Secure Shell

See also: Secure shell tunneling

Secure Shell (SSH) is a cryptographic network protocol for secure data communication, remote shell services or command execution and other secure network services between two networked computers that connects, via a secure channel over an insecure network, a server and a client (running SSH server and SSH client programs, respectively). The protocol specification distinguishes between two major versions that are referred to as SSH-1 and SSH-2.

The best-known application of the protocol is for access to shell accounts on Unix-like operating systems, but it can also be used in a similar fashion for accounts on Windows. It was designed as a replacement for Telnet and other insecure remote shell protocols such as the Berkeley rsh and rexec protocols, which send information, notably passwords, in plaintext, rendering them susceptible to interception and disclosure using packet analysis. The encryption used by SSH is intended to provide confidentiality and integrity of data over an unsecured network, such as the Internet.

Internet protocol suite
Application layer
DHCP DHCPv6 DNS FTP HTTP IMAP IRC LDAP MGCP NNTP BGP NTP POP RPC RTP RTSP RIP SIP SMTP SNMP SOCKS SSH Telnet TLS/SSL XMPP (more)
Transport layer
TCP UDP DCCP SCTP RSVP (more)
Internet layer
IP IPv4 IPv6 ICMP ICMPv6 ECN IGMP IPsec (more)
Link layer
ARP/InARP NDP OSPF Tunnels L2TP PPP Media access control Ethernet DSL ISDN FDDI (more)

Definition

SSH uses public-key cryptography to authenticate the remote computer and allow it to authenticate the user, if necessary. There are several ways to use SSH; one is to use automatically generated public-private key pairs to simply encrypt a network connection, and then use password authentication to log on.

Another is to use a manually generated public-private key pair to perform the authentication, allowing users or programs to log in without having to specify a password. In this scenario, anyone can produce a matching pair of different keys (public and private). The public key is placed on all computers that must allow access to the owner of the matching private key (the owner keeps the private key secret). While authentication is based on the private key, the key itself is never transferred through the network during authentication. SSH only verifies whether 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.

Key management

On Unix-like systems, the list of authorized public keys is stored in the home directory of the user that is allowed to log in remotely, in the file ~/.ssh/authorized_keys. This file is only respected by ssh 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 (some software like Message Passing Interface (MPI) stack may need this password-less access to run properly). 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 side, ask for the password and obtain it (man-in-the-middle attack). However this is only possible if the two sides have never authenticated before, as SSH remembers the key that the remote side once used. Password authentication can be disabled.

Usage

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.

The standard TCP port 22 has been assigned for contacting SSH servers,.

An SSH client program is typically used for establishing connections to an SSH daemon accepting remote connections. Both are commonly present on most modern operating systems, including Mac OS X, most distributions of GNU/Linux, OpenBSD, FreeBSD, NetBSD, Solaris and OpenVMS. Notably, Windows is one of the few modern desktop/server OSs that does not include SSH by default. Proprietary, freeware and open source versions of various levels of complexity and completeness exist.

SSH is important in cloud computing to solve connectivity problems, avoiding the security issues of exposing a cloud-based virtual machine directly on the Internet. An SSH tunnel can provide a secure path over the Internet, through a firewall to a virtual machine.

History and development

Version 1.x

In 1995, Tatu Ylönen, a researcher at Helsinki University of Technology, 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 and rsh protocols, which did not provide strong authentication nor guarantee confidentiality. 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 Secure Communications evolved into increasingly proprietary software.

It is estimated that, as of 2000, there were 2 million users of SSH.

Notable vulnerabilities

In 1998 a vulnerability was described in SSH 1.5 which allowed the unauthorized insertion of content into an encrypted SSH stream due to insufficient data integrity protection from CRC-32 used in this version of the protocol. A fix known as SSH Compensation Attack Detector was introduced into most implementations. Many of these updated implementations contained a new integer overflow vulnerability that allowed attackers to execute arbitrary code with the privileges of the SSH daemon, typically root.

In January 2001 a vulnerability was discovered that allows attackers to modify the last block of an IDEA-encrypted session. The same month, another vulnerability was discovered that allowed a malicious server to forward a client authentication to another server.

Since SSH-1 has inherent design flaws which make it vulnerable, it is now generally considered obsolete and should be avoided by explicitly disabling fallback to SSH-1. Most modern servers and clients support SSH-2.

Version 1.99

In January 2006, well after version 2.1 was established, RFC 4253 specified that an SSH server which supports both 2.0 and prior versions of SSH should identify its protoversion as 1.99. This is not an actual version but a method to identify backward compatibility.

