Secure Shell

Source: Wikipedia, the free encyclopedia.
"SSH" redirects here. For other uses, see SSH (disambiguation).
Secure Shell
Protocol stack
Purposesecure connection, remote access
Developer(s)Tatu Ylönen, Internet Engineering Task Force (IETF)
Introduction1995
OSI layerTransport layer through application layer
Port(s)22
RFC(s)RFC 4250, RFC 4251, RFC 4252, RFC 4253, RFC 4254
Internet protocol suite
Application layer
Transport layer
Internet layer
Link layer

The Secure Shell Protocol (SSH Protocol) is a

network protocol for operating network services securely over an unsecured network.[1] Its most notable applications are remote login and command-line
execution.

SSH was designed for

passwords
.

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 public network in an unsecured way poses a great risk of 3rd 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.[2]

Finnish computer scientist Tatu Ylönen designed SSH in 1995 and provided an implementation in the form of two commands, ssh and slogin, as secure replacements for rsh and 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 systems.

SSH applications are based on a

SSH server.[3] 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.[1]

Definition

SSH uses public-key cryptography to authenticate the remote computer and allow it to authenticate the user, if necessary.[3]

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 keys, i.e. 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 ~/.ssh/authorized_keys.[4] 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 -i for ssh). The ssh-keygen utility produces the public and private keys, always in pairs.

Use

SSH is typically used to log into a remote computer's

Secure Copy Protocol (SCP).[3]

SSH uses the

Windows prior to Windows 10 version 1709 do not include SSH by default, but proprietary, freeware and 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[5] 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 FISH protocol to provide a split-pane GUI with drag-and-drop. The open source Windows program WinSCP[6] provides similar file management (synchronization, copy, remote delete) capability using PuTTY as a back-end. Both WinSCP[7] and PuTTY[8] 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.[9]

The

SCTP rather than TCP as the connection oriented transport layer protocol.[12]

Historical development

Version 1

In 1995, Tatu Ylönen, a researcher at

FTP[14] and rsh protocols, which did not provide strong authentication nor guarantee confidentiality. He chose the port number 22 because it is between telnet (port 23) and ftp (port 21).[15]

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.[16]

In December 1995, Ylönen founded SSH Communications Security to market and develop SSH. The original version of the SSH software used various pieces of

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.[17]

Version 2

In 2006, after being discussed in a working group named "secsh",[18] a revised version of the SSH protocol, SSH-2 was adopted as a standard.[19] 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 codes like MD5 or SHA-1, which can be negotiated between client and server. SSH-2 also adds stronger encryption methods like AES which eventually replaced weaker and compromised ciphers from the previous standard like 3-des.[20][21][19] New features of SSH-2 include the ability to run any number of shell sessions over a single SSH connection.[22] Due to SSH-2's superiority and popularity over SSH-1, some implementations such as libssh (v0.8.0+),[23] Lsh[24] and Dropbear[25] eventually supported only the SSH-2 protocol.

Version 1.99

In January 2006, well after version 2.1 was established,

RFC 4253 specified that an SSH server supporting 2.0 as well as prior versions should identify its protocol version as 1.99.[26] This version number does not reflect a historical software revision, but a method to identify backward compatibility
.

OpenSSH and OSSH

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.[27] This served as a code base for Björn Grönvall's OSSH software.[28] Shortly thereafter, OpenBSD developers forked 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.[29]

As of 2005[update], 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.[30] 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[31][32][33] by PhD student François Michel and Professor Olivier Bonaventure and its code has been made open source.[34] 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:

However, the name SSH3 is under discussion, and the project aims to rename itself to a more suitable name.[35] The discussion stems from the fact that this new implementation significantly revises the SSH protocol, suggesting it should not be called SSH3.

Uses

xeyes
.
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

VPN is possible, but presently only with the OpenSSH
server and client implementation.

File transfer protocols

The Secure Shell protocols are used in several file transfer mechanisms.

Architecture

Diagram of the SSH-2 binary packet.

The SSH protocol has a layered architecture with three separate components:

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

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.[42][43] A fix known as SSH Compensation Attack Detector[44] was introduced into most implementations. Many of these updated implementations contained a new integer overflow vulnerability[45]
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 IDEA-encrypted session.[46] The same month, another vulnerability was discovered that allowed a malicious server to forward a client authentication to another server.[47]

Since SSH-1 has inherent design flaws which make it vulnerable, it is now generally considered obsolete and should be avoided by explicitly disabling fallback to SSH-1.[47] Most modern servers and clients support SSH-2.[48]

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,

CTR, counter mode, instead of CBC mode, since this renders SSH resistant to the attack.[49]

Suspected decryption by NSA

On December 28, 2014

CIA hacking tools BothanSpy and Gyrfalcon suggested that the SSH protocol was not compromised.[51]

Terrapin attack

Main article: 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.[52][53] 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.[54][55] The ssh developers have stated that the major impact of the attack is to degrade the keystroke timing obfuscation features of ssh.[55] 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

Internet standard
.

