The story
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-sniffing attack at his university network. The goal of SSH was to replace the earlier rlogin, telnet, FTP 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).
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 Communications Security evolved into increasingly proprietary software.
It was estimated that by 2000, the number of users had grown to 2 million.
Version 2
In 2006, after being discussed in a working group named "secsh", a revised version of the SSH protocol, SSH-2 was adopted as a standard. 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 3DES. New features of SSH-2 include the ability to run any number of shell sessions over a single SSH connection. Due to SSH-2's superiority and popularity over SSH-1, some implementations such as libssh (v0.8.0+), Lsh and Dropbear 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. This version number does not reflect a historical software revision, but a method to identify backward compatibility.
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. This served as a code base for Björn Grönvall's OSSH software. 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.
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. 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.
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Future
In 2023, an alternative to traditional SSH was proposed under the name SSH3 by PhD student François Michel and Professor Olivier Bonaventure and its code has been made open source. 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 delays 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. The discussion stems from the fact that this new implementation significantly revises the SSH protocol, suggesting it should not be called SSH3.
In 2026, researchers from Stanford University introduced Hop, an open-source transport and remote access protocol designed to modernize remote terminal sessions and address protocol complexity in legacy SSH. Hop replaces SSH's traditional Trust-On-First-Use (TOFU) model with automated certificate verification using ACME. To protect against active network scanning and metadata leakage, the protocol supports both a standard discoverable handshake and a "hidden mode" that conceals server and client identities from unauthenticated observers. Hop also features a cryptographically mediated delegation scheme that allows secure session forwarding through jump hosts without exposing client private keys. Additionally, Hop provides built-in network resilience, supporting seamless client roaming across IP address changes and preserving session state during intermittent connection loss.
From Wikipedia’s article on SSH Communications Security, shared under CC BY-SA 4.0.5