OpenSSH and OSSH

In 1999, developers wanting a free software version to be available went back to the older 1.2.12 release of the original SSH program, which was the last released under an open source license. Björn Grönvall's OSSH was subsequently developed from this codebase. Shortly thereafter, OpenBSD developers forked Grönvall's code and did extensive work on it, creating OpenSSH, which shipped with the 2.6 release 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, coming by default in a large number of operating systems. OSSH meanwhile has become obsolete. OpenSSH continues to be maintained and now supports both 1.x and 2.0 versions.

Version 2.x

"Secsh" was the official Internet Engineering Task Force's (IETF) name for the IETF working group responsible for version 2 of the SSH protocol. In 2006, a revised version of the protocol, SSH-2, was adopted as a standard. This version is incompatible with SSH-1. SSH-2 features both security and feature improvements over SSH-1. Better security, for example, comes through Diffie-Hellman key exchange and strong integrity checking via message authentication codes. 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 Lsh and Dropbear, only support SSH-2 protocol.

Vulnerabilities

In November 2008, a theoretical vulnerability was discovered for all versions of SSH which allowed recovery of up to 32 bits of plaintext from a block of ciphertext that was encrypted using what was then the standard default encryption mode, CBC. However, to date, no practical solutions exist to exploit the problem.

OpenSSH

In the case of using the standard OpenSSH configuration, the attacker's success probability for recovering 32 bits of plaintext is $2^{-18}$. The OpenSSH 5.2 release modified the behavior of the OpenSSH server to further mitigate against this vulnerability.

Internet standard documentation

The following RFC publications by the IETF "secsh" working group document SSH-2 as a proposed Internet standard.

• RFC 4250, The Secure Shell (SSH) Protocol Assigned Numbers
• RFC 4251, The Secure Shell (SSH) Protocol Architecture
• RFC 4252, The Secure Shell (SSH) Authentication Protocol
• RFC 4253, The Secure Shell (SSH) Transport Layer Protocol
• RFC 4254, The Secure Shell (SSH) Connection Protocol
• RFC 4255, Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints
• RFC 4256, Generic Message Exchange Authentication for the Secure Shell Protocol (SSH)
• RFC 4335, The Secure Shell (SSH) Session Channel Break Extension
• RFC 4344, The Secure Shell (SSH) Transport Layer Encryption Modes
• RFC 4345, Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol

It was later modified and expanded by the following publications.

• RFC 4419, Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol (March 2006)
• RFC 4432, RSA Key Exchange for the Secure Shell (SSH) Transport Layer Protocol (March 2006)
• RFC 4462, Generic Security Service Application Program Interface (GSS-API) Authentication and Key Exchange for the Secure Shell (SSH) Protocol (May 2006)
• RFC 4716, The Secure Shell (SSH) Public Key File Format (November 2006)
• RFC 5656, Elliptic Curve Algorithm Integration in the Secure Shell Transport Layer (December 2009)

Uses

Example of tunneling an X11 application over SSH: the user 'josh' has SSHed from the local machine 'foofighter' to the remote machine 'tengwar' to run xeyes.

Logging into OpenWrt via SSH using PuTTY running on Windows.

SSH is a protocol that can be used for many applications across many platforms including most Unix variants (Linux, the BSD's including Apple's OS X, & Solaris), as well as Microsoft Windows. Some of the applications below may require features that are only available or compatible with specific SSH clients or servers. For example, using the SSH protocol to implement a VPN is possible, but presently only with the OpenSSH server and client implementation.

• For login to a shell on a remote host (replacing Telnet and rlogin)
• For executing a single command on a remote host (replacing rsh)
• Secure file transfer
• In combination with rsync to back up, copy and mirror files efficiently and securely
• For forwarding or tunneling a port (not to be confused with a VPN, which routes packets between different networks, or bridges two broadcast domains into one).
• For using as a full-fledged encrypted VPN. Note that only OpenSSH server and client supports this feature.
• For forwarding X from a remote host (possible through multiple intermediate hosts)
• For browsing the web through an encrypted proxy connection with SSH clients that support the SOCKS protocol.
• For securely mounting a directory on a remote server as a filesystem on a local computer using SSHFS.
• For automated remote monitoring and management of servers through one or more of the mechanisms discussed above.
• For development on a mobile or embedded device that supports SSH.

File transfer protocols using SSH

There are multiple mechanisms for transferring files using the Secure Shell protocols.

• Secure copy (SCP), which evolved from RCP protocol over SSH
• rsync, intended to be more efficient than SCP
• SSH File Transfer Protocol (SFTP), a secure alternative to FTP (not to be confused with FTP over SSH)
• Files transferred over shell protocol (a.k.a. FISH), released in 1998, which evolved from Unix shell commands over SSH

Architecture

Diagram of the SSH-2 binary packet.