The protocol specifications were later updated by the following publications:

In addition, the OpenSSH project includes several vendor protocol specifications/extensions:

See also

References

  1. ^
    doi:10.17487/RFC4251. RFC 4251
    .
  2. ^ "Missouri University S&T: Secure Telnet".
  3. ^
    doi:10.17487/RFC4252. RFC 4252
    .
  4. ^ "How To Set Up Authorized Keys". Archived from the original on 2011-05-10.
  5. ^ Win-32 OpenSSH
  6. ^ "WinSCP home page". Archived from the original on 2014-02-17.
  7. ^ "WinSCP page for PortableApps.com". Archived from the original on 2014-02-16.
  8. ^ "PuTTY page for PortableApps.com". Archived from the original on 2014-02-16.
  9. ^ Amies, A; Wu, C F; Wang, G C; Criveti, M (2012). "Networking on the cloud". IBM developerWorks. Archived from the original on 2013-06-14.
  10. ^ "Service Name and Transport Protocol Port Number Registry".
  11. ^ "Service Name and Transport Protocol Port Number Registry". iana.org. Archived from the original on 2001-06-04.
  12. S2CID 8415240
    .
  13. ^ Tatu Ylönen. "The new skeleton key: changing the locks in your network environment". Archived from the original on 2017-08-20.
  14. ^ Tatu Ylönen. "SSH Port". Archived from the original on 2017-08-03.
  15. ^ Ylönen, Tatu. "The story of the SSH port is 22". www.ssh.com. Retrieved 2023-11-30.
  16. ISBN 978-0-596-00011-0
    .
  17. ^ Nicholas Rosasco and David Larochelle. "How and Why More Secure Technologies Succeed in Legacy Markets: Lessons from the Success of SSH" (PDF). Quoting Barrett and Silverman, SSH, the Secure Shell: The Definitive Guide, O'Reilly & Associates (2001). Dept. of Computer Science, Univ. of Virginia. Archived (PDF) from the original on 2006-06-25. Retrieved 2006-05-19.
  18. ^ IETF (Internet Engineering Task Force): datatracker for secsh
  19. ^ a b RFC4252: The Secure Shell (SSH) Authentication Protocol, Jan 2006
  20. ^ O'Reily: Secure Shell, The Definitive Guide
  21. ^ RFC4250: The Secure Shell (SSH) Protocol: Assigned names, Jan 2006, page 16
  22. ^ "SSH Frequently Asked Questions". Archived from the original on 2004-10-10.
  23. ^ "libssh".
  24. ^ "A GNU implementation of the Secure Shell protocols". Archived from the original on 2012-02-04.
  25. ^ "Dropbear SSH". Archived from the original on 2011-10-14.
  26. doi:10.17487/RFC4253. RFC 4253
    .
  27. ^ ssh-1.2.13 now available: copying policy changed (permission now required to sell ssh commercially, use is still permitted for any purpose)
  28. ^ OSSH sources
  29. ^ "OpenSSH: Project History and Credits". openssh.com. 2004-12-22. Archived from the original on 2013-12-24. Retrieved 2014-04-27.
  30. ^ "OSSH Information for VU#419241". CERT Coordination Center. 2006-02-15. Archived from the original on 2007-09-27. Either way ossh is old and obsolete and I don't recommend its use.
  31. ^ "Remote terminal over HTTP/3 connections". datatracker.ietf.org. 2024-08-01.
  32. ^ "Secure shell over HTTP/3 connections". www.ietf.org. 2024-02-28.
  33. ^ "Towards SSH3: how HTTP/3 improves secure shells". arxiv.org. 2023-12-12.
  34. ^ "ssh3". github.com. 2024-07-12.
  35. ^ "Secure shell over HTTP/3 connections". datatracker.ietf.org. 2024-02-28.
  36. ISBN 978-0133085044
    .
  37. doi:10.17487/RFC8709. RFC 8709
    .
  38. ^
    doi:10.17487/RFC5656. RFC 5656
    . Retrieved 12 November 2012.
  39. ^ Miller, D.; Valchev, P. (September 3, 2007). The use of UMAC in the SSH Transport Layer Protocol. I-D draft-miller-secsh-umac-00.
  40. doi:10.17487/RFC4253. RFC 4253
    .
  41. doi:10.17487/RFC5647. RFC 5647
    .
  42. ^ "SSH Insertion Attack". Core Security Technologies. Archived from the original on 2011-07-08.
  43. US CERT. Archived
    from the original on 2010-07-10.
  44. ^ "SSH CRC-32 Compensation Attack Detector Vulnerability". SecurityFocus. Archived from the original on 2008-07-25.
  45. ^ "Vulnerability Note VU#945216 - SSH CRC32 attack detection code contains remote integer overflow". US CERT. Archived from the original on 2005-10-13.
  46. ^ "Vulnerability Note VU#315308 - Weak CRC allows last block of IDEA-encrypted SSH packet to be changed without notice". US CERT. Archived from the original on 2010-07-11.
  47. ^ a b "Vulnerability Note VU#684820 - SSH-1 allows client authentication to be forwarded by a malicious server to another server". US CERT. Archived from the original on 2009-09-01.
  48. ^ "How to use SSH keys for authentication". Up Cloud. 17 September 2015. Retrieved 29 November 2019.
  49. ^ a b "Vulnerability Note VU#958563 - SSH CBC vulnerability". US CERT. Archived from the original on 2011-06-22.
  50. Spiegel Online. December 28, 2014. Archived
    from the original on January 24, 2015.
  51. ^ Ylonen, Tatu (3 August 2017). "BothanSpy & Gyrfalcon - Analysis of CIA hacking tools for SSH". ssh.com. Retrieved 15 July 2018.
  52. ^ "Terrapin Attack". terrapin-attack.com. Retrieved 2023-12-20.
  53. ^ Jones, Connor. "SSH shaken, not stirred by Terrapin downgrade vulnerability". www.theregister.com. Retrieved 2023-12-20.
  54. ^ Jones, Connor. "SSH shaken, not stirred by Terrapin downgrade vulnerability". www.theregister.com. Retrieved 2023-12-20.
  55. ^ a b "OpenSSH 9.6 release notes". openssh.com. 2023-12-18.

Further reading

Wikimedia Commons has media related to SSH.
Wikibooks has a book on the topic of: Internet Technologies/SSH
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