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Glossary: SSH Client

SSH (Secure Shell) is a cryptographic network protocol that transports data securely over an unsecured network. As the name implies, its main purpose is to provide a secure connection to a remote shell account.

The ssh-client is the software, which the user runs on his local computer to connect to the remote computer. Once connected, the ssh-client enables the user to run programs or perform other tasks on the remote server.

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The Need for Encrypted Access to the Shell

When you're working with client/server computing, a general terminal or a terminal emulator is used to send and receive data from the host.

There are several different methods that can be used to connect the two, but in the past, the most commonly used method of connecting a terminal to a server over a network were telnet and rsh (remote shell), which exchange data with the server without using encryption.

Today however, unencrypted transmission via telnet or rsh presents a serious security risk, because anybody with access to the network can monitor or record all details of the session, including username and password, which would subsequently allow the listener full access to the remote compuer.

Because of the ever-increasing demand for security, the SSH (Secure Shell) protocol was developed. SSH uses advanced encryption technology to encrypt every all aspects of the communication between the user (called SSH client) and the server. Should an unauthorized third party be able to intercept traffic somewhere along the communication path, they will see nothing but completely useless data.

Technical SSH Basics

The basics of the SSH protocol are laid out in RTF 4253. The document describes SSH as a secure transport protocol that provides strong encryption, cryptographic host authentication, and integrity protection.

Or, as RFC 4253 states in its intro:

The Secure Shell (SSH) is a protocol for secure remote login and other secure network services over an insecure network.

This document describes the SSH transport layer protocol, which typically runs on top of TCP/IP. The protocol can be used as a basis for a number of secure network services. It provides strong encryption, server authentication, and integrity protection. It may also provide compression.

Key exchange method, public key algorithm, symmetric encryption algorithm, message authentication algorithm, and hash algorithm are all negotiated.

This document also describes the Diffie-Hellman key exchange method and the minimal set of algorithms that are needed to implement the SSH transport layer protocol.

It hence defines ways to create a shared encryption key in the possible presence of a listener, host and user authentication methods (i.e. ways in which users and server can prove that they are who they claim to be), and possible data compression to more effectively transmit data.

Assymetric Encryption

Asymmetric Encryption Asymmetrical encryption differs from symmetrical encryption in the fact that two different keys are used. One (any) of those two is used to encrypt the data and then the other is used to decrypt it. The benefit of this technique is that you can give the other party a key to encrypt messages to you, but anyone knowing that key will still not be able to decrypt the message again. Such a key is called the public key. The other key, which is not made public and which is used to decrypt the messages is called the private key.

This also works in the other direction. A message encrypted using the private key can only be decrypted using the public key. With SSH this fact can be used to prove identity. If a message is decryptable using the public key, it proves that whoever encrypted the message, is in possession of the private key.

Symmetrical Encryption

Symmetrical encryption is a form of encryption where a key can be used to encrypt messages to the other party, and also to decrypt the messages received from the other participant. What makes the encryption mmetrics the fact that the same key is used for encryption and decryption.

Symmetric encryption usually requires little computing power and is hence used to encrypt larger blocks of data. With SSH, it is used to encrypt the whole data stream.

Key Exchange

An especially challenging part of encrypting such communication, is the need to negotiate a shared secret (an encryptino key) over a channel that might already be monitored by a third party.

Think of the problem as such: You need to agree with someone else on a password, but you can only talk to each other about it in the presence of an enemy.

SSH answers this challenge through the initial key exchange phase of the connection using the older Diffie-Hellman kex method. Newer versions now also support ED25519 elliptic curve kex. It is a specific implementation of the Edwards-curve Digital Signature Algorithm (EdDSA), which itself is a variant of Schnorr's signature system with Twisted Edwards curves (math heavy details can be found in the upcoming IETF standard for ED25519.

SSH Client Features and Requirements

In other words, there are many benefits to using a SSH client. On top of the encryption of the data transfer and secure key exchange, the SSH protocol also offers verification that you are connected to the correct computer.

This may seem surprising, but it makes perfect sense. Keep in mind that if somebody were able to control any part of the communication path, they could actually reroute the traffic to another computer. This could then play the role of the computer which you actually wanted to connect to (this is called a man-in-the-middle attack), and could either display fake data or obtain information from the client computer. A feature called known_hosts can prevent this.

A SSH client should also support a variety authentication methods. These include username/password, public/private key, and various custom formats. The latter might include a system where the server could obtain information that only the authorized users know, e.g. by using a SecurID card or by sending an access code to the user's mobile phone.

To be able to connect to various different servers, the ssh client it has to support latest key exchange and encryption protocols, because what seemed unbreakable five years ago, is considered less so today. Most server continually switch to more advanced encryption methods, ssh clients need to support these as well.

Other typical must have features for a ssh client would be:

  • Port forwarding (tunneling connections from client to server through the ssh channel)
  • PKCS#11 authentication (this allows authentication through hardware, e.g. smart cards)
  • ED25519, ECDSA, RSA and DSA public key authentication
  • Dynamic port forwarding (SOCKS like)
  • SFTP file transfer
  • X11 forwarding (allows to run x-windows programs on the remote server)
  • UTF8 support in terminal emulation
  • Connection through proxy

SSH Also Requires a Good Terminal Emulation

However, SSH only covers the actual transmission of data between the client and server. But the secure shell client is usually a terminal emulator, i.e. a software that allows a remote computer to receive keyboard input from, and send formatted text (color, cursor placement, etc.) to the user's computer.

Obviously, the client still needs to be able to perform the functions of a terminal emulator (supporting various terminal emulations, file transfer, printing), but also extra functions like , logging, script-automation, host directory and so on.

The ZOC Terminal does all that and more. Together with ssh features like latest encryption and public key authentication, port forwarding, tunneling, smart card authentication, etc. this makes ZOC an SSH client that you should try.


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