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Secure Shell (SSH) is a cryptographic network protocol that provides secure remote login and other secure network services over an insecure network. The protocol was designed as a replacement for older, less secure protocols like Telnet and rlogin, which transmitted data in plaintext, exposing sensitive information to potential interception. SSH, on the other hand, encrypts all communications, ensuring confidentiality, integrity, and authentication for network connections.

The SSH protocol was first documented in January 2006, with the core protocol being outlined in several RFCs, most notably RFC 4251 and RFC 4253. These documents describe the architecture and transport layer of SSH, respectively. RFC 4251 details the protocol's general architecture, including its support for different encryption algorithms, while RFC 4253 outlines how the SSH protocol operates over TCP/IP networks, ensuring secure connections by encrypting all communications between client and server.

One of the key features of SSH is its use of strong encryption algorithms. It supports multiple cryptographic algorithms, such as AES and Triple DES, which protect the data being transmitted between the client and server. During the connection establishment process, SSH uses a secure Diffie-Hellman key exchange algorithm to generate encryption keys dynamically. This ensures that even if an attacker intercepts the data, they cannot decrypt it without the session keys.

In addition to encrypting communications, SSH also provides mechanisms for authenticating the client and server. This is typically done using public-key cryptography, where the server has a public key that is shared with clients. The client can verify the server's authenticity by comparing the received key with a known key. Alternatively, SSH supports password-based authentication, though public-key authentication is generally considered more secure.

SSH operates through a series of channels, as described in RFC 4254, which details the SSH connection protocol. Each connection consists of multiple channels that are multiplexed into a single SSH session. These channels can be used for interactive login sessions, file transfers via protocols like SCP and SFTP, and tunneling arbitrary data over encrypted SSH connections.

RFC 4253 also specifies the use of port 22 for SSH connections, which has been officially assigned by the IANA as the default port for SSH services. However, administrators can configure SSH to run on different ports to enhance security by obscurity.

SSH has evolved significantly over the years and is now widely used in a variety of network environments, including cloud services, remote server management, and secure file transfers. It is supported on almost all modern operating systems, including Linux, macOS, and Windows.

For further technical information, you can refer to: - RFC 4251: https://www.rfc-editor.org/info/rfc4251 - RFC 4253: https://www.rfc-editor.org/info/rfc4253 - Wikipedia on SSH: https://en.wikipedia.org/wiki/Secure_Shell

Secure Shell (SSH) is a foundational protocol for secure remote access, encrypting and authenticating communication between clients and servers. Its adoption has helped ensure that sensitive information remains protected when transmitted over insecure networks. The various RFCs that define SSH, including RFC 4251 and RFC 4253, have contributed to making SSH the standard for secure network communications in the modern internet landscape. Through encryption, strong authentication mechanisms, and support for secure file transfers, SSH continues to be a critical tool in ensuring network security.

Snippet from Wikipedia: Secure Shell

The Secure Shell Protocol (SSH Protocol) is a cryptographic network protocol for operating network services securely over an unsecured network. 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 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, like 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.

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 client–server architecture, connecting an SSH client instance with an SSH server. 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.

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