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The Secure Shell Protocol (SSH) 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 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.

SSH was designed on Unix-like operating systems, as a replacement for Telnet and for unsecured remote Unix shell protocols, such as the Berkeley Remote Shell (rsh) and the related rlogin and rexec protocols, which all use insecure, plaintext transmission of authentication tokens.

SSH was first designed in 1995 by Finnish computer scientist Tatu Ylönen. 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.

Definition

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

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. 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.

SSH also supports password-based authentication that is encrypted by automatically generated keys. In this case, the attacker could imitate the legitimate server side, ask for the password, and obtain it ( man-in-the-middle attack). However, this is possible only if the two sides have never authenticated before, as SSH remembers the key that the server side previously used. The SSH client raises a warning before accepting the key of a new, previously unknown server. Password authentication can be disabled from the server side.

Use

SSH is typically used to log into a remote machine and execute commands, but it also supports tunneling, forwarding TCP ports and X11 connections; it can transfer files using the associated SSH file transfer (SFTP) or secure copy (SCP) protocols. SSH uses the client–server model.

An SSH client program is typically used for establishing connections to an SSH daemon, such as sshd, accepting remote connections. Both are commonly present on most modern operating systems, including macOS, most distributions of Linux, OpenBSD, FreeBSD, NetBSD, Solaris and OpenVMS. Notably, versions of Windows prior to Windows 10 version 1709 do not include SSH by default. Proprietary