The SSH Protocol (SSH, The Secure Shell: The Definitive Guide)

1.3. The SSH Protocol

SSH is a protocol, not a product. It is a specification of how to conduct secure communication over a network.[2]

[2]Although we say "the SSH protocol," there are actually two incompatible versions of the protocols in common use: SSH-1 (a.k.a SSH-1.5) and SSH-2. We will distinguish these protocols later.

The SSH protocol covers authentication, encryption, and the integrity of data transmitted over a network, as shown in Figure 1-2. Let's define these terms:

Authentication: Reliably determines someone's identity. If you try to log into an account on a remote computer, SSH asks for digital proof of your identity. If you pass the test, you may log in; otherwise SSH rejects the connection.
Encryption: Scrambles data so it is unintelligible except to the intended recipients. This protects your data as it passes over the network.
Integrity: Guarantees the data traveling over the network arrives unaltered. If a third party captures and modifies your data in transit, SSH detects this fact.

Figure 1-2. Authentication, encryption, and integrity

In short, SSH makes network connections between computers, with strong guarantees that the parties on both ends of the connection are genuine. It also ensures that any data passing over these connections arrives unmodified and unread by eavesdroppers.

1.3.1. Protocols, Products, Clients, and Confusion

SSH-based products -- i.e., products that implement the SSH protocol -- exist for many flavors of Unix, Windows, Macintosh, and other operating systems. Both freely distributable and commercial products are available. [Section 13.3, "Table of Products"]

The first SSH product, created by Tatu Ylönen for Unix, was simply called "SSH." This causes confusion because SSH is also the name of the protocol. Some people call Ylönen's software "Unix SSH," but other Unix-based implementations are now available so the name is unsatisfactory. In this book, we use more precise terminology to refer to protocols, products, and programs, summarized in the sidebar "Terminology: SSH Protocols and Products", In short:

Protocols are denoted with dashes: SSH-1, SSH-2.
Products are denoted in uppercase, without dashes: SSH1, SSH2.
Client programs are in lowercase: ssh, ssh1, ssh2, etc.

Terminology: SSH Protocols and Products

SSH

A generic term referring to SSH protocols or software products.

SSH-1

The SSH protocol, Version 1. This protocol went through several revisions, of which 1.3 and 1.5 are the best known, and we will write SSH-1.3 and SSH-1.5 should the distinction be necessary.

SSH-2

The SSH protocol, Version 2, as defined by several draft standards documents of the IETF SECSH working group.[Section 3.5.1, "Protocol Differences (SSH-1 Versus SSH-2)"]

SSH1

Tatu Ylönen's software implementing the SSH-1 protocol; the original SSH. Now distributed and maintained (minimally) by SSH Communications Security, Inc.

SSH2

The "SSH Secure Shell" product from SSH Communications Security, Inc. (http://www.ssh.com). This is a commercial SSH-2 protocol implementation, though it is licensed free of charge in some circumstances.

ssh (all lowercase letters)

A client program included in SSH1, SSH2, OpenSSH, F-Secure SSH, and other products, for running secure terminal sessions and remote commands. In SSH1 and SSH2, it is also named ssh1 or ssh2, respectively.

OpenSSH

The product OpenSSH from the OpenBSD project (see http://www.openssh.com/), which implements both the SSH-1 and SSH-2 protocols.

OpenSSH/1

OpenSSH, referring specifically to its behavior when using the SSH-1 protocol.

OpenSSH/2

OpenSSH, referring specifically to its behavior when using the SSH-2 protocol.