computer science

OpenSSH

OpenSSH is a computer science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand OpenSSH rather than just read about it. In short: OpenSSH (also known as OpenBSD Secure Shell) is a suite of secure networking utilities based on the Secure Shell (SSH) protocol, which provides a secure channel over an unsecured network in a client–server architecture. OpenSSH started as a fork of the free SSH program developed by Tatu Ylönen; later versions of Ylönen's SSH were proprietary software offered by SSH Communications Security.

Key takeaways

OpenSSH belongs to computer science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect OpenSSH to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of OpenSSH from memory before moving on to harder problems.

Reference excerpt

OpenSSH (also known as OpenBSD Secure Shell) is a suite of secure networking utilities based on the Secure Shell (SSH) protocol, which provides a secure channel over an unsecured network in a client–server architecture. OpenSSH started as a fork of the free SSH program developed by Tatu Ylönen; later versions of Ylönen's SSH were proprietary software offered by SSH Communications Security. OpenSSH was first released in 1999 and is currently developed as part of the OpenBSD operating system. OpenSSH is not a single computer program, but rather a suite of programs that serve as alternatives to unencrypted protocols like Telnet and FTP. OpenSSH is integrated into several operating systems, namely Microsoft Windows, macOS and most Linux operating systems, while the portable version is available as a package in other systems.

History OpenBSD Secure Shell was developed by OpenBSD developers as an alternative to the original SSH software by Tatu Ylönen, which is now proprietary software. Although source code is available for the original SSH, various restrictions are imposed on its use and distribution. OpenSSH was created as a fork of Björn Grönvall's OSSH that itself was a fork of Tatu Ylönen's original free SSH 1.2.12 release, which was the last one having a license suitable for forking. The OpenSSH developers claim that their application is more secure than the original, due to their policy of producing clean and audited code and because it is released under the BSD license, the open-source license to which the word open in the name refers. OpenSSH first appeared in OpenBSD 2.6. The first portable release was made in October 1999. Developments since then have included the addition of ciphers (e.g., ChaCha20-Poly1305 in 6.5 of January 2014), cutting the dependency on OpenSSL (6.7, October 2014) and an extension to facilitate public-key discovery and rotation for trusted hosts (for transition from DSA to Ed25519 public host keys, version 6.8 of March 2015). On 19 October 2015, Microsoft announced that OpenSSH will be natively supported on Microsoft Windows and accessible through PowerShell, releasing an early implementation and making the code publicly available. OpenSSH-based client and server programs have been included in Windows 10 since version 1803. The SSH client and key agent are enabled and available by default, and the SSH server is an optional Feature-on-Demand. In October 2019 protection for private keys at rest in RAM against speculation and memory side-channel attacks were added in OpenSSH 8.1.

Development

OpenSSH is developed as part of the OpenBSD operating system. Rather than including changes for other operating systems directly into OpenSSH, a separate portability infrastructure is maintained by the OpenSSH Portability Team, and "portable releases" are made periodically. This infrastructure is substantial, partly because OpenSSH is required to perform authentication, a capability that has many varying implementations. This model is also used for other OpenBSD projects such as OpenNTPD. The OpenSSH suite includes the following command-line utilities and daemons:

scp, a replacement for rcp. sftp, a replacement for ftp to copy files between computers. ssh, a replacement for rlogin, rsh and telnet to allow shell access to a remote machine. ssh-add and ssh-agent, utilities to ease authentication by holding keys ready and avoid the need to enter passphrases every time they are used. ssh-keygen, a tool to inspect and generate the RSA, DSA and elliptic-curve keys that are used for user and host authentication. ssh-keyscan, which scans a list of hosts and collects their public keys. sshd, the SSH server daemon. The OpenSSH server can authenticate users using the standard methods supported by the SSH protocol: with a password; public-key authentication, using per-user keys; host-based authentication, which is a secure version of rlogin's host trust relationships using public keys; keyboard-interactive, a generic challenge–response mechanism, which is often used for simple password authentication, but which can also make use of stronger authenticators such as tokens; and Kerberos/GSSAPI. The server makes use of authentication methods native to the host operating system; this can include using the BSD Authentication system or pluggable authentication modules (PAM) to enable additional authentication through methods such as one-time passwords. However, this occasionally has side effects: when using PAM with OpenSSH, it must be run as root, as root privileges are typically required to operate PAM. OpenSSH versions after 3.7 (16 September 2003) allow PAM to be disabled at run-time, so regular users can run sshd instances. On OpenBSD, OpenSSH uses a dedicated sshd user by default to drop privileges and perform privilege separation in accordance with the principle of least privilege, applied throughout the operating system including the Xenocara X server.

… excerpt ends here. Continue reading the full article.

Illustrations

Worked examples

Example 1 — a first encounter with OpenSSH

Start with the simplest possible case. Write down what OpenSSH claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In computer science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to OpenSSH before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about OpenSSH ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of OpenSSH

In research: OpenSSH appears in computer science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses OpenSSH in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: OpenSSH is common in secondary-school and first-year university syllabi. It links to neighbouring topics Cross-platform free software, Cryptographic software, Free network-related software, so understanding it makes those chapters shorter.
In everyday life: Look for OpenSSH outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study OpenSSH in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what OpenSSH means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain OpenSSH out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is OpenSSH in simple terms?

Why does OpenSSH matter?

Because it connects several computer science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study OpenSSH?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on OpenSSH.