The SSH-2 protocol has an internal architecture (defined in RFC 4251) with well-separated layers. These are:

• The transport layer (RFC 4253). This layer handles initial key exchange as well as server authentication, and sets up encryption, compression and integrity verification. It exposes to the upper layer an interface for sending and receiving plaintext packets with sizes of up to 32,768 bytes each (more can be allowed by the implementation). The transport layer also arranges for key re-exchange, usually after 1 GB of data has been transferred or after 1 hour has passed, whichever is sooner.
• The user authentication layer (RFC 4252). This layer handles client authentication and provides a number of authentication methods. Authentication is client-driven: when one is prompted for a password, it may be the SSH client prompting, not the server. The server merely responds to the client's authentication requests. Widely used user authentication methods include the following:
  • password: a method for straightforward password authentication, including a facility allowing a password to be changed. This method is not implemented by all programs.
  • publickey: a method for public key-based authentication, usually supporting at least DSA or RSA keypairs, with other implementations also supporting X.509 certificates.
  • keyboard-interactive (RFC 4256): a versatile method where the server sends one or more prompts to enter information and the client displays them and sends back responses keyed-in by the user. Used to provide one-time password authentication such as S/Key or SecurID. Used by some OpenSSH configurations when PAM is the underlying host authentication provider to effectively provide password authentication, sometimes leading to inability to log in with a client that supports just the plain password authentication method.
  • GSSAPI authentication methods which provide an extensible scheme to perform SSH authentication using external mechanisms such as Kerberos 5 or NTLM, providing single sign on capability to SSH sessions. These methods are usually implemented by commercial SSH implementations for use in organizations, though OpenSSH does have a working GSSAPI implementation.
• The connection layer (RFC 4254). This layer defines the concept of channels, channel requests and global requests using which SSH services are provided. A single SSH connection can host multiple channels simultaneously, each transferring data in both directions. Channel requests are used to relay out-of-band channel specific data, such as the changed size of a terminal window or the exit code of a server-side process. The SSH client requests a server-side port to be forwarded using a global request. Standard channel types include:
  • shell for terminal shells, SFTP and exec requests (including SCP transfers)
  • direct-tcpip for client-to-server forwarded connections
  • forwarded-tcpip for server-to-client forwarded connections
• The SSHFP DNS record (RFC 4255) provides the public host key fingerprints in order to aid in verifying the authenticity of the host.

This open architecture provides considerable flexibility, allowing SSH to be used for a variety of purposes beyond a secure shell. The functionality of the transport layer alone is comparable to Transport Layer Security (TLS); the user authentication layer is highly extensible with custom authentication methods; and the connection layer provides the ability to multiplex many secondary sessions into a single SSH connection, a feature comparable to BEEP and not available in TLS.

Enhancements

These are intended for performance enhancements of SSH products:

• SSH-over-SCTP: support for SCTP rather than TCP as the connection oriented transport layer protocol.
• ECDSA: support for elliptic curve DSA rather than DSA or RSA for signing.
• ECDH: support for elliptic curve Diffie-Hellman rather than plain Diffie-Hellman for encryption key exchange.
• UMAC: support for UMAC rather than HMAC for MAC/integrity.

See also

• Ident
• Web-based SSH – Access to SSH servers through standard web browsers
• Autossh - a tool to maintain a constant SSH connection, restarting it as necessary
• Comparison of SSH clients
• Comparison of SSH servers
• Corkscrew – a tool that enables a user to run SSH over HTTPS proxy servers
• Public-key cryptography

Further reading

• Daniel J. Barrett, Richard E. Silverman, and Robert G. Byrnes, SSH: The Secure Shell (The Definitive Guide), O'Reilly 2005 (2nd edition). ISBN 0-596-00895-3
• Michael Stahnke, Pro OpenSSH, Apress 2005 ISBN 1-59059-476-2
• Tatu Ylönen (12 July 1995). "Announcement: Ssh (Secure Shell) Remote Login Program". comp.security.unix.  Original announcement of Ssh
• Himanshu Dwivedi; Implementing SSH, Wiley 2003. ISBN 978-0-471-45880-7
• This article is based on material taken from the Free On-line Dictionary of Computing prior to 1 November 2008 and incorporated under the "relicensing" terms of the GFDL, version 1.3 or later.

External links

• Old homepage for IETF 'secsh' working group, which has concluded (for SSH-2)
• SSH Protocols