archive-date=2007-09-27|website=CERT Coordination Center|quote=Either way ossh is old and obsolete and I don't recommend its use.}}</ref> OpenSSH continues to be maintained and supports the SSH-2 protocol, having expunged SSH-1 support from the codebase in the OpenSSH 7.6 release.

=== Future ===

In 2023, an alternative to traditional SSH was proposed under the name '''SSH3'''<ref>{{cite web|date=2024-08-01|website=datatracker.ietf.org|title=Remote terminal over HTTP/3 connections|url=https://datatracker.ietf.org/doc/draft-michel-remote-terminal-http3}}</ref><ref>{{cite web|date=2024-02-28|website=www.ietf.org|title=Secure shell over HTTP/3 connections|url=https://www.ietf.org/archive/id/draft-michel-ssh3-00.html}}</ref><ref>{{cite arXiv|date=2023-12-12|title=Towards SSH3: how HTTP/3 improves secure shells|eprint=2312.08396 |last1=Michel |first1=François |last2=Bonaventure |first2=Olivier |class=cs.NI }}</ref> by PhD student François Michel and Professor Olivier Bonaventure and its code has been made open source.<ref>{{cite web|date=2024-07-12|website=github.com|title=ssh3|url=https://github.com/francoismichel/ssh3}}</ref> This new version implements the original SSH Connection Protocol but operates on top of [[HTTP/3]], which runs on [[QUIC]]. It offers multiple features such as:

* Faster session establishment, reducing the number of [[Round-trip delay]]s from 5-7 to 3.
* High security: while SSHv2 relies on its own protocols, SSH3 leverages [[TLS 1.3]], [[QUIC]], and [[HTTP]].
* UDP port forwarding
* X.509 certificates
* OpenID Connect

However, the name SSH3 is under discussion, and the project aims to rename itself to a more suitable name.<ref>{{cite web|date=2024-02-28|website=datatracker.ietf.org|title=Secure shell over HTTP/3 connections|url=https://datatracker.ietf.org/doc/draft-michel-ssh3/}}</ref> The discussion stems from the fact that this new implementation significantly revises the SSH protocol, suggesting it should not be called SSH3.

==Uses==
[[File:X11 ssh tunnelling.png|right|250px|thumb|Example of tunneling an [[X11]] application over SSH: the user 'josh' has "SSHed" from the local machine 'foofighter' to the remote machine 'tengwar' to run [[xeyes]].]]
[[File:OpenWrtPuTTY.png|right|250px|thumb|Logging into [[OpenWrt]] via SSH using [[PuTTY]] running on [[Windows]].]]
SSH is a protocol that can be used for many applications across many platforms including most [[Unix]] variants ([[Linux]], the [[BSD]]s including [[Apple Inc|Apple]]'s [[macOS]], and [[Solaris (operating system)|Solaris]]), as well as [[Microsoft Windows]]. Some of the applications below may require features that are only available or compatible with specific SSH clients or servers. For example, using the SSH protocol to implement a [[VPN]] is possible, but presently only with the [[OpenSSH]] server and client implementation.
* For login to a shell on a remote host (replacing [[Telnet]] and [[rlogin]])
* For executing a single command on a remote host (replacing [[Remote shell|rsh]])
* For setting up automatic (passwordless) login to a remote server (for example, using [[OpenSSH]]<ref>{{cite book|last1=Sobell|first1=Mark|title=A Practical Guide to Linux Commands, Editors, and Shell Programming|date=2012|publisher=Prentice Hall|location=Upper Saddle River, NJ|isbn=978-0133085044|pages=702–704|edition=3rd}}</ref>)
* In combination with [[rsync]] to back up, copy and mirror files efficiently and securely
* For [[Port forwarding|forwarding]] a port
* For [[Tunneling protocol|tunneling]] (not to be confused with a [[VPN]], which [[VPN#Routing|routes]] packets between different networks, or [[VPN#OSI Layer 1 services|bridges]] two [[broadcast domain]]s into one).
* For use as a full-fledged encrypted VPN. Note that only [[OpenSSH]] server and client support this feature.
* For forwarding [[X Window System|X]] from a remote [[Host (network)|host]] (possible through multiple intermediate hosts)
* For browsing the web through an encrypted proxy connection with SSH clients that support the [[SOCKS|SOCKS protocol]].
* For securely mounting a directory on a remote server as a [[file system]] on a local computer using [[SSHFS]].
* For automated remote monitoring and management of servers through one or more of the mechanisms discussed above.
* For development on a mobile or embedded device that supports SSH.
* For securing file transfer protocols.

===File transfer protocols===
The Secure Shell protocols are used in several file transfer mechanisms.
* [[Secure copy]] (SCP), which evolved from [[rcp (Unix)|RCP]] protocol over SSH
* [[rsync]], intended to be more efficient than SCP. Generally runs over an SSH connection.
* [[SSH File Transfer Protocol]] (SFTP), a secure alternative to [[FTP]] (not to be confused with [[FTP over SSH]] or [[FTPS]])
* [[Files transferred over shell protocol]] (FISH), released in 1998, which evolved from [[Unix shell]] commands over SSH
* [[Fast and Secure Protocol]] (FASP), aka ''Aspera'', uses SSH for control and UDP ports for data transfer.

=={{Anchor|ARCHITECTURE}}Architecture==
[[File:Ssh binary packet alt.svg|right|250px|thumb|Diagram of the SSH-2 binary packet.]]

The SSH protocol has a layered architecture with three separate components:
* The ''transport layer'' ({{IETF RFC|4253}}) typically uses the [[Transmission Control Protocol]] (TCP) of [[TCP/IP]], reserving [[port number#Common port numbers|port number]] 22 as a server listening port. This layer handles initial key exchange as well as server authentication, and sets up encryption, compression, and integrity verification. It exposes to the upper layer an interface for sending and receiving plaintext packets with a size of up to 32,768 bytes each, but more can be allowed by each implementation. The transport layer also arranges for key re-exchange, usually after 1 GB of data has been transferred or after one hour has passed, whichever occurs first.
* The ''user authentication layer'' ({{IETF RFC|4252}}) handles client authentication, and provides a suite of authentication algorithms. Authentication is ''client-driven'': when one is prompted for a password, it may be the SSH client prompting, not the server. The server merely responds to the client's authentication requests. Widely used user-authentication methods include the following:
** ''password'': a method for straightforward password authentication, including a facility allowing a password to be changed. Not all programs implement this method.
** ''publickey'': a method for [[public-key cryptography|public-key-based authentication]], usually supporting at least [[Digital Signature Algorithm|DSA]], [[ECDSA]] or [[RSA (algorithm)|RSA]] keypairs, with other implementations also supporting [[X.509]] certificates.
** ''keyboard-interactive'' ({{IETF RFC|4256}}): a versatile method where the server sends one or more prompts to enter information and the client displays them and sends back responses keyed in by the user. Used to provide [[one-time password]] authentication such as [[S/Key]] or [[SecurID]]. Used by some OpenSSH configurations when [[Pluggable authentication modules|PAM]] is the underlying host-authentication provider to effectively provide password authentication, sometimes leading to an inability to log in with a client that supports just the plain ''password'' authentication method.
** [[GSSAPI]] authentication methods which provide an extensible scheme to perform SSH authentication using external mechanisms such as [[Kerberos 5]] or [[NTLM]], providing [[single sign-on]] capability to SSH sessions. These methods are usually implemented by commercial SSH implementations for use in organizations, though OpenSSH does have a working GSSAPI implementation.
* The ''connection layer'' ({{IETF RFC|4254}}) defines the concept of channels, channel requests, and global requests, which define the SSH services provided. A single SSH connection can be multiplexed into multiple logical channels simultaneously, each transferring data bidirectionally. Channel requests are used to relay out-of-band channel-specific data, such as the changed size of a terminal window or the exit code of a server-side process. Additionally, each channel performs its own flow control using the receive window size. The SSH client requests a server-side port to be forwarded using a global request. Standard channel types include:
** ''shell'' for terminal shells, SFTP and exec requests (including SCP transfers)
** ''direct-tcpip'' for client-to-server forwarded connections
** ''forwarded-tcpip'' for server-to-client forwarded connections
* The [[SSHFP]] DNS record (RFC 4255) provides the public host key fingerprints in order to aid in verifying the authenticity of the host.

This open architecture provides considerable flexibility, allowing the use of SSH for a variety of purposes beyond a secure shell. The functionality of the transport layer alone is comparable to [[Transport Layer Security]] (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.

==Algorithms==
* [[EdDSA]],<ref>{{cite ietf|rfc=8709|title=Ed25519 and Ed448 Public Key Algorithms for the Secure Shell (SSH) Protocol|first1=B.|last1=Harris|first2=L.|last2=Velvindron|date=February 2020}}</ref> [[ECDSA]], [[RSA (cryptosystem)|RSA]] and [[Digital Signature Algorithm|DSA]] for [[public-key cryptography]].<ref name=RFC5656>{{cite ietf|rfc=5656|last1=Stebila|first1=D.|last2=Green|first2=J.|title=Elliptic Curve Algorithm Integration in the Secure Shell Transport Layer|date=December 2009|access-date=12 November 2012}}</ref>
* [[ECDH]] and [[Diffie–Hellman]] for [[key exchange]].<ref name=RFC5656 />
* [[HMAC]], [[AEAD]] and [[UMAC (cryptography)|UMAC]] for [[Message authentication code|MAC]].<ref>{{cite ietf|last=Miller|first=D.|author2=Valchev, P.|title=The use of UMAC in the SSH Transport Layer Protocol|draft=draft-miller-secsh-umac-00|date=September 3, 2007}}</ref>
* [[Advanced Encryption Standard|AES]] (and deprecated [[RC4]], [[3DES]], [[Data Encryption Standard|DES]]<ref>{{cite ietf|rfc=4253|title=The Secure Shell (SSH) Transport Layer Protocol|first1=T.|last1=Ylonen|first2=C.|last2=Lonvick|publisher=IETF}}</ref>) for [[symmetric encryption]].
* [[AES-GCM]]<ref>{{cite ietf|rfc=5647|title=AES Galois Counter Mode for the Secure Shell Transport Layer Protocol|first1=K.|last1=Igoe|first2=J.|last2=Solinas|date=August 2009}}</ref> and [[ChaCha20-Poly1305]] for [[AEAD]] encryption.
* [[Secure Hash Algorithm|SHA]] (and deprecated [[MD5]]) for [[key fingerprint]].

==Vulnerabilities==
===SSH-1===
In 1998, a vulnerability was described in SSH 1.5 which allowed the unauthorized insertion of content into an encrypted SSH stream due to insufficient data integrity protection from [[CRC-32]] used in this version of the protocol.<ref>{{cite web|url=http://www.coresecurity.com/content/ssh-insertion-attack|title=SSH Insertion Attack|website=[[Core Security Technologies]]|url-status=live|archive-url=https://web.archive.org/web/20110708192336/http://www.coresecurity.com/content/ssh-insertion-attack|archive-date=2011-07-08}}</ref><ref>{{cite web|url=http://www.kb.cert.org/vuls/id/13877|title=Vulnerability Note VU#13877 - Weak CRC allows packet injection into SSH sessions encrypted with block ciphers|website=[[US CERT]]|url-status=live|archive-url=https://web.archive.org/web/20100710040357/http://www.kb.cert.org/vuls/id/13877|archive-date=2010-07-10}}</ref> A fix known as SSH Compensation Attack Detector<ref>{{cite web|url=http://www.securityfocus.com/bid/2347/discuss|title=SSH CRC-32 Compensation Attack Detector Vulnerability|website=[[SecurityFocus]]|url-status=live|archive-url=https://web.archive.org/web/20080725110345/http://www.securityfocus.com/bid/2347/discuss|archive-date=2008-07-25}}</ref> was introduced into most implementations. Many of these updated implementations contained a new [[integer overflow]] vulnerability<ref>{{cite web|url=http://www.kb.cert.org/vuls/id/945216|title=Vulnerability Note VU#945216 - SSH CRC32 attack detection code contains remote integer overflow|website=US CERT|url-status=live|archive-url=https://web.archive.org/web/20051013074750/http://www.kb.cert.org/vuls/id/945216|archive-date=2005-10-13}}</ref> that allowed attackers to execute arbitrary code with the privileges of the SSH daemon, typically root.

In January 2001 a vulnerability was discovered that allows attackers to modify the last block of an [[International Data Encryption Algorithm|IDEA]]-encrypted session.<ref>{{cite web|url=http://www.kb.cert.org/vuls/id/315308|title=Vulnerability Note VU#315308 - Weak CRC allows last block of IDEA-encrypted SSH packet to be changed without notice|website=US CERT|url-status=live|archive-url=https://web.archive.org/web/20100711103528/http://www.kb.cert.org/vuls/id/315308|archive-date=2010-07-11}}</ref> The same month, another vulnerability was discovered that allowed a malicious server to forward a client authentication to another server.<ref name="cert1">{{cite web|url=http://www.kb.cert.org/vuls/id/684820|title=Vulnerability Note VU#684820 - SSH-1 allows client authentication to be forwarded by a malicious server to another server|website=US CERT|url-status=live|archive-url=https://web.archive.org/web/20090901012536/http://www.kb.cert.org/vuls/id/684820|archive-date=2009-09-01}}</ref>

Since SSH-1 has inherent design flaws that make it vulnerable, it is now generally considered obsolete and should be avoided by explicitly disabling fallback to SSH-1. Most modern servers and clients support SSH-2.<ref name="cert1" />

===CBC plaintext recovery===
In November 2008, a theoretical vulnerability was discovered for all versions of SSH which allowed recovery of up to 32 bits of plaintext from a block of ciphertext that was encrypted using what was then the standard default encryption mode, [[Block cipher modes of operation#Cipher-block chaining (CBC)|CBC]].<ref name="SSH CBC vulnerability">{{cite web|url=http://www.kb.cert.org/vuls/id/958563|title=Vulnerability Note VU#958563 - SSH CBC vulnerability|website=US CERT|url-status=live|archive-url=https://web.archive.org/web/20110622005639/http://www.kb.cert.org/vuls/id/958563|archive-date=2011-06-22}}</ref> The most straightforward solution is to use [[Block cipher modes of operation#Counter (CTR)|CTR]], counter mode, instead of CBC mode, since this renders SSH resistant to the attack.<ref name="SSH CBC vulnerability"/>

===Suspected decryption by NSA===
On December 28, 2014 ''[[Der Spiegel]]'' published classified information<ref name=Spiegel2014>{{cite news|url=http://www.spiegel.de/international/germany/inside-the-nsa-s-war-on-internet-security-a-1010361.html|title=Prying Eyes: Inside the NSA's War on Internet Security|date=December 28, 2014|work=[[Spiegel Online]]|url-status=live|archive-url=https://web.archive.org/web/20150124202809/http://www.spiegel.de/international/germany/inside-the-nsa-s-war-on-internet-security-a-1010361.html|archive-date=January 24, 2015}}</ref> leaked by whistleblower [[Edward Snowden]] which suggests that the [[National Security Agency]] may be able to decrypt some SSH traffic. The technical details associated with such a process were not disclosed. A 2017 analysis of the [[CIA]] hacking tools ''BothanSpy'' and ''Gyrfalcon'' suggested that the SSH protocol was not compromised.<ref>{{cite web|first=Tatu|last=Ylonen|url=https://www.ssh.com/ssh/cia-bothanspy-gyrfalcon|title=BothanSpy & Gyrfalcon - Analysis of CIA hacking tools for SSH|website=ssh.com|date=3 August 2017|access-date=15 July 2018}}</ref>

=== Terrapin attack ===
{{main|Terrapin attack}}
A novel man-in-the-middle attack against most current ssh implementations was discovered in 2023. It was named the [[Terrapin attack]] by its discoverers.<ref>{{Cite web |title=Terrapin Attack |url=https://terrapin-attack.com/ |access-date=2023-12-20 |website=terrapin-attack.com}}</ref><ref>{{Cite web |last=Jones |first=Connor |title=SSH shaken, not stirred by Terrapin downgrade vulnerability |url=https://www.theregister.com/2023/12/20/terrapin_attack_ssh/ |access-date=2023-12-20 |website=www.theregister.com |language=en}}</ref> However, the risk is mitigated by the requirement to intercept a genuine ssh session, and that the attack is restricted in its scope, fortuitously resulting mostly in failed connections.<ref name="El Reg">{{Cite web |last=Jones |first=Connor |title=SSH shaken, not stirred by Terrapin downgrade vulnerability |url=https://www.theregister.com/2023/12/20/terrapin_attack_ssh/ |access-date=2023-12-20 |website=www.theregister.com |language=en}}</ref><ref name=":1">{{Cite web |date=2023-12-18 |title=OpenSSH 9.6 release notes |url=https://www.openssh.com/txt/release-9.6 |website=openssh.com}}</ref> The ssh developers have stated that the major impact of the attack is to degrade the [[Keystroke dynamics|keystroke timing]] obfuscation features of ssh.<ref name=":1" /> The vulnerability was fixed in OpenSSH 9.6, but requires both client and server to be upgraded for the fix to be fully effective.

==Standards documentation==
The following [[Request for Comments|RFC]] publications by the [[IETF]] "secsh" [[working group]] document SSH-2 as a proposed [[Internet standard]].
* {{IETF RFC|4250}} – ''The Secure Shell (SSH) Protocol Assigned Numbers''
* {{IETF RFC|4251}} – ''The Secure Shell (SSH) Protocol Architecture''
* {{IETF RFC|4252}} – ''The Secure Shell (SSH) Authentication Protocol''
* {{IETF RFC|4253}} – ''The Secure Shell (SSH) Transport Layer Protocol''
* {{IETF RFC|4254}} – ''The Secure Shell (SSH) Connection Protocol''
* {{IETF RFC|4255}} – ''Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints''
* {{IETF RFC|4256}} – ''Generic Message Exchange Authentication for the Secure Shell Protocol (SSH)''
* {{IETF RFC|4335}} – ''The Secure Shell (SSH) Session Channel Break Extension''
* {{IETF RFC|4344}} – ''The Secure Shell (SSH) Transport Layer Encryption Modes''
* {{IETF RFC|4345}} – ''Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol''

The protocol specifications were later updated by the following publications:
* {{IETF RFC|4419}} – ''Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol'' (March 2006)
* {{IETF RFC|4432}} – ''RSA Key Exchange for the Secure Shell (SSH) Transport Layer Protocol'' (March 2006)
* {{IETF RFC|4462}} – ''Generic Security Service Application Program Interface (GSS-API) Authentication and Key Exchange for the Secure Shell (SSH) Protocol'' (May 2006)
* {{IETF RFC|4716}} – ''The Secure Shell (SSH) Public Key File Format'' (November 2006)
* {{IETF RFC|4819}} – ''Secure Shell Public Key Subsystem'' (March 2007)
* {{IETF RFC|5647}} – ''AES Galois Counter Mode for the Secure Shell Transport Layer Protocol'' (August 2009)
* {{IETF RFC|5656}} – ''Elliptic Curve Algorithm Integration in the Secure Shell Transport Layer'' (December 2009)
* {{IETF RFC|6187}} – ''X.509v3 Certificates for Secure Shell Authentication'' (March 2011)
* {{IETF RFC|6239}} – ''Suite B Cryptographic Suites for Secure Shell (SSH)'' (May 2011)
* {{IETF RFC|6594}} – ''Use of the SHA-256 Algorithm with RSA, Digital Signature Algorithm (DSA), and Elliptic Curve DSA (ECDSA) in SSHFP Resource Records'' (April 2012)
* {{IETF RFC|6668}} – ''SHA-2 Data Integrity Verification for the Secure Shell (SSH) Transport Layer Protocol'' (July 2012)
* {{IETF RFC|7479}} – ''[[Ed25519]] SSHFP Resource Records'' (March 2015)
* {{IETF RFC|5592}} – ''Secure Shell Transport Model for the Simple Network Management Protocol (SNMP)'' (June 2009)
* {{IETF RFC|6242}} – ''Using the NETCONF Protocol over Secure Shell (SSH)'' (June 2011)
* {{IETF RFC|8332}} – ''Use of RSA Keys with SHA-256 and SHA-512 in the Secure Shell (SSH) Protocol'' (March 2018)
* {{IETF RFC|8709}} – ''Ed25519 and Ed448 Public Key Algorithms for the Secure Shell (SSH) Protocol'' (February 2020)
* [https://datatracker.ietf.org/doc/html/draft-gerhards-syslog-transport-ssh draft-gerhards-syslog-transport-ssh] – ''SSH transport mapping for SYSLOG'' (July 2006)
* [https://datatracker.ietf.org/doc/html/draft-ietf-secsh-filexfer draft-ietf-secsh-filexfer] – ''SSH File Transfer Protocol'' (July 2006)
* [https://datatracker.ietf.org/doc/html/draft-ietf-sshm-ssh-agent draft-ietf-sshm-ssh-agent] - SSH Agent Protocol (March 2025)

In addition, the [[OpenSSH]] project includes several vendor protocol specifications/extensions:
* [https://cvsweb.openbsd.org/cgi-bin/cvsweb/~checkout~/src/usr.bin/ssh/PROTOCOL?content-type=text/plain OpenSSH PROTOCOL overview]
* [https://cvsweb.openbsd.org/cgi-bin/cvsweb/~checkout~/src/usr.bin/ssh/PROTOCOL.certkeys?content-type=text/plain OpenSSH certificate/key overview]
* [https://cvsweb.openbsd.org/cgi-bin/cvsweb/~checkout~/src/usr.bin/ssh/PROTOCOL.u2f?content-type=text/plain OpenSSH FIDO/u2f support]

==See also==
* [[Brute-force attack]]
* [[Comparison of SSH clients]]
* [[Comparison of SSH servers]]
* [[Corkscrew (program)|Corkscrew]]
* [[Ident protocol|Ident]]
* [[OpenSSH]]
* [[Secure Shell tunneling]]
* [[Web-based SSH]]

==References==
{{Reflist}}

==Further reading==
*{{cite book |author-link=Daniel J. Barrett |first1=Daniel J. |last1=Barrett |first2=Richard E. |last2=Silverman |first3=Robert G. |last3=Byrnes |title=SSH: The Secure Shell (The Definitive Guide) |publisher=O'Reilly |year=2005 |isbn=0-596-00895-3 |edition=2nd }}
*{{cite book |author-link=Michael W. Lucas|first=Michael W.|last=Lucas| title=SSH Mastery |edition=2nd |publisher=Tilted Windmill Press |year=2018 |isbn=9781642350029 |url=https://mwl.io/static/books/ssh-mastery-2nd-edition.html}}
*{{cite book |first=Michael |last=Stahnke |title=Pro OpenSSH |publisher=Apress |year=2005 |isbn=1-59059-476-2 }}
*{{Cite news|url=https://groups.google.com/group/comp.security.unix/msg/67079d812a19f499?dmode=source&hl=en
|author=Tatu Ylönen
|title=Announcement: Ssh (Secure Shell) Remote Login Program
|publisher=comp.security.unix
|date=12 July 1995 }} Original announcement of Ssh
*{{cite book |first=Himanshu |last=Dwivedi |title=Implementing SSH |publisher=Wiley |year=2003 |isbn=978-0-471-45880-7 }}

==External links==
{{Commons category|SSH}}
{{Wikibooks|Internet Technologies/SSH}}
* [https://www.snailbook.com/protocols.html SSH Protocols]
* {{cite IETF |rfc=7076 |title=P6R's Secure Shell Public Key Subsystem |date=November 2013 |author1=M. Joseph |author2=J. Susoy}}
* [http://alvarestech.com/temp/ssh/old/ssh-1.0.0.tar.gz Original SSH source tarball]

{{URI scheme}}
{{TLS/SSL}}
{{Authority control}}

[[Category:Secure Shell| ]]
[[Category:Application layer protocols]]
[[Category:Finnish inventions]]
[[Category:Software developed in Finland